The Rant

Frankly there are times where trying to be reasonable and courteous in the discussion of matters of diving doesn't seem to do any good. Well. That is it doesn't seem to do my blood pressure any good... or so it feels. This is where I let off steam on subjects I feel strongly about and some I don't. I will leave you to decide which is which.

PS: They are in no particular order and there will be some overlap between sections but I'll try not to repeat myself too much (unless I really want to).

Dry suits BCD Jackets/Wings Twinsets Blobs
Reels Weighting and Lift Computers and Tables Depth and Depth limits
Ponies Gases and Gas Switching Rebreathers Masks
Dumpable Weight P-Valves 300 Bar Training
Argon Long hoses and the Hog rig Failure points Air Horns
Marshalling DIR Go Pro Suit as buoyancy
Technology or skills? Gradient Factors Nitrox Airway separation
Gas Calculations Fill Rates Oxygen Cells Gills
Fatalities Seasickness Gloves Narcosis
BOV/DSV Conservatism Solo Diving BSAC
Alpinist Stages Rebreather types Internet Divers
Helium Spanners and O-rings Bailout Temp Stick
Oxygen Clean Bungeed wings Blinded by a torch? Reverse profiles
Gas boosting Gas Planning Cleaning Crack bottles
Scrubber duration Nipples Trim Pressure and height
Breathing Pressure Fins Technical diving
Deep air - - Strokes

Most of the physical constants I quote come from the CRC Handbook of Chemistry and Physics 77th Edition (1997) aka The Old Rubber Book.
Throughout I will use SI derived units except where I believe I need an old one to give the right feel. So, sometimes I will use tons, knots or mph but normally meters, seconds and bar. I will assume that atmospheric pressure is 1 bar although the barometer on my wall tells me that right now it is 0.993 bar. I will assume that 10 meters of sea water adds another 1 bar although I know it is really salinity dependant and should be 9.81*1.027 meters per bar. Please don't give me grief about psi, feet or ATA as me laughing at you may cause offence.

Dry suits
I live on the south coast of England and diving is supposed to be fun. I may personally dive outside what is known as recreational diving but it is still amateur and diving for my own enjoyment, so many divers seem to forget this. Once I have booked and paid for the trip then nothing will change the price so I can dive completely on whim. OK so if nobody is forcing me into the water why should I be cold?

Down to my dry suit. I have a dry suit to keep me warm. Getting cold sucks. Getting cold also triggers those little receptors in your brain that say 'Bad News'. Their function is to train you not to place yourself at risk of freezing so places you got cold get black balled. This, I think, is why a good percentage of UK divers have given up within three years. Their wetsuits allow the aversion therapy effect of cold water to extracted its due.

Tendons like steel wires Seals
OK so what do we want from a dry suit? Well first and foremost it must be dry. Don't assume this is always true. The seals on a suit are a common problem. If you are of the more chubby build with smooth rounded flesh (bioprene) you can use either latex seals or neoprene seals. In fact neoprene, being an insulator is nicer on your neck as it helps keep things warmer. However if you are a scrawny runt like me who can see the ligaments in his wrists and neck standing out like wires you need a latex seal that snaps round you like an elastic band. My current rig has a neoprene collar round the latex seal. It seems a nice idea but I can't admit that I notice any difference.

I used to use cheap Boots talc on my latex seals for everything but now only use it for storage. Avoid the baby talcs and any additives as they are usually oils and latex hates oils. Do inspect your seals regularly as putting a thumb through a perished seal as you put the suit on sucks. Now I have some barrier cream and it stops the 'hanged man' scar back at work on Mondays. I've also had very happy dives on Vet Lube (dirt cheap) or the quick and simple to obtain 'Gloop' from the dive shop. These are especially good when you come to disrobe as a dab on a finger slipped under the seal and wiped round makes things so easy to slide off.

One thing you will need is to do is stretch new latex seals. If they're the right size they will be horribly tight for the first few hours and while the advice is often to trim them down a bit that just hastens the day when the go all sloppy on you. My preference is to stick something in that measures the same circumference as my wrists and neck and let the latex get used to what I want then cut it. That seems to work for me.

The dump lets the expanding gas out of the suit as you ascend and allows you to control the total buoyancy. There are two sorts of dump. The one we used to use and the one we use now. They are called the cuff dump, simulating a sloppy wrist seal, and the auto, or shoulder, dump. The cuff dump is a one way valve letting gas out so if you raise your arm the air runs up it and bubbles out. Drop you arm below your body and it is trapped. If you have been using a cuff dump for years you might as well carry on but if you are starting out from new there is no choice. Go for the Auto.

New seals on 'stretch to fit' So what is so auto about an auto dump? An auto dump is just a one way valve outwards for air with a wind up spring so you can shut it off when hanging about on the surface waiting for the boat to pick you up. When, on the way down, the gas in your suit compresses and you feel the suit beginning to squeeze a bit you push the injector to add some more and when you ascend the gas in the suit expands and pushes its way out. Easy if your weighting is right as auto equates to no think, it just happens.

However then somebody says 'You should use your suit for buoyancy'. So you have to put more gas in and the auto dump correctly lets it out again. So you are told to wind up the spring a bit. This has the effect of making the suit run a constant amount of over pressure so your buoyancy goes totally to pot as you ascend. Using you suit for buoyancy is a useful drill to practice as it converts ARGH! My BCD has failed! I'm going to die! into Oh Bother. Put some more air in the suit. I wonder what it is going to cost to fix this BCD. BUT you don't have to do it all the time. I'll come back to this in a later section I expect but for now I'll leave it that I open the dump right up on my predive checks and close it when I surface après dive so I can blow the suit up like the Michelin man for maximum warmth in case I'm last in the queue to be picked up.

Ice dive - Yes, it is me Undersuits
A dry suit doesn't keep you warm. Yes the original ones were made of thick neoprene but it didn't work. They compressed as you descended and it was the whole sorry wet suit story all over again. You have to wear something underneath. Because it is dry inside that is something that is fluffy and traps air and does not let it circulate. Air is a fantastic insulator but it must be still air to work. To stop your fluffy undersuit getting compressed into a very flat and unfluffy thing you have to top up the air in the suit as you go deeper and it compresses but that's it.

Now once you have the idea of an undersuit you can start to improve it by adding features. The first is to coat the fibres that trap the air in a hydrophobic (water hating) film so it is very hard to saturate it if the suit leaks. This can make a big difference if you tear a hole in things. Secondly add a hydrophilic (water loving) layer next to the skin to wick away sweat and any minor seepage so you don't get cold wet patches.

Now out of the water an undersuit feels slightly strange. The arms are too short because your wrists must stick out to go through the suit cuffs. Also the neck feels funny as you can't have a collar or it would get into the neck seal. It's not really quite as warm as you might expect as it is the wrong shape but it is the right shape to dive. When you try and wear a nice warm jumper to add to things it all bunches up under the neck seal and lifts it off your neck and you get this nasty trickling sensation. I've seen this done...

To the suit I added booties. Socks are never enough to keep your feet warm and cold feet bring on the classic divers calf cramp that they taught you to pull on the end of your fin to relieve in beginner classes. I used to have a real DM demonstration quality cramp release before I got my booties. Now it has withered away like all those other skills I don't use.

Oh and beware of fluffy linings stuck in suits. They are the very dickens to launder unless you want to push the whole thing in the washing machine in which case it leaps about the kitchen like a thing possessed if you fail to stop the spin cycle.

Torn neck seal. Inflators
Not much to say about these. This is where you put the air in to stop the suit shrink wrapping you as you descend and the air within it compresses. I'm told it can be handy to have the nipple on the wing and the nipple on the suit the same but I never bothered. I feel slightly squeamish at the idea that if one of my buoyancy sources has failed that I am going to disconnect the working one? No if the drysuit fails I'm out on the wing and vice versa. Nothing that works is going to get unplugged. After all I only need to add gas to the suit as I descend. If I have lost my suit inflation am I really going to carry on down? That was my backup buoyancy.

You might want to revise this doing complex cave penetrations with serious ascents and descents on the route as out might be down but if you're reading me for advice I'm guessing you're not ready for that yet.

There is another point but it might be easier to fix. All my Apeks suit hoses are equipped with a big chunky pull to release sleeve. This is good news when you get an inflator jam open fault as you can just grab and pull. If I were to swap to a wing/BCD style nipple there I fear I would have to swap to the much smaller pull to release sleeve they use. I don't like that idea as a drysuit runaway is a serious problem calling for a very fast response. A big chunky release would not fit onto either my BCD or either of my wings, there just isn't room so the more I have thought about it the more I think I'm going to stick to the standard fittings.

I have three different rigs I dive with my drysuit hence the fill hose can be coming from three different directions so I have one of the Apeks 350° swivelable ones so I don't have to slack it off and reseat it every time I change.

Front entry zips look the business so my new suit has one but what else do you take a buddy along for? My old shoulder to shoulder type was never a problem and it was not a consideration in selecting the new suit just an option.
The only important thing about zips seems to be to use bee's wax on them. I've tried some spray stuff (dire) and some sort of foamy stuff with a brush on the top of the pot (even worse). Wax is a fraction of the price too. Do it regularly and then some more. I have got to the point of keeping a stick of bees-wax in the bottom of one of my suit pockets so it's always there when I unload when dekitting. A quick swipe takes seconds and the zip runs like magic. I didn't always do this and I paid the penalty.

Pockets are a good idea. When you upgrade from a simple BCD to a more technical rig the big thing you miss is pockets so having some put on your dry suit helps. If you RIB dive you don't want them on the front but on the sides or you stuff gets squashed as you clamber back over the side. Big is good so beware of the documentation style flat ones. Having a ring you can bolt snap your car keys too helps avoid the problem of how far you have to dekit in the car park before you can get a place to put things down.

I carry the spare torch, spare spare torch (a tiny one to read the depth timer and tables), tables, spare blob with the spool, yellow blob and a spare mask in my pockets and it's getting a bit crammed.

Pesky mice Storage
I've had a problem with mice chewing at a suit that I left on the floor. It wasn't a very big hole but I got very wet. Hang the suit up by the boots with the zip open. You can buy a hanger to do this or make one. This means air can circulate and it can dry out properly. Don't ask me how to store it in the 'off' season because once you have a dry suit there isn't an off season. If you store it damp and rolled up and when you come to use it it has fuzzy bits growing on it that's your problem not mine.

There have been three choices for what to make your drysuit out of over time so I will deal with them in order.

Uncompressed Neoprene. This was the first material and it has largely gone out of favour now because it's junk. Neoprene is an insulator so the suit provided additional warmth when shallow but, like your wet suit, it compressed at depth and provided less and less insulation. Annoyingly this was compression you could not deal with by adding air to the suit so it didn't gain from the idea of being dry.

Membrane or Trilaminate. This gave up the idea of the suit material insulating and focused on being waterproof and neutral. All your warmth came from the undersuit that you wore. I have two of these.

Compressed Neoprene or Crushed or the high density stuff some people use. This is an attempt to recover some of the insulating properties of the neoprene suit but by pre-compressing the bubbles to lessen the changes caused by depth on it. I can see the point but I'm not convinced enough to make it a reason to choose. As far as I can see it is just Neoprene pretending to be Membrane. If you are worried about floods the little extra insulation compressed neoprene gives you can easily be multiplied by buying the right undersuit. Neoprene suits tend to have neoprene seals and Membrane suits latex by default but you can swap things round. If you have a suit that fits it doesn't really matter much. The real problem is that you don't use foamy stuff like neoprene for waterproof seals anywhere else so why in a dry suit? Add what salt water does as it dries out in the surface pores of the suit and they soon look very tacky unless you rinse them in fresh water each and every time you dive them.

Your drysuit needs to fit. This is why the companies offer made-to-measure services and actively promote them. However you need to be aware of two points on the fit of a suit.

Firstly you must not compress you undersuit too much. You are paying for all that nice insulating air trapping fibres so don't squeeze all the air out to put the suit on and then expect to stay warm. It may have looked quite the dive god in the shop and disguised the tummy a bit but this is not a fashion accessory, it is here to keep you warm.

Secondly you need to be able to move about in it. Technical divers worry about getting a hand to their tank valves and that can be hard enough without a suit on and impossible in a restrictive suit. Also not being able to put your own fins on because you can't bend over far enough will the cause of endless amusement to your mates but the day a fin strap pops off in the water you are faced with a more serious problem.

Using a dry suit
A quick rundown on why and how we use drysuits.

Dry suit. A bag to keep your temperature sensitive bits in that stays dry on the inside so you can wear some nice warm clothing.
Problem. Nice warm clothing is only nice and warm because it is nice and fluffy ie: full of air. As you descend the air in it compresses and it becomes progressively less fluffy and hence less warm.
Solution. Add air to the suit as you descend to keep it at roughly the same volume, fluffiness and warmth.
Snag. As you re-ascend you need to progressively dump the excess air in the suit or it expands and you leave the realms of fluffy and become a balloon leading to a very rapid and rather unhealthy rush for the surface.
If you are diving a single cylinder you can probably just get away with putting a bit too much air in your suit at the start and not putting any in your BCD because your tank only changes in weight by about 3Kgms between the start of the dive and the end. Whether you choose to do this routinely or not is up to you but it is a good trick to have in the mental toolbox as I've had the inflator of a BCD come off in my hand and if all you do is sigh and switch to buoyancy by suit it's just a trip to the shop not a serious problem.
My personal take is to put the right amount of air in the suit to keep it comfortable and do the actual control on the BCD. Getting excess air out of a suit does require encouraging it to head for the dump and the bit in your boot, the other side of your macho strap on knife, will take its time especially if you are holding a nice, stylish horizontal position in the water.
Weighting. This is key with a dry suit. Get the weight right and the auto dump really is auto - you just inject air to keep the suit comfortable on the descent and the ascent looks after itself.
My method for this is to ascend and do a pretend stop at about one meter depth and keep handing off the lumps of lead until I can't do it comfortably anymore because the BCD is empty and the suit has to be encouraged to squeeze a bit. Then work out the weight of the gas left in the tank and add that to the weight belt so I can do the same on virtually empty. Be sure that you are coming into the stop from below so you are hitting the real case of what air you can get out of the suit ascending not what gets squeezed out on the surface talking to the guys taking the lead from you.
Also beware. Your nice new undersuit is at its fluffiest so you are going to have to revise down the amount of weight you need over the first couple of hours wearing it. This change can easily run to several kilos. I pool dived a new undersuit and got it right and ten minutes later dropped two kilos, another quarter of an hour and another two went and more. I thought it was stable but I wore it for a couple of days boat handling for other divers and when I finally sea dived it I was still overweight.

Your drysuit needs looking after. The zip, well the metal zips, need lubricating. Not on the tiny metal teeth that actually engage and do the seal that nobody sees but on the outside where the zip 'closer' slides along. Taking a thing like that and putting it in the sea is looking for trouble so look after it. The conventional wisdom is sticks of bee's-wax but there are several commercial products ready to take your money. Personally I like the bee's-wax as, since lubricating my front entry suit regularly makes it so much easier, so a stick of wax in a pocket means it gets a quick wipe over after I do it up.

The other thing that needs lubricating is you. Sliding a dry hand into, or worse out of, a dry seal is hard and stresses the seal. For latex you can use talc as it is a super lubricant. Avoid the 'scented' or such versions as the oils involved are not good for latex but get the cheap 'baby's bottom' version. Alternatively both neoprene and latex work well with water based 'personal' lubricants. KY gel if you're posh, vet lube if you remember to buy it in advance with commercial products like 'Gloop' coming about half way on price. I find a squirt of lube on a finger that is then run under the seals helps no end when you're struggling to get out of the suit after a couple of hours in the water.

Drysuit diving isn't a black art, it isn't even hard. The main trick, as with all buoyancy work, is to correct early based on how fast you are moving not how close you are to your target. That way a descent rounds out nicely with a deep breath to bring you to a halt and as you exhale you are left just hanging above the wreck. The only time I actually fiddle with my autodump is when I hit the surface where I shut it right up or the waves massage all the air out and leave you rather unfluffy and chilly.
One word of warning however. Every dry suit newbie thinks their dump isn't working fast enough and you see long discussions and some quite fanciful solutions being presented. Why do they feel this? Because they are trying to dump too much gas too late. Now with a BCD you can get away with dumping gas in a last minute panic when you realise you are going too fast to stop easily but in a dry suit the air is all spread out keeping you warm. You adjust buoyancy to set the speed of an ascent/descent not the final destination. So, when you see the depth of your safety stop coming up you slow it down and slow it down so you virtually just ease into it with no overshoot. If you try and zoom up to it and stop dead you are going to be disappointed.

Nothing is watertight forever and, although if you're careful leaks just result in a slightly damp dive occasionally some individual is going to have a catastrophic flood. So there are two consequences. The first, obviously, is that you don't stay so warm. Yes a good undersuit will still do its best to keep the water still but you are going to lose heat more rapidly and if you are committed to a lot of deco stops this may be a problem. Secondly you are losing a lot of nice buoyant air and replacing it with neutral water so you effectively become 'heavier'.

I did a simple test in the pool using my Otter 'Commercial 300' undersuit, Weezle overboots and a big pile of weights. I adjusted myself for neutral trim in the shallow end with no scuba kit other than a mask, fins and a pocketed weight belt. Then I unzipped the suit, let it fill up and then unloaded weights onto the side until I was back to neutral. The difference between zipped up and dry and unzipped and soaked was 7kgms. This means that after a total flood I would need 7Kgm of extra buoyancy in the BCD/wing to get back to neutral to ascend. This is the sort of number you need to factor into your buoyancy plans. Try it but remember to bring a big tough plastic sack to take the undersuit home in.

I've used the term squeeze and every drysuit owner knows just what I mean but why does the suit squeeze? It's not quite as obvious as you think. Consider a ball inside a spherical balloon. As you increase the outside pressure the suit, being flexible, transfers that pressure to the inside gas and the 'suit' closes in on the ball. The increased depth reduces the gap but the air is never eliminated so the suit never touches the ball so no squeeze. How come there is a problem?
Well there are two factors in play here. The first is the undersuit. This is nice springy cloth that tries to maintain its volume leaving less for elsewhere but the main contribution comes from the fact that the suit material may be flexible but it does not change its dimensions. This means that in our ball in a balloon example the balloon cannot just shrink around the ball but it must fold and crease up to enclose less space. The squeeze you feel is those folds digging into your soft bits.

I like waistcoat style BCDs. I know that's not a very hip thing to say in tecky circles but for diving a single cylinder rig they are tried and tested gear, they work and, what's more, they have pockets. I still have the Scubapro Club jacket I bought way back when I started diving and it still works just as well now as then so it goes on holiday with me and does training with club beginners. However there are a few things I see people doing that I think are odd though...
Have you ever watched a diver stop fining and suddenly slump to feet down? I see it quite often and it normally correlates to first stage behind the shoulders tank position. I find that a steel tank certainly, and normally ali too, needs to be set so the valve and first stage are behind your head so the tank is well up your back for a diver to naturally hang horizontally in the water. Secondly I do like a string operated pull to dump on the left shoulder. I'm not quite so happy about pull to dump on the inflator hose as one once came off in my hands - dumping the contents of the jacket. Clips that separate the shoulder from the chest section are good too as you can easily get out of the whole thing in the water to pass it off to a RIB crew.

However let's just consider the physics of the whole BCD/Wing controversy and see if we can shed any light:

A BCD and a Wing What is a Wing? It's the one o the right. Well normally it is a single inflatable bag, either shaped like a horseshoe or it is an oval donut shape. The idea is to provide a reasonably large buoyancy compensating bag with the bulk of its volume either side of your cylinder or cylinders and connected across the top and, if possible, across the bottom. A wing may be sold with a harness and a means of attaching a cylinder or cylinders. Also, to reassure the pedants that I haven't lost the plot, a Wing is a BCD (Bouyancy Control Device).

What is a waistcoat style BCD? Well it does more than a Wing because it almost invariably designed to attach itself to the diver and attach the (normally single) cylinder. The key trick is that its inflatable sections are within not only the back but also the sides and the fronts of the shoulders. Where the wing put everything behind you the... Heck I'm going to call it a BCD, the BCD lets the gas go round you.

So what happens? Well in the water air moves strongly to the highest point so if you're trimmed out nicely horizontal on both the buoyancy is behind you back, well above your back in truth. This is why both wings and BCDs have those nice pull the string dumps behind your shoulder and behind your hips. Wringing air out of the inflator hose needs a good pull up, behind your head, to provide an uphill all the way out path. Remember that neither of them are tightly inflated balloons so air will only escape if it can go up. To me this says that under water they are pretty much identical which matches my experience.

However when you turn to the head up position to ascend and to hold yourself on the surface the differences start to show. On the wing the gas is constrained to stay pretty much in the same place and just rise to the top of the wing while on the BCD a good deal of the 'top' is at the front. Why is a BCD designed that way? Well the idea is to mimic the design of a life jacket and ensure your head and most importantly your face is out of the water pretty much regardless of how you rig it. It's not that a Wing tries to drown you just that this is not a priority in it's design and you can rig it (cylinders low, wing very high) so it tends to want to be on top.

A Wing is just designed with different priorities in mind. They usually have more volume to give more lift and are often very wide so they can accommodate a twinset between the two sides without the tanks stopping the bags fully inflating. Blowing a BCD up until the over pressure dump squeaks is common but few people would find a wing comfortable like that.

My preference is to use a Wing on the twinset or the rebreather and a BCD on a single. I have them and that works for me. Of course there are several people that that tell me they can make a wing that is equally good on a single or a twin but after exhaustive research I discover that what they really mean is equally bad. There are just too many tradeoffs to make that one work. Of course there are some people who insist that they must have kit that is the same under all configurations. Since I dive lots of different configurations I find that firstly rather amusing but then, when I consider how they will cope with a fault, rather worrying.

So he pins me in a corner "Right. What's all the hassle about twinsets? Two tanks? I have two tanks and I use them for two dives. Didn't I see you do two dives on one fill? What's the difference?"

The twinset is about redundancy. In diving things break and, sadly, if you're not able to supply a solution to the problem you drown. The problem is that in some situations the surface, in terms of an emergency ascent, is not available to you. If you are in a wreck or a cave, if you have significant deco racked up or if you are just too deep you must do something to manage faults.

So what is the simplest fault? It is common too. It is a regulator freeflow. We all practiced this in the pool before an instructor ever took us into wild water. You can get a few breaths from the gas rushing past your reg to help you on your way up but you get up and out at that level.
The best solution is to turn off the cylinder. Let the reg warm up a bit as most freeflows are icing somewhere in the system and turn it on again and carry on. If you're on a single that needs a buddy and they might not be instantly available so a pony might help. However the twins thinking is to split your gas into two equal halves and carry it in two tanks. If you equip each tank with its own regulator set if one goes wrong you can breathe from the other.

In its simplest form that is all a twinset is. Two completely independent cylinders. So you ensure that there is always a good reserve of gas in both sides you adopt a gas management plan that involves swapping between the two at intervals so that at whatever moment a failure strikes there is always a way out.

Manifold However, although independent twins are used in some circumstances (some solo divers like them), there are some things we can do to improve on them.
Joining the two tanks below the valves with a manifold which itself contains a valve allows you to dispense with the side to side swapping and ensures that you use your gas evenly. The downside is that you need to react to a freeflow promptly as it can empty both your tanks but the trick with the manifold valve is to only just open it so it can equalise the tanks as you breathe but not keep up with a freeflow. This keeps the loss from the good side down and if your problem drill starts with close the manifold valve the remaining half a turn whatever has happened cannot cost you more than half your gas.

Normally closing the pillar valve feeding the offending regulator set solves the problem and you can open the manifold and have access to all your gas through the good regulator. The manifold valve only really becomes into its own when a pillar valve goes - a most unlikely event but who wants that said at their inquest? However since the tanks are behind you accurate diagnosis is not easy so the manifold gets shut first and only opened when the problem is sorted out.

Valve diagram On the usual two dive boat trip the twinset does involve you taking the dive two gas along on dive one but that is not a bad thing. Despite much discussion we tend to still dive the deepest dive first and make it the significant dive of the day with the second as an optional extra. This means we not only take the gas along as spare but we remove much of the gas limits from the first dive and can split the gas maybe 60/40% if we want. After all who minds cutting short a scenic drift over a bunch of weedy boulders because you wasted the gas diving the deep wreck?

The key trick with the twinset is practising the shutdown drill that enables you to get payback from the valves. Your hands need to know the drill and it needs to be fast. I practice the sequence of close manifold, close primary reg breathing it dry, swap to secondary (on the necklace), close secondary and open primary (simultaneously is easy on my inverted twins) breathe the secondary dry and swap back to the primary, open the secondary and finally open the manifold valve. I can't see any reason why I'd ever need more than a small part of that drill but the hands know where the valves are.

As a final note let's just discuss the manifold. Some people don't have the picture in their minds. The manifold joins the two cylinders below the pillar valves. If the manifold is open the two tanks are joined and both pillar valves draw from the common volume. Either pillar valve can be shut to close off a failing regulator and you can still breathe all the remaining gas on the other DV.
If however the fault is more systematic, if the pillar or the valve itself has failed, then that side is bound to leak away all its gas so shutting the manifold valve saves half the supply. Continue to breathe the side that is losing gas until it runs out so at least you get some of it and then move over to the remaining side. If you are diving rule of thirds (bring one third of you total gas back home) and if you are slick with your valves you should retain enough gas to finish the dive without resorting to your buddy.
You may go an entire diving lifetime and never have a fault that needs you to close the manifold but it is still worth practising. Certainly I have had regulators freeflow, but if you insist on quarry diving in February you must expect that occasionally, but I've never had a pillar or valve fault. However they do happen and it's always nice not to die.

DSMB UK divers tend to use Delayed Surface Marker Buoys. They suit the way we dive. The problems we face aren't uncommon. They will get a mention again and again in this page: Tide, Visibility and Cold. The DSMB addresses the problem that due to the vis being only a couple of meters on a good day finding your way back to the shot line to ascend back to your boat might be a good trick but it's a bad plan. As having a string up to the surface makes a controlled ascent much easier and, as slow ascents are good for your life expectancy and general health, take the string with you.
I deploy the DSMB from the bottom or from the wreck and if I can tie the reel down I prefer too. Then I have a rate I turn the handle at to give me the 9 or 10 meter per minute, or slower, ascent rate that I like.

I use the Buddy DSMBs although any self sealing one would do for what I want. My primary blob is a self inflator with a small gas bottle on it. When in full tecky mode (all the toys) I have that blob and reel hanging behind my right hip and a non-self inflator with a 20 meter spool of line in my pocket. Add a bright yellow blob with a piston clip to run up an existing string as a HELP indicator normally to imply "Please send my drop tank down". (I don't carry it if there isn't a tank available.)

If you are using blobs there are a few things you need to be clear about:
Firstly: Blobs are expendable! If the reel jams or some problem develops where the blob starts pulling you towards the surface you need to be able to just let it go without undoing any clips or such. A blob being pulled out of your hands is annoying but being pulled up is a route to a helicopter ride. Never clip a reel to yourself. Just rely on your fingers to hold it. Clips don't let go under tension. You can hardly miss this big red/orange thing bobbing about on the surface when you get back to the boat so it won't stay lost.
Secondly: Even if you do have a self inflator practice using it normally. Learn to blow it up with exhaust bubbles or the octo or something. You should be able to get three fast breaths into it and if you are reasonably deep as the gas expands this will blow it up tight by the time it has got to the surface. Don't rely on the crack bottle. The number of times they have failed on me (normally my fault) is horrible.
Thirdly: Watch your buddy when they deploy a blob. I sometimes pop out my wreck reel and hitch it onto their handle so if we get a jam he can let go and I can run out line. I have had a buddy hold onto one of my fins as I sorted out a slightly inflated blob with a problem. He had his other hand on the wreck and I greatly appreciated the assist.
Fourthly: Make sure the skipper knows what your blob procedures will be. Some like one blob per diver if you're deco diving so they can count you off the bottom and know you are on schedule. If you are reeling as a pair warn them. If a good diver looks at his schedule and says he starts his ascent after 28 minutes he will hit it accurately.

Ascending in darkness with 2 meters vis you really appreciate the DSMB. There is no way you can tell how fast you are rising but keeping the string tight tells you you are holding level. From the skipper's point of view he can stay over the wreck but when the blobs appear he can count them and then follow them as the tide starts to run as you do your stops. When you finally break surface he is there watching for the heads to appear beside the markers.

Putting up a DSMB at depth
You want the reel held so the line and any tape loop is tight so it can't tangle round your reg but you are ready to thumb down the release as it starts up.
You tip your head on one side if you have two exhaust valves on the reg like my Apex50s so all the exhaled breath comes out of the top one.
You make sure the open end is well over the reg and give it three sharp breaths then move it away in front of you so you can see it is clear. This means that when the gas hits the top of the unreeling sausage and up becomes an imperative it is all held out clear from you. Three breaths doesn't look big deep but by the time it gets to the surface it is full.
Then thumb the ratchet and off it goes.
Keep a (gloved) finger on the reel so it doesn't overshoot and 'bird's nest' and be prepared that if the reel jams you can give it one jerk to try and free it then you must let go. (This has only happened to me twice, both times down to knowingly reeling in badly and not bothering to run it out and do it up again.)
As soon as it stops crank in the slack so you think you have stood it up and 'feel it' (you should feel the waves if there are any).
Then signal your buddy and start to ascend.

Reel and blob I know that half a turn every two seconds on my reel is roughly a 10m/minute ascent rate so I stay a little negative all the way up. Again hand hold it so if something does snag the line you can let go. I've never had this but I've seen my buddy pulled to up a bit and then he let go and the reel vanished. He shrugged and I deployed my DSMB and we carried on with the ascent. We recovered his with the boat hook later, I think we just drifted through the shot line but the tide was running strongly by then.

I carry a self inflate (bulky) as the device of choice on an 85m+ reel and have a blow it up with your exhaust gas model with a 20m spool (no moving parts) in a pocket. Most deep water charter skippers assume they are part of your kit and drop you on slack water and then, when you've finished your wreck tour, you blob off and do your stops as the tide begins to run and he just follows the cluster of orange markers.

Technical divers use the code that running a yellow blob up the same string means 'HELP! I'm in the poo' and we often carry a drop tank of gas to enable a guy with a problem to finish their deco. I have a yellow with just a P-clip on it to do that but I've never had to use it yet. Oh and I also have a drop tank of 50% and the rig to run it down the line because without that you're wasting your time putting up a yellow.

An additional note on colours.
A friend, who is a professional skipper who picks up divers and equipment virtually every day in what ever conditions apply and who probably knows more about practical diving than I ever will commented:

I would say the worst colour we have found to look for is white - a stealth buoy, in other words a shotline with a Sofnolime tub tied to it, is not easy to see.

Some SMBs are awful - the older buddy bi-coloured ones where the yellow half is very yellow, not fluorescent, are hard to see sometimes. Any SMB shaped like a liftbag is a bit of a problem as well as it can be discounted as a liftbag and ignored if the skipper is having a bad time, or if you have drifted a long way it can look like a lobster-pot buoy. The narrow Halcyon ones are not visible and should only be used in a team when one person has put up a decent sized bag.

(No credit as I edited it rather a lot and it's not fair to apply somebody else's name to my version of their words.)

It wasn't quite what I expected but looking back at pictures I've taken from dive boats I think I can see what's being said. The sea can look white, green, black or even occasionally blue so you want a colour and an intensity that contrasts. Bright, like white or yellow, isn't enough when the sky is bright as the sea reflects it, dark doesn't work on a gloomy day but the sea is never anything like orange or red.


Reel I recognise three sorts of reels - others may differ.
A DSMB reel which has a ratchet which I can thumb down to let it fly and then wind myself back up the line click by click and only need one hand to hold the reel while the other busies itself taking care of the other chores.
A Wreck or Cave reel that if it has a brake has a friction brake on the spindle so I can drag it out and wind it up as I lay the line I will retreat on. As cave line can be rope it might be a big reel.
A Spool which has no moving parts but is ultra compact and, provided the line doesn't snag, has nothing to go wrong.

It is worth unreeling and rewinding a line before a deep dive. All my jams have been down to a cack-handed reel up on one dive getting twisted up and then the next deployment snagged. If you do a slack wind most of the ascent and then go tight at some point the tight bit binds down into the pile of slack and is almost bound to snag up.

It is also a good idea to have a handle that is an enclosed loop so your hand traps the reel but can still be used (see picture). A handle that you must hold commits that hand from when you launch the blob to when you pass it up to the boat bitch.

Weighting and Lift
There is some drivel talked about weighting. It is really very simple. You need enough lead to get down and to stay down and no more. Lift gets the same stupid treatment but is just as easy. You need enough lift to put you back on the surface in an emergency.

OK lets put some numbers to that. The worst point for weight is doing the last part of the ascent when all that gas you were carrying is gone. Now diving a 12L/232bar single cylinder set you carry 3.3Kgs of gas so you need enough lead on your belt to hold a stop right down to empty. Now I do mean empty here. I know you are a good little bunny and you will normally bring a reserve 50 or so bar back to the boat but that is a reserve. It is no good saving your buddy from certain death and then bending him because once you both started breathing the reserve you couldn't hold the stop. That's not a reserve that's a passenger. You can't use it.

The best way to do this is in the pool. Put on the full clobber with all your thermals et. al. put your weights in the BCD pockets and dive the shallow end. Fill the suit and BCD and then empty them to descend. This means that any pockets of trapped air are filled just as they would be on the ascent. Now start unloading your weights onto the pool floor and see what you can get down too. Then note the SPG reading so you know how much gas you have.

Weights Let's say you end up with 7.5Kgs of lead and 120bar of gas and you can still exhale to go down. Less weight and you are pinned to the surface. Well in a 12L 232bar of air is 3.3Kgs weight so 3.3 divide by 232 multiply by 120 is 1.7Kgs of gas weight. So now you know that in fresh (pool) water you need 7.5+1.7 is 9.2Kgs of lead. As you only have the old stuff in pounds multiply by 2.2 and round it up to 20lbs. I confess I'm too stupid to keep swapping units so I put all my weights in turn on the kitchen scales and stamped them with their weight in Kilos.

Good. Now we need to turn that into sea water weight. Sea water is 2.7% more dense than fresh water so we need to increase your total weight by 2.7%. Wait until everybody else is out. Get the bathroom scales. Switch them to kgs if they are the fancy digital ones. Put the full kit on including your suit, tanks and freshly calculated freshwater weight belt. Stand on the scales and note the number then scurry off and change before anybody catches you playing dressing up. It has been suggested that you only need to hold these items but very few people can read a set of scales holding a tank in one hand, with the suit under their arms and a BCD clutched in their teeth. If it reads 110Kgs you need 110*0.27 = 3Kgs of lead extra. So your seawater belt is 9.2+3=12.2Kgs. 12 will do.

Actually if you always assume you need 3Kgs more in the sea you won't be far wrong and it's a good starting point if you don't fancy all the rigmarole with scales et. al. So where does this place us against the PADI Hold a normal breath with the jacket deflated and you should float eyes level with the water? Well it's just about the same. Eyes level places you about a kilo positive so when you exhale and lose about 4 of your 5 litres of lung capacity you end up 3Kgs negative and on the way down. It's a good trick but it only works with a single and if you don't understand what hold a normal breath means or if you just can't stop fining, and lots of people don't seem to be able too, you are instantly wrongly weighted.

Right so now we have the lead right how much lift do you want? Well lift is not a problem a beginner worries about as a BCD has enough for a single cylinder rig but when you go to twins and stages it starts to be a worry.

Again the maths is simple. What is the worst case? It is that you start down the shot line in full kit for a deep dive and, as you pause to see if your buddy is following, you turn and your drysuit neck seal splits. You are suddenly very cold, very wet and very negative as all that nice, warm, insulating, buoyant air bubbles up past your ears. You need enough lift to put you straight back on the surface.

So now, assuming you were correctly weighted before, you are negative by the total weight of the gas you are carrying, say 6.6Kg in the twinset and 1.9Kgs in each of your stages, plus the lift your drysuit provided and which you just lost. Well I'll tell you the drysuit is 7Kgs and the gas about another 10Kgs then you want 3 or 4Kgs to float your head above water so until you start to get into the realms of twin 18s all you need is a 22Kg wing. There are quite a few 40 pound wings, 18Kgs, about and with these you are into ditching stages territory and although you life is far more precious than your equipment you know as well as I do you will hesitate to drop something with a three figure price tag. If you are into diving twinsets stages will happen so plan for them. Is 22Kgs enough? Well when you start carrying two 12L stages for 100 meter dives so much of your gas is helium you actually carry less mass. Nobody ever needs a 100lb wing. Really.

20 meters Depth and Depth limits
How serious are those depth limits the certifying bodies stick on our cards? Do we really need to take them as gospel or are they just guidelines?
The picture is Holland Road Baptist Church in Hove. It was built in 1887 when spires were trendy so it has a spire. Look at the cars, you all know what a car is like, and then look at the spire. This is 20 meters. If you have a problem at the peak depth of an Open Water or Ocean Diver qualification this is about how far you have to swim before you can breathe again.

To picture my 60 meter Normoxic trimix qualification we need a block of flats. About 15 stories. I won't bother with a picture as I'm sure you can find one locally.

(Sorry. That was a lie. I know you will have difficulty finding a 15 story block of flats unless you live in a seriously high-rise neighbourhood. Try counting a few.)

Please bear this in mind when we discuss ponies, twinsets and such. It is so easy to get familiar with the numbers and forget what it looks like. Just because I have 59.6 meters in my log doesn't make 20 meters any less daunting than Holland Road's spire. So try it. Find something that is 20 meters tall that you can stand on. Then look straight down. This helps keep a sense of perspective to the idea that twenty meters 'isn't deep'. It's worth doing regularly.

It may not be trendy to say it in tecky circles but I quite like the old pony bottle. OK it's not much gas so I'd only use one on a planned no-stop dive but it has its place and can convert a disaster into an annoyance.
Pony They come in for a fair amount of criticism because they are small but as a get you out tool they do the job. The common feature of the too small shouters is to postulate that you panic and your breathing rate goes through the roof. Well if you haven't got a pony you will panic. If you have you have the solution right to hand so you stuff it in your mouth and breathe normally.

The three big problems with ponies are:
Firstly ponies that don't work. You didn't fill it, you didn't check it, you didn't turn it on. If you have a pony you need to keep it in a top quality state as when you need it you really need it. This means you buy it a good reg, you service it, you test breathe it regularly and you top it up again before the testing reduces the pressure much.
Secondly you remember that depth is the enemy of the pony. The deeper you are the more gas you use so while a pony is masses for an ascent and you can normally pop in a safety stop, if you stay at 35 meters you will eat it up. As soon as you put the pony reg in your mouth you are on your way up and out. If your buddy has waltzed off and got lost that is their fault. You are not going to go and take a serious risk by looking for them. You are on your last gas and you are going to get home alive now.
Thirdly you want it all accessible. You want to be able to see the gauge. Saying I don't need to see it, I'm on my way up cuts no ice with a nice 3 at 5 safety stop. You don't want the reg to go hard in your mouth just because you didn't know how much you had when you could have been in the boat. Conversely if it's all gone badly wrong already you might just need that safety hang to pull you back from your day getting even worse.

I bought a 3L/300bar tank with a TX40/DS1 reg set right back at the beginning. The DS1 was a mistake but it's now doing suit inflate duties on an Argon bottle so it didn't go to waste. That rig dived with me and my son for many years and, although we never needed it, it was the insurance in case something broke. I now use two 3L/232bar replacements and clip them onto the rebreather if I am diving with a diver without spare gas.

A note on gas consumption.
People like to quote the "If you panic you might be breathing 100L/minute. How long will your pony last then?" line.

100L/minute? I hooked up the pony to the big blending gauge and measured the pressure, then I screwed in the Apex 50 regulator set, sat on the floor and maxed out my lungs for 60 seconds. It was a good thing I was on the floor because my head was spinning at the end of that but I am sure I could not have got more gas out of the cylinder in the time. Once I was sure I was back to breathing normally again it was back on the blending gauge and it worked out to a few decimals under 100L/min. Since all the limiting factors depend on the viscosity of gases and these increase with pressure this number must go down at depth.

No, I don't think it was realistic but it was a test to draw a maximum line. I tend to think that if you have a pony going OOG may be a nervy moment but getting the pony reg in your mouth will settle things very quickly. It's if you go OOG and you don't have a pony that you are really going to panic but then your breathing rate will be 0L/min.

Oh and when you're planning a pony set up: I weighted mine, a Faber 3L/232 steel, with an Apeks regulator set on it and in the water butt it came out as 1.7Kgs so that is all the weight you get to remove and all the weight you need to move about to stay in balance. In the simplest case take 2Kgs off your belt on the same side that the pony is going. That's the empty weight by the way. Since it only carries 800gms of gas it's not a problem.

Gases and Gas Switching
Air is a bad gas to dive. OK. Perhaps I'd better explain a bit. Nitrogen is a bad gas to dive and air tends to have rather a lot of it. Now I'm not talking about drills in the pool or a pootle about at 10 meters or less but for most diving nitrogen is the pits. Nitrogen is biologically inert so all that can happen is that it dissolves in the water and fats that make up most of your body and when you ascend it will undissolve. If you stay well away from the no decompression limits of your tables the amount of gas that dissolves is not huge and should not cause you many problems but as soon as you get near to the no stop limits you are in the position where you are as full of gas as a can of coke. If you handle it gently it opens and tastes fizzy. If you let some seven year old (mental age) shake it up before they pull the ring then you will be cleaning it off the ceiling.

These are the dreaded bends. They are named after the 'Grecian Bend' of society ladies in the mid 1800s (:an affected carriage of the body, the upper part being inclined forward) and seen in the workers who dug bridge foundations and such in pressurised tunnels and caissons. They worked a shift under enough pressure to keep the water out and then walked into an airlock and were depressurised quickly and walked out. It used to hurt and they were often folded up by the pain. However they were normally OK again in time for their shift the next day and it was well paid work. Air dissolved in them and when they came out it undissolved. If you were unlucky you got trapped bubbles and it hurt. Probably little bits of you died but it felt a lot better when they pumped up the pressure the next day.
See stuff on decompression diving for a fuller explanation.

So Nitrox, air but with the oxygen content boosted is better. Oxygen is biologically active. A bubble of oxygen can spontaneously be absorbed as any tissue, if its blood supply is blocked, will become oxygen deficient and the bubble will redissolve to supply it. If a lot of the bubble is oxygen that might be enough to let the bubble move and be removed. Even if you don't get as far as bubbles more oxygen means less problems from nitrogen.

Oxygen has one big problem. It is poisonous. It is poisonous in two ways. Long term breathing too much oxygen under pressure leaves your lungs raw and creaky but far worse, in the short term too much oxygen blows your nervous system and leads to unconsciousness and convulsions.
Now a convulsion to a diver, who needs to keep that regulator in their mouth, is bad news of the terminal kind. We have tables to work out our oxygen exposure and the limits have been crept down over the years so now we tend to think that 1.4bar of oxygen is the max or perhaps 1.6 if you are pushing it. I follow the guidelines that I run my rebreather at 1.3 because the tables say I can do a three hour dive on that with margin. This limits the oxygen percentage we can have by the depth of the dive so nitrogen creeps back in more and more.

Nitrogen has another nasty effect that is narcotic and an Anaesthetic. It not only bends you but makes you drunk. People report silly things when under narcosis (narked). "Oh poor fish. Do you need air? Here have my regulator. I don't need it" at one extreme and claustrophobia, fear and general paralysis of action at the other. Below 25 meters everybody is narked, below 45 meters everybody is narked badly. Some people say they can become accustomed to the effects but most proper tests reveal no such acclimatisation - they are just learning to ignore it but their performance in dealing with a problem is no better. Deep diving we need to get rid of the nitrogen, oxygen won't do so we turn to helium.

Helium is not your friend. Yes helium can regain your brain from the blur of nitrogen narcosis but helium is a fast, unforgiving gas to dive. Helium gasses on fast and off fast and in large quantities. This means that decompression stops, even very short ones are significant. Deviations from the plan are significant. Safety margins are important. Accurate mixes are important. Helium makes you pay for that clear head. If you get bent on helium you are seriously bent. You don't mess about with helium.
However if you work out how much nitrogen you are breathing at depth as a partial pressure and then work out what depth you would be diving at to get that much nitrogen on air that gives you a feeling for the gain in clear headedness you get. It's not accurate but it's about right. Think of it as diving to 70m with you head at 30m on air - that's 43% helium for you.

A note on Helium and bends
The real problem with helium is that it is a 'fast' gas.
So what do I mean by fast?

On the Bühlmann model there are 16 'compartments' which represent overlapping tissue types. It isn't that your spleen is a compartment four and your leg is a compartment six just that all 16 overlap enough for all cases to get represented. Also when one compartment feeds another it is just represented by a slightly longer compartment. The system dates from Haldane and it's good physics.

For nitrogen they have one set of 'half times' for each compartment representing how long gases take to dissolve and release but for helium they are about a third as long.

This implies that helium dissolves in your tissues at nearly three times the rate so, in very approximate terms, a dive on helium is three times as long. However, as your deco is three times as long too, it evens out and a helium dive isn't all that much different from a Nitrox dive.

Until you miss stops. Then those ten minutes you missed on your 40 meter dive is effectively half an hour missed on a dive with three times the bottom time and your problem has moved up an order of magnitude.

Conversely when they haul your messed up carcass into a chamber all those helium bubbles redissolve at three times the rate and you then are back to doing the re-deco at 'normal' rates. Your untreated bend is much worse but, if they get to it in time, the prognosis is much better than the same level of bent on nitrogen.

One of the consequences of this is that your ascent needs to be spot on. The deeper stops can have some compartments off gassing while others are still on gassing so hitting those numbers is much more important. The fact that only a two minute stop is called for doesn't make it trivial (think six minutes). I much prefer a computer on helium precisely because in the deep ascent stage of the dive it is recalculating 'on the fly'. If I were diving tables I would select a far more conservative algorithm because I know I'm not going to hit my numbers to the second and all my errors are multiplied by at least three.

PS: I have stated in the past that you gas on 'more' helium and I think I mislead people. You gas on about the same total volume.

Computers and Tables
BSAC tables - OSTC3 computer When you start diving you learn to use tables. This is a good thing as it means that from day one you are planning your decompression. OK, not quite planned decompression as planning not to do decompression but it's the same idea. Then you rapidly discover that you get longer dive times if you switch to a computer as it gives you credit for not staying at your planned maximum depth for the entire dive and the tables tend to languish in the dive box unwanted and unloved. Then, you migrate to more technical diving and you start cutting tables with stops in on your PC and writing them up on your slate and following them rigorously. At this point you start to wonder what use a computer is, and in many circles it is elitist to deny owning one.

Let's be up front about it. I like computers. For no stop or simple one gas diving a computer is good kit giving a plan on your wrist and, providing you are watching it, it can keep you within no-stop limits when tables would be screaming in terror and hiding under the bed. When it comes to more exotic diving I bring out the more exotic computers and dive them.

I do not subscribe to the tables are best mantra even though for serious decompression I have been taught at times to dive the plan on my slate. However I'm convinced that tables are not the best way to dive a rebreather. As I have said before I dive with a computer and a back-up computer and I got that from a technical diving instructor and it made good sense. The crucial thing I did learn from him was that you need to practice switching computers to bailout mode, something I had never done before, because just thinking you know how to do it won't cut it when the problem strikes.

The only table I carry is a little snippet with the depth/times I can bailout from using my standard bailout mixes. Since I fill these when the tanks come back from test they never change so they are very predictable. This is the limit of my dive planning: give me the planned depth for the dive and I can sling the appropriate cylinders into the car and I'm off. How near I go to that max-bailout bottom time, or if I choose to overrun it, is a decision I make on the dive.

OK, that will make me evil and looking for a Darwin Award in some people's eyes but I see it as a sensible way to dive CCR. Actually my 'standard bailout' for a planned no (or small) stop dive is nothing external. I can breathe the dil and ascend on my independent suit inflate bottle if things go wrong with the loop. I am not convinced by people who clutter themselves up with large external tanks for relatively minor dives. This just seems to negate all the advantages of a rebreather and conflicts with what I was taught in Mod1.

Rebreather in cold dark water This is not the place where I am going to rave about people who decry certain makes of rebreather nor the attitude of some that all rebreathers are death traps. What I want to do is put the case for rebreathers as I see it. Now I am not bothered to justify the expense. Diving is a black hole for money so if you want one I know you'll buy it, that is just the way it is and it's not my fault. What I am interested in is what does the rebreather do well.

So we'll start with a quick recap - what is a rebreather? A rebreather takes the exhaled gas that normally bubbles off in Open Circuit (OC) equipment and goes to waste and recycles it. Naturally you need to replenish the oxygen in that gas or the world will gently fade away. There are various methods of controlling the mix you breathe that range from the simple oxygen only rebreathers, through the more complex semi closed systems up to the high-tech fully closed rebreathers with on-line oxygen measuring plus manual or fully electronic oxygen injection.

Now having an Inspiration CCR and a fully closed Oxygen rebreather I tend to view semi-closed systems as just drills I practice to manage faults but they have their place in the diving world. However I'm not planning on commenting on them in detail as I've never dived anything that primitive.

If you want to see my write-ups on my equipment with all the whinges look at my Inspiration pages and the dreaded IDA64.

Time for the sales pitch - what are the advantages?
Best mix for depth. Since you are blending the gas you breathe as you go you can always get the highest oxygen which means the lowest inert gas content so you have the shortest time to surface available. OK we don't always go for the highest oxygen we tend to go for 1.3bar partial pressure so we have some margin but that gives us 3 hours a dive on NOAA CNS tables which would be a lot less if we went to, say, 1.4bar.

Ridiculously low gas consumption. When I moved the suit inflation off the diluent tank and on to a separate 'Argon' bottle I moved into the 30bar each side (oxygen and diluent) range for an hour plus of diving. This means that I routinely put a helium mix in because I'm not going to need to fill things very often. Remember this is 30bar on 3 Litre tanks. We are talking less than five UK pound a fill here for a weekend's diving.

It's well lighter than the twinset. I'm getting to be an old buffer and lugging the twins about was getting to be a pain. The rebreather is not much less but it's significant. About the same as a 15 or maybe a 15 and a pony.

It's safer. Hey? What? Haven't I heard about all those accidents, fatal accidents, on rebreathers?
Haven't you heard about all those fatal accidents technical diving? The problem is that a rebreather fault can be less obvious and the world just fades away but if you are monitoring properly you catch the fault early and fix it. The rebreather gives you something OC scuba can never give - time. If I look at my oxygen read outs and wonder why I only have 1.2bar not the 1.3 it will take several minutes for the level to fall to 0.4bar where the alarm sounds or 0.2bar that matches the air you are breathing now or 0.15bar where you start to go wobbly. I have drilled driving the thing manually so I can control it. Even if the displays are blank, ie: both my computers have failed simultaneously, I can still run the thing as a semi closed rebreather and stretch my diluent out to four times what it would be diving OC so my 3L tank becomes a 12L and my 7L stage becomes a 28L twinset. Diving OC the one thing you don't have is time. When a fault develops your next breath depends on you fixing it.

Mask with half lenses Right back in our first scuba course we learnt to select a mask. You went through the box and tried them until you found one that fitted. OK?
Right check the memory. You place the mask against your face. You breathe in gently through your nose and if the mask sticks with just the lightest of breath then it fits. That means it fits your face. Get the mask that fits your face as a bad fitting mask becomes evil after an hour in the water. There can be two masks from the same manufacturer, with the same name on them, to all appearances identical and one fits and one doesn't.

Don't tighten a mask any more than you have too. That rapidly gets to be uncomfortable and anyhow an over tight mask always leaks. You just want it tight enough to stay snugly up against your face while you pop the regulator in and out of your mouth. Remember the regulator changes the shape of your face so taking your snorkel or similar with you when mask shopping helps.

Mask clearing is again something we learnt as beginners. The idea is that you are going to blow air into the mask with your nose and that will cause it to move forwards, stretching the strap slightly and bubble out all round. By holding a finger on the top we just stop that bit moving forward, you don't even have to press just stop it moving, and that only allows air to escape at the bottom across your cheeks. If you tip your head back a bit that is where the is water so it naturally goes out first. There is a complication with the long masks like the Cressi Big Eyes, these don't really flex enough to clear easily but the air bubbles out at the side. I have one but I just tip my head so the water runs to the side then clear it there. Easy. ... but I do get some strange looks.

You go to the shop and, starting at the cheapest work up until you find the best possible fit in a colour you can stand. If it's a make nobody's ever heard of don't worry.
Best fit? Take the mask and fold the strap forward so it is not involved in the fitting. Place the mask on your face. Wiggle it. Does it press anywhere or is it just gentle contact all round? Suck in very gently through your nose. Do you feel it bunch up a bit? Again all round? No tight spots? Now just hold the slight negative on your nose, let go and breath in and out through your mouth. You are leak testing so does it leak and fall off?
This one feels good? Try it again with a mouthpiece in your mouth.
Still good? Keep hold of the mask and give the box to the shopkeeper and say "I'll have this one please." Not this make. Not this model. This particular piece of silicon and plastic.
Then when you wear it you can set the strap as loose as anything and it still seals, it still fits and it doesn't leak. As I said at the beginning of this section after an hour or so in the water an badly fitting mask becomes evil and you will hate it.
Also all masks leak if you tighten them up. They are designed to seal to your face and the strap just stops them drifting away with the tide.
If you buy a good fitting mask by mail order you did the wrong thing. You ought to have bought a lottery ticket. It was your day.

If, like me you are in need of your glasses to do anything get lenses fitted to a mask you have selected. Most shops are quite happy to take the mask you select and send it off to be lensed. Get the stick in ones not the clip in ones and if you need a distance and a reading prescription pay the extra for two lens sections.

Dumpable Weight
Why do you want to dump weight? You learnt this on rescue training. You dump a diver's weight belt to pin them on the surface. What happens if you release their weight belt at depth? Probably a lung over expansion injury and they die. Read the accident statistics. A diver was non-responsive at depth so they dumped his weights and killed him. Of course it doesn't say they killed him but if the cause of death was a burst lung he must have been alive when they sent him up so...
Right so now you've gathered that I'm not a fan of dumpable weight we can talk about belts, harnesses and integration.

RIB diving. Here you want to be able to hand off your weights to the boat. The problem is that when you become separated from your rig if the weights are still with you either you or the dive set is suddenly heavy. This could be either dangerous or expensive. Passing up the weights first will leave both parts pretty buoyant. On a RIB you need accessible weight and you need to make it clear to the crew what you are handing them.

For most other diving I prefer to attach it to me solidly. On the Rebreather I swap between a pocketed belt and integrated pockets depending on what I'm diving. On the twinset most of the weight is in the 300bar tanks and it's always the belt on a single.

I have a harness, I won it shooting my mouth off on the letters page of a magazine, I don't think I'd have bought one and I don't get on with it very well. A harness, however, solves a serious problem for one type of diver. Well one shape of diver actually. The shape with no hips.
I can pull a weight belt up above my pelvis, tighten it and snap the catch and it is going to stay. If you have a 42 42 42 build it probably won't.

My weight belt is a pocketed thing. It is more comfortable than having raw lumps of lead strapped round your tum and even more so with the shot pouches that came with the harness in it. I confuse matters as I reverse the clip on my belt so the weight goes on my strong arm handing it off. This has caused a problem once and there is a weight belt somewhere off the Brothers Islands in the Red Sea to prove it.

Dehydration is blamed for a lot of DCI hits when the diver was otherwise well within the tables or the computer's plan for the dive. However staying well topped up with water when you have some serious diving planned can give rise to other problems. It is said that you can go in a wet suit although I'm far to inhibited for that but wearing my lovely fluffy undersuit it would be just daft. So what we need is a P-valve.

Start with Male Incontinence Sheaths. Every chemist stocks them because it's a problem we might all face when we get old. Plumb this to a fitting that connects through the suit with or without additional fittings so the waste products are dumped overboard.

OK. The first time you use it it's a bit unnerving. You open the external valve, you hesitate but there is no sudden cold feeling so no leaks and it didn't run back in. Then go, hesitate again but there is no sudden warm feeling so no leaks that way either. Sigh of relief.

There is some debate as to what sort of plumbing you need. Mine is the simple sort as sold by O3 with collapsible pipe work and just a screw seal on the outside. The alternative thinking says that you want a one way out valve to stop the cold water getting back into your equipment and a second one way valve to allow some air from your suit to enter the system to prevent a very unpleasant case of squeeze.
But, just as you screw your face up and shudder, I need to point out that I've taken my simple p-valve to 50meters and never been squeezed that I've noticed and I don't have any one way valves. I suspect that we're dealing with nice sloppy rubbery things that don't crease up like a squeezing drysuit so nothing is uncomfortable. Well it works for me.
One thing you do need to do is run the scissors round if you are a hairy type. Peeling the self adhesive stuff off can be a bit... tedious... otherwise. If you see a guy in the changing rooms with his face to the wall hacking away with his sea-snips somewhere inside his undersuit don't ask. The only problem I have had with one is diving on a cold Stoney morning I stepped into the gents to pop on the consumables and it was so cold I needed a size smaller than I was carrying. Oh and always assemble the clip onto the catheter before putting it on. Somehow it becomes ten times harder if you forget.

Update! Custom Divers now produce a thing called a 'Hee-wee'. I've got one. I'm not describing it here in detail but Google will find it for you. You won't believe how small the 'small' diaphragm is but once you have mastered putting it on it is really comfortable and it is so easy to take off as there are no adhesive bits. A long term burn-test was starting to feel a bit constricting after about six hours which covers a day's gas diving in one hit. As a further plus it is washable so you don't need a big box of the consumables.

There is some rubbish posted about 300bar cylinders on the internet and a lot of it by people who really should know better. The first tale is that you can't get it. Well I can get it free because the club compressor was replaced and now does 300 and anywhere locally where I want to pay can do 300bar. Then comes the fiction that you can't get Nitrox fills or at least you can't get accurate Nitrox fills. More hogwash. Any blender with an ounce of sense and more than two days experience can do 300bar Nitrox fills. You just aim a bit low if you're partial pressure blending either getting the offset from your tables or by experience. I'm lazy so I use my own blending software. See here for the IPAQ and Android versions and here for the Windows version.

Then they start wittering on about how you don't get as much as you think. Well this is true if you just multiply it up but that would mean you are as stupid as they are. My 10/300s hold as much as a set of 12/232s. OK it's 0.133% down but if you just scale the pressures it should be over 7% up.

Then comes the cry of anguish that they are so heavy. We'll ignore the point that everything is neutral in the water because steel on your back equates to lead off your weight belt and climb the ladder back into the boat. My twin tens displace virtually 4L of water less than my mates twin 12s so by the time we count in the lead I must be nearly 4 kilos lighter. I don't know about you but the ladder is the killer. Dragging stuff from the car into the shop for a fill the extra 4 kilos of steel really don't hurt.

The real red rag to me is the guys who are six inches taller than me, half my age, have never seen inside a cardiac ward who say "but it's too heavy for me." Well give up diving wimps. Leave it to the girls and old men who can handle it.

While I'm on the subject of pressure...

...Can I complain about how divers have no clue about what 'pressure' means. We talk glibly about 230bar or 300bar like they are trivial numbers. Hundreds of bar are a serious business.

What is a bar? Well other than 14.5psi...
A bar is 100 kilopascals, a Pascal is one Newton per square meter, a Newton is the amount of force required to accelerate a mass of one kilogram at a rate of one metre per second per second, a kilogram is...
That didn't help did it?
Try one bar is roughly one kilogram of force per square cm.
That is about a 2Kg diving weight sitting on a postage stamp.

Let's start big.
The pressure in the breech of a big navel gun or, for that matter a field gun, is about 3000 bar. Special guns are up to 4500 bar.
Just 0.07 bar was the pressure drop fuelling Hurricane Georges in September 1998. Georges did $5.1 billion worth of damage and killed 602 people.
A typical old steam railway locomotive ran at 15-20 bar. Those things always impress me. Watch one pulling out of a station pulling grief knows how many hundreds of tons of stock. And it's doing it all on one cylinder! Sure it has two but they are arranged as double acting (they push and pull) and the linkages are set in quadrature so they take it in turns to drive. Then the linkages are a leverage step down so more distance and less force. All on 20 bar or less.

Am I making my point? Just because diving equipment manufacturers have tamed this monster and packaged it in nice easy to use gadgets does not stop it being a monster. Every now and then a tank gives up the game and bursts and hopefully nobody is in the way. The energy packed into a filled scuba tank is comparable to that in a hand grenade.

Training is good. Learning by experience is far more expensive. It is so much better to have an instructor say "Well that isn't such a good idea. The last guy that tried that died." A lot of diving procedures were developed in the days of flaky, home made equipment and a lot of our procedures were tested quite often by people who really needed them. They do work. Modern gear has taken scuba from heroics to hobby. Kev and Sandra can do scuba, leaving little Joaquin and Margarita in the crèche at the Club Chav resort hotel, and have a killer time. Once you start doing it on your own, and I count diving with a buddy with only your skill set as 'on your own' in this context, you need to have already learned to save your own neck.

This goes through all levels of diving. If you want to dive trimix in your rebreather find an instructor who has been there, got in the poo and dug themselves out. If they think you should do an exercise again (and again) then trust them. They have a reason. Somebody who fails you and makes you do a part of the course again is to be treasured. They care.

Long Hoses and the Hogarthian Rig
The story, as I heard it, was that Florida Cave diving started with a serious fatality rate and there was a significant chance that it was going to be regulated either by the state or by the landowners. Several people put effort into developing a diving protocol encompassing equipment, training, skills and procedures to try and ensure that divers got back alive. The system involved a scuba configuration named after its first populariser William 'Bill' Hogarth Maine. Rather than call it the 'Bill rig' they settled for the 'Hog rig'.

Now in conventional beginner level scuba you know where the way out is. It is straight up. All you need is a reasonable speed ascent and you have solved a lot of the problems. In a cave this just isn't so. Above you is rock so you need to know the way back. You need a torch and you need a line with clear direction markers on it. Then, as a line in a silt-out may be invisible you need procedures to get your hands back on a lost line so you can feel for the direction marker.

Then what about an Out Of Air emergency? Firstly you need to have the discipline not to swim your self into an OOA state and a rig that, if a fault dumps your air, enables you to save what gas you can and as a final resort both you and your buddy should be equipped to get out of a cave sharing air. To try and make it possible to survive a failure the Hog rig is a twinset with a sealable manifold joining the two halves and two hoses, one seven feet long that wraps around you and is the one you normally breathe from and the other is shorter and the regulator is held just below your chin on a necklace.

The long hose is the get out of the cave trick. Let us imagine the worst case. You have blown something on your rig, you are suddenly OOA and the visibility is now so churned up be the huge surge of bubbles that was the air that was going to keep you alive for the rest of your dive that you can't even see your gauges. The two buddies immediately converge. Since they both have a hand on the line and they knew if they were leading or following this isn't too hard. Now the OOA diver feels for the buddy's head and takes the regulator from his mouth. This is the only one he can find fast. This is on the long hose that runs behind your head so you virtually just 'nod' to release it. Breathe and live. The buddy reaches up and puts his secondary from the necklace in his own mouth. No discussion. Start for home.

The first idea in this system is that the OOA diver goes first as they exit. If it is narrow and he stops the guy behind must stop so the hose never gets pulled out of his mouth. This means it has to reach one whole diver's length ahead.

The second idea is that if you need air take the reg in my mouth. I may have several regs on various deco mixes festooned about my person but only I know which is which. You need air fast so go for the one reg that I promise is switched on, is working and is the right mix for the depth. That is the one in my mouth. This cuts through any complicate decision making. Take that reg. Breathe. Live.

Argon Argon
I'm still not quite sure on this one. The numbers don't quite stack up the way I'd like. I bought a separate suit inflation bottle when I started putting trimix in the rebreather. Helium conducts heat six times better than air so it is bad news pumping that into a suit that is supposed to keep you warm. Suddenly the suit works a lot worse just when you are doing deeper for longer and want it more.
However a lot of people said Just put air in the bottle. It makes no difference.
Now certainly that returns you to the situation you had before and avoids blowing your suit up with cold, expensive helium but is there an advantage with going the extra step to Argon?

Well the numbers aren't so obvious. Argon's thermal conductivity is 17.9 mW/meter Kelvin while Air is 26.2 and Helium is a frightening 156.7 (at 1 bar/300K). Hence Argon only offers 1.46 times the insulation so I don't feel a swoosh of warm as I push the injector. In fact I don't feel anything different. However what I do notice is that after an hour in 5°C water my lips can be too numb to talk properly but the rest of me isn't particularly cold. It didn't used to be like that so I'll stick to the Argon when it's cold.

I've read some reports where military divers were tank tested to see if Argon suit inflate would maintain their core temperature longer and the results were inconclusive. They didn't gain much time so we're not talking huge benefits. Also you need to flush the air out of the suit a bit although the percentage argon once you are deep, due to the compression of the gas, is automatically high.
Beware of welding Argon. Some is 100% but at Combro we use AP's Coogar which has a big lump of carbon dioxide in it. This is not so good in a dry suit as it reacts with any water there, eg perspiration, to produce a mild acid that effects some people badly. Have you ever had the desperate desire to scratch doing stops? This is the formula for just that.

Failure points
Failed I've actually used the word fail quite a lot in this document. This is not because diving equipment fails a lot but because the consequences of a failure can be so serious. A small problem with a valve that might be a minor annoyance at home is desperate when it controls what you breathe.
This means that divers have to be trained and prepared to manage failures in their equipment and need to look after their equipment to ensure that critical failures are minimised.

Any dive equipment must be thoroughly appraised as to what might fail and how do you cope with it. Quite a bit of this is covered in basic training and the novice needs to follow the skills they are taught to minimise risk. However when a diver progresses to more complex diving the buck stops with them. It is your life and your life support equipment must be your own personal concern. You may off load the configuration to trusted mentors and the servicing to a trusted shop but you have to choose them.

One of the favourite tools for planning is counting "failure points". This is an assessment of a rig or a change to a rig that weighs the benefits of an addition against the new ways things can break. While this is one tool in your mental tool-box it needs to be used selectively not just grabbed at as the final argument that says something is a bad idea because it "adds a failure point" when that failure can be insignificant, uncommon or easily managed and the change is otherwise beneficial as a convenience or confidence boost.

An example of this was my second SPG (pressure gauge) on the twinset. There are straight forward procedures to manage with one and the addition of a high pressure hose, rotating O-rings connecting the head and the pressure gauge itself can all fail so you have added things to go wrong. However I had had a pressure gauge jam on me and not indicate the gas correctly. It is my preference to view myself as part of the kit and assume that I am one of the weakest links in the chain. A second SPG warns me of several self induced problems as well as mechanical failures and reduces task loading as I am not required to remember the previous pressure reading to detect a jam.

Failure point counting has its place in assessing a rig but it is not the be-all-and-end-all defining method.

Air Horns
On about our tenth dive, and our first away from the PADI fold, we hit a snag. The sea state was up a bit when Adrian dived but when they surfaced it was not what I would call diveable. Fortunately he was diving with an experienced Buddy as I had trashed a regulator on the previous dive. Although they could see the RIB they were diving from those in the RIB could not see them.
Now this was Portland, and to this day I'd say that Portland is not the place that you are going to have an incident just because you are not picked up cleanly but they managed it.
Adrian was not a strong finner in those days and they were carried on the ebb the tide out of the harbour mouth and further and further away. Diving inside the harbour nobody had thought to carry a flag but when they saw a fishing boat they waved at it and Adrian attempted to blow the whistle attached to his jacket. He said some very rude things about that whistle later but his buddy was a bit more forthright. It gurgled and it hissed but it never whistled. They resorted to shouting and thankfully he heard/saw them, pulled over, listened to their tale of woe and radioed the RIB.
Well the story may have ended happily but more by luck then anything else. So now we carry surface detection aids and a dirty great whistle on the BCD inflator blown by back gas. It's called a "Dive Alert" and the bumph says you can hear it for a mile. Probably. The only time I have been fool enough to blow it without ear defenders on I couldn't hear much for 10 minutes... It gets checked regularly.

I bought the Buddy-Blast version for the Inspiration as the Auto Air on the wing has a funny Buddy fitting. If I ever have to use one in anger I'm putting my ears under water as I push the button.

Oh a final note on other surface detection devices.
Flag: good. EPIRB: got one, hope I never have to try it out.
One item that often gets suggested is the old AOL CD as a reflector to attract attention. It sounded good until I thought about it. No sunlight, nothing to reflect. But if we have sunlight we have no mist so it is a clear day. If they look my way they can see me. So they aren't seeing me because the sea is rough and I only see them from the top of the wave. So any other detection aid like a flag or a DSMB works. And they work in other cases too.

Horn 1Horn 2

Marshalling is a very military concept. You write down all the parameters of the divers (qualifications, next of kin, gas in and out, dive plan et. al.) so it is available if required. However this raises the problem of what is that information for? Are you recording or authorising?

Naturally if the divers are students and you are operating under a QA standard either professionally or for a club you want those details to prove you are adhering to standards so if an accident occurs it was an accident not your negligence. However if you are just providing the boat it is the divers themselves who must call to do or not do the dive. If it is my trip I do not wish to be responsible for your accident and, in fact, if you ask me if you should dive I will always say 'no'. I reason that if you are unsure enough to ask that is reason enough not to dive. There will always be another day and another dive so why risk a dive you are unsure of and almost certainly won't enjoy.

This means that on my trips the only record is the skippers. I don't need to know what gas you are carrying or what your plan is. Of course we will have discussed the dive plan so we don't overstay the time but I do not approve your plan. Well. Unless I'm marshalling for the club.

DIR is a rather holistic approach to diving embracing fitness, training, equipment configuration, team work and attitude. It grew, I believe, out of the danger that Florida cave diving would be virtually shut down due to the high fatality rate so people devised an equipment and procedural plan to try to ensure diving there would be much safer.
DIR is always identified with the manifolded twinset for gas redundancy, the long hose on the primary regulator needed to get an out of air buddy out of a narrow cave and the *take the regulator in my mouth* approach to gas sharing. It goes well beyond that but you need a DIR trained diver to explain things which I am not. If you are considering any form of technical diving DIR should be on your reading list but not everybody wants to swallow the whole package. It does tend to attract its zealots who endlessly debate the details with fundamentalist glee which puts a lot of people off.
The best book on my shelf on the subject is "Doing it Right. The Fundamentals of Better Diving" by Jarrod Jablonski ISBN 0-9713267-0-3. I bought it at a dive show but I suspect you can find it in the usual places, maybe even the library.

OK that's a good start: so what don't I like about DIR?
Well, like so many things, it has engendered a whole mass of elitist hangers on. People who feel big because they can claim they are *Doing it Right* not because they want to do things well but because that means that other people are doing it wrong. And they can say so. Loudly.
The attitude of many of the DIR minded instructors and divers I have met around the country deserves nothing but praise, they are trying to promulgate a system of diving that they see as enhancing safety and teamwork. However the internet, that final bastion of the close minded bigot, brings me into contact with some people I would prefer to avoid. Now most training systems like this would condemn adherents with a name calling, insult shouting, "Do as we say or %$&*^%$& off" mentality but sadly here, in the 1990s and early 2000s, it went right to the top.
As I say DIR should be on your reading list if you are moving into technical diving, GUE courses can help you acquire skills, Halcyon equipment is good if rather overpriced but call me a stroke because I don't carry my kit in the canonical positions handed down from on high and I will laugh in your face.

The guy who sent me hate-email when he read my web-page on my inverted twinset got pretty much that. He did get asked some questions, the answers were on the page he was complaining about, but he quickly gave up. He could use rude words and make sweeping statements but all he knew about diving was what people had told him and contradictions didn't seem to matter. Conversely maybe he sobered up and realised what a load of slime he had been writing.

There is no one true way in diving. It is a changing, evolving sport. Use an equipment set-up that suits you, your body and the type of diving you want to do. If that is one set way as you are diving in a team and you want to be interchangeable even in the water fair enough. If not and somebody shows you a better way to do something adopt it. If your buddy can't cope with that but can't give a good reason why it's wrong - get a better buddy. Nothing is cast in stone. Nothing is forever except getting it wrong.

PS: DIR and rebreather users: Most of the rebreather world always remembers the description that the Buddy Inspiration as "Should be sold with a shovel" from the mouthpiece of DIR at the time. This does tend to mean that the letters DIR are treated with contempt in these circles and that is a large percentage of the technical diving fraternity. Perhaps it would be better to let the acronym die and let GUE teach their style of diving without the acrimony that the name DIR brings. Certainly I treat in your face internet DIR divers with the contempt due to duckspeak while, for GUE trained friends, I have nothing but admiration.

Suit as buoyancy
"I was told to use the BCD on the surface and the suit for buoyancy in the water".
This is a good skill to have so learn it and practice it but when doing real diving keep it for the backup when the BCD breaks. Set the autodump (you do have an auto dump?) wide open and now ignore it. Add air to the suit to keep it comfortable but not so there is a huge blob of air moving about. Trim out so you naturally hang level (or feet actually higher because you bend your knees) and you will be warm, comfortable and stream-lined so fining is easy. Use your lungs and the BCD to control your buoyancy. If something goes wrong and you are ascending too fast pull and hold the BCD dump and as soon as you stop rising bang some air back in and trim for neutral hover regardless of depth. Once you have your act back together resume your dive.

You will have moments like this in your first year and the BCD is a buoyancy control device so it dumps fast. Get to know the pull dump at the bottom too. You will not stop an accelerating ascent on the suit as the air is spread out in a bag that is full of body and fluffy suit and can't get out fast enough.

Let's think through the numbers:
You start the dive with a 12L/232bar cylinder. That is 3.3Ks of air. If you are going to do a nice slow ascent having used most of your 50 bar reserve (perhaps you have your buddy on your octo) you are neutral then so you started the dive you were 3.3Kgs negative. 3.3Kgs of buoyancy is a 3.3 Litre bubble so look at the milk in the fridge and imagine 3.3L loose in your suit. Is it at your shoulders? your bottom? your feet? This is 3.3L over and above what it takes to keep the suit comfortable. It is just flipping from one high point to the next. 3.3L is the top end of what is manageable but as soon as you go above that, a 15L/232bar is 4.1 and as 12+12 twinset is 6.6Kgs, it really isn't.

The suit as buoyancy only really works on a single and you are pushing your luck even then. However when you learn to dive a dry suit learn to control buoyancy with the suit. This becomes your get-out-of-jail-free card to manage a BCD fault so it is a skill to acquire but, in my opinion, not a skill to use every dive. You may chose to do your ascent, after most of the gas has been consumed, on the suit but that is a personal decision.

Some people will say that having two sources of buoyancy to adjust makes the job to hard for poor beginners but that really isn't true. Set the autodump wide open, put some gas in when the suit starts to compress on you and otherwise ignore it. Auto dumps really are auto used like that. Anyway what's the problem with two sources of buoyancy to control? Many times in a dive I may decide the suit is a bit tight and I put a bit extra in and bubble some off from the wing. It's hardly a skill because I'm controlling my position in the water by how I breath (well not on the rebreather as how I breath does not effect buoyancy so I have to get the suit and the wing spot on).

I'd say use the BCD, that's what it's there for, but be able to do both.

Go Pro
This was actually written for somebody I considered was being a bit simplistic in their ambition to become a diving instructor when they has barely started diving:

Instructing and diving are totally different. I'm PADI trained and I know the way the system sweeps you along but once you reach RD level you have options ahead and 'Go Pro' is only one. It doesn't work out well for many people.

I used to be an Institute of Advanced Motorists Observer on motorcycles (called Observer not Instructor for, I suspect, insurance reasons). Training somebody up to a standard where they pass the pursuit test with a Police motorcycle instructor was a personal thrill but it bore no resemblance to riding a bike fast and well myself. I would never want to be a professional bike instructor as I like to set my own pace and work the victim, oops, student hard and send them off for their test pretty assured that they will ace it. No way would a customer put up with what I, as an unpaid volunteer, could hand out but nobody ever complained and I just got recommendations.

For diving however I don't see it. If I want to instruct I'd go the club route and keep my lucrative full time job but only if it is extra time in the water. I'm not losing the diving that I, like you, love to mess about shallow doing the drill again, not because they got it wrong, but because I think they need a couple more goes to be relaxed with it.

Ask the question "What do I want to spend my time in the water doing?". There are the people who get their kicks out of 'Colourful things' who are best suited to warm water holidays. Camera geeks who have masses of fun but are best left alone as they are oblivious to the world. Wreckies, both the salvagers and the lookers, archaeologists (specialist wreckie) and finally instructors. Pick one or several of those options and have a great time the rest of your life. OK I confess I'm a wreckie with aspirations to being an Archaeologist so my subsequent training is to enable me to get to the wrecks I want to go to and to gain the archaeological techniques needed to understand them.

There is the side issue that setting your life's ambition to be a dive instructor is aiming rather low. It's not hard, it doesn't pay well, there are lots of them and not many people can make a life's work out of it. The drop-out-rate is huge as it just isn't something you can do week by week for years. Frankly the people I see having the most fun instructing are the totally 'people' oriented people who would be teaching and helping people on their weekends even if they had never discovered diving. If that's not you get real now. This is something that is built in to your basic character and no training will make it happen.

If you want to do it then do it. But don't burn any boats as they say.

Technology or skills?
The question if often posed: Should divers substitute technology fixes for good in water skills?
Now this is a loaded question normally used to decry some latest device that bypasses a skill that was once trained. Yes. We all approve of divers having the skill set to manage a dive on much simpler equipment than many of us carry but does that mean we should avoid the gadgets and devices that are getting more common?

I say we should embrace good technology:
The weakest link in my dive kit is me. If I can just use me to monitor the technology that is good in my book.

Hence the tables and depth/timer back up to the computer.
The self seal blob backs up the self inflate.
The CCR is all electronic with manual and SCR modes as a fallback.
The twinset was manifolded with an isolator not independents.
Autodump not cuff dump.
Octopus not buddy breathing.
probably more...

Gradient Factors
Originally written on Yorkshire Divers

There are lots of mathematical explanations of Erik Baker's Gradient Factors and as a mathematician I like them but let me try and do a words explanation.

There is Professor A A Bühlmann's deco model. Tried tested. You can 'overpressure' yourself, ie let the gas tensions in your tissues go right up to the Bühlmann a/b limits and that draws the line between 'probably not bent' and 'at risk'. That is '100'. Fast out - probably not bent.

Then there is a another way of using Bühlmann's compartment model and that is never to let the tissue inert gas tensions exceed the absolute pressure. This is '0' definitely not bent but so slow. You are decompressing solely on the oxygen content of your breathing mix.

A first stab at a conservatism system is to pick a number between 0 and 100 and plan on that. However that doesn't really address the problems of the compartment model. Better is more conservative deep, so you introduce short deep stops to suppress bubble formation. This is where the gas flow graphs are all steep so small changes in physiology make big differences so there is more likely hood of an error (the maths is OK, it's your body that is at fault) so use 50 here. Stay well away from the bent/at risk line.

Shallow, where the graphs are flatter, you can be less conservative, say use 80, because off gassing is nice and predictable here. Between your deepest stop at 50 and the surface at 80 factor the conservatism by depth so half way it is (50+80)/2 = 65.

Right so how does this work out in practice?

Start from the bottom at highest conservatism so deco stops are called to keep you well away from bubbles. This means that blood jostling about doesn't flash up much in the way of microbubbles and any that do form stay small and get flushed out in the lungs. However shallow the conservatism drops off so you don't do an extra hour of stops where microbubbles and turbulent flow have ceased to be a problem.

So what do you choose in a GF set? 100/100 is raw Bühlmann. Fast out and probably safe for any one dive but if you do 100 dives a year you can probably expect to get bent a couple of times and you will usually feel pretty rough after a dive. 0/0? Take a lot of gas. You won't get bent but don't expect to be invited to dive with those guys again. 30/70, 40/80? The smaller, deep number will switch in deep stops and the larger, shallow number is still more conservative than raw Bühlmann but not drag out the time needlessly.

Being more conservative deep does not extend your dive time much, these are typically two minute stops and having run times for leaving depths is a good tool to control your ascent rate (slow is good). Hanging on a string at 6 meters is very predictable so you don't need so much extra here.

GF is a good system. If I'm cutting tables I use 50/80 or 50/70 but I confess I do most of my diving on a computer set to zero conservatism and I add deep stops/extra stop time in my head. The computer is on, effectively, 100/100 so if something all goes horribly wrong I can abandon the slate and do shortest time to surface.

Well it looks like BSAC are going to make Nitrox part of entry level diving so it seems reasonable to discuss it here.

I like Nitrox. It has its down side but the advantages really stack up. Nitrox is not the deep water diving gas, that is Trimix, but it offers better no-stop time and more safety for diving down to about 35 meters.

The problem is Nitrogen. Nitrogen is a nasty gas to dive. It does nothing good for you. It dissolves in your blood and if you overstay your time or ascend too fast it can spontaneously undissolve and produce bubbles that block things up. We have lots of vital organs and a bubble blocking something reasonably trivial like the blood supply to a piece of skin may be bad but a bubble in the brain has the potential to be worse than death. Nastier still the Nitrogen, when dissolved, acts as an aesthetic and takes your brain away. I have dives below 40m on air in my log but none in my memory. The only thing in Nitrogen's favour is that it is dirt cheap as you get 790ccs free with every Litre of Air.

OK so the simplest way to reduce the Nitrogen in your breathing gas is to replace it with Oxygen. Now Oxygen isn't the nicest stuff but we'll come back to that. Your body can cope with Oxygen. Dissolved Oxygen is fuel for your cells so even if a bubble does form and the blood supply is reduced then the tissues rapidly become oxygen starved and the oxygen in the bubble gets used up supplying the short-fall and that reduces the bubble in the process. An excess of dissolved oxygen is rapidly depleted as the cells use it and don't bother getting it from the blood. Decompression calculations virtually ignore oxygen because in live animal tests (goats) forced oxygen bends spontaneously cleared.

Right so why don't we dive 100% oxygen all the time? Well sadly oxygen has its dark side too. There has been a lot of work done with oxygen only systems as the military like the idea that you can recycle it giving no nasty tell-tale bubbles. However pure oxygen systems, once you got to 40 feet (13 meters) and below, often put the diver into convulsions. That was 2.3 bar of Oxygen and convulsions, if you have to keep a mouth-piece in your mouth to breathe, can be annoyingly fatal. If you test your divers you can get ones who can reliably do 45 feet (15m) and below but it's not a good idea.

Quick Maths Recap.
The pressure at the surface of the air we breathe is 1 bar.
As we descend we gain an extra 1 bar every 10 meters so at 35 meters we have 4.5 bar of pressure. Pressure = depth/10 + 1
Partial Pressure is the fraction of the total pressure made up of the gas in question.
So at 30m (4 bar pressure) with 30% Oxygen 70% Nitrogen you have an Oxygen pp of 4 * 0.30 = 1.2 bar and a Nitrogen pp of 4 * 0.7 = 2.8 bar.
If you want to know the partial pressure pp = total_pressure * fraction
If you want to know the fraction f = partial_pressure/total_pressure
If you want to know the total pressure p = partial_presure/fraction.

The current thinking is that 1.6bar of oxygen is a good, safe limit. 1.4bar is better. There are tables that you can work out your exposure on if you are planning a dive but on conventional scuba mixes those limits will keep you out of trouble.

OK so this sets a Maximum Operating Depth (MOD) for any gas containing oxygen.
If we are talking about Air with 21% Oxygen the total pressure at that depth will be max_ppO2/fractionO2 = 1.6/0.21 = 7.6bar so 66 meters.
If we prefer 1.4bar max Oxygen then 1.4/0.21 = 6.7bar so 57 meters.
For N% nitrox convert N to a fraction (35% => 0.35) and divide it into 1.6 (or 1.4 or whatever limit you want). That gives the depth in bar. Subtract 1 for the atmosphere at the surface and multiply by 10 to get depth in meters.
eg: 35% -> 0.35 -> 1.6/0.35 = 4.57bar -> 35.7meters

We tend not to push Oxygen limits. It is so easy to stay safe. You may be a hard man who can take 2bar for extended periods but the only way to find out is to do the dive and not drown. So what? You keep diving deeper until you don't come back and they engrave your personal limit on your tomb stone? Lacks something that does.

So what advantages do we get from Nitrox?
Well the whole trick is that we can ignore oxygen in the deco calculations. So let's consider a 35% mix. That's 35% oxygen and 65% Nitrogen.
Always work out the MOD. MOD = 1.6/0.35 = 4.57bar = 35meters.
Let's say we're diving 30 meters.
At 30m our ppN2 will be 4.0bar * 0.65 = 2.6bar and that is the equivalent of diving on Air to 23meters (2.6/0.79 = 3.29bar) so we can plan the no stop time with Air tables as if we were at 23m.
Now my favourite Air tables give me 17 minutes no stop at 30m and 36 minutes no stop at 23m. We have probably gone from being limited by the no stop time to being limited by the gas consumption.

Not so bad a deal.

How do we use it in practice?
We are diving the City of Waterford out of Brighton.
The guide book lists it as 28meters.
We are diving on a high slack of 5meters so 33meters to the sand.
MOD? Let's go for 36m so that's 4.6bar so 1.6/4.6=0.347 so 35%.
Get 35%, measure it on an oxygen meter you trust and sticker 35% 36M on the tank. This trick is important as 35% might just kill you at 45meters.
The figures are going to be quite near my example above and when we dial up 35% on our Nitrox computer it will work out our real dive times and, as the City of Waterford is about 8m high in places, we should get a good 40 minute dive with the opportunity to cut-and-run if there is a problem as we have never gone into deco.

Airway Separation
We had finished the pool session for the evening and adjourned to the bar. They were doing OK overall but I had seen them having problems tonight. I asked what the difficulty was. It was water up the nose causing sneezing, spluttering, must stand up and stop this drill syndrome. Yup. That's what I saw. It happened every time you tried.

I mentioned 'air way separation'. I knew they didn't have a clue so I laid it on a bit thick, telling them they had a valve in their head that could switch their breathing between mouth, nose, both, neither, to get to the "I can't do that" stage. They couldn't. They were convinced.

Yes you can. You do it all the time you just don't know you do. Let's practice. Hold your breath and blow. Right now do it with your mouth open so your cheeks don't blow up. See? I can see your chest tighten. That's the off position. The important thing about 'off' is not to do it ascending. Best not to do it ever as it can catch you out. Well unless you are snorkelling.

Now breathe in and out through your nose with your mouth open. See? Pinch your nose a bit with your fingers so you can feel it is only going that way. Easy.

Now the one you want in the pool. Just breathe through your mouth. Close it down to a kiss so you can feel it is only going through there not your nose. Like when you blow up a balloon. Now the problem is to just hold that configuration because you tend to blow out like that and then relax it as you breathe in. Blow-suck-blow-suck on just the kiss. If you do that on a just a reg no water enters your nose. If you need to clear your nose either pop a hand up and pinch it so you can blow out or switch to nose breathing and back mid-exhale. Don't stop though or you always get some back.

I've done lots of mask off drills and the 'blow through a kiss' mentality sets things up just fine. Head down is too easy and head level works too. I suspect that if I looked up I might regret it.

It's not quite that. It's one of those things you explain and watch the eyes so you know when to stay on a point and when to move on. I expect you could formalise it but I've never tried. It's a "realise you can" sort of thing not something you learn.

Gas Calculations
A lot of people complain that when they get to doing nitrox and trimix diving they get bogged down with the calculations. There are so many formula and they all look so much the same that it's hard to remember which one to use and to be sure that you've actually got all the details right. This shouldn't be like that. Let me introduce you to depth in bar. This is the magic trick that makes all the formula simple.

We all know that the pressure rises as we descend through the water and in metric units it is very easy to work out. You start with 1 bar of air pressure at sea level and gain an extra 1 bar for every 10 meters you descend.
So the depth in bar is the absolute pressure at that depth. So divide by ten and add one.
0 meters = 1 bar
7 meters = 1.7 bar
18 meters = 2.8 bar
72 meters = 8.2 bar
That was easy wasn't it?
Now do it the other way.
3.6 bar = 26 meters ... OK I won't bother with another list as you've got the idea.

So how does this make life easy?
What is the oxygen partial pressure of 27% Nitrox at 26 meters.
26 meters is 3.6 bar deep. 27% of 3.6 bar is 0.27*3.6 = 0.97 bar
Yes that simple.

What about EADs? Equivalent air depths.
What do we want here? We want to know how deep we would be diving on air to be breathing the same amount of nasty nitrogen as we have in our Nitrox so we know what depth to use in our tables.
Lets say we are diving 35% O₂ at 27 meters.
27 meters = 3.7 bar.
How much nitrogen? 65%. So 65% of 3.7 bar is 2.4 bar.
If we were breathing air with 79% nitrogen how deep would that be?
We multiplied to go from depth to fraction so to go from fraction to depth we divide.
2.4/0.79 = 3.0 bar deep. So 20 meters. So although we are at 27 meters we can plan our dive times as an air dive to 20 meters. That should help increase the no stop time.

OK what about the MOD for our Nitrox? The Maximum Operating Depth before oxygen toxicity becomes a problem and nitrox stops being friendly. If we have 40% O₂ what do we write on the cylinder?
Let's say you use the rule of never break 1.4 bar O₂. I know there are others.
1.4/0.4 = 3.5 bar so 25 meters.

Do you see how everything so far has reduced to a single steps of multiply and divide? All the business of multiplying and dividing by ten and adding ten, or was it one, or was it subtract, has gone. You couldn't remember them anyway and who wants failure points in their mathematics?

Let's do a big one. We are diving a rebreather with an Oxygen setpoint of 1.3 bar and a diluent of 18/40 trimix and we are at 60 meters. What is going on?
Right 18/40 trimix is 18% Oxygen, 40% helium and hence the remainder is 42% nitrogen.
We are 7 bar deep so that is what our gases must add up to.
Well the oxygen is easy. If the rebreather is working it is 1.3 bar.
Right 5.7 bar to go.
Now the rest of the gas came out of the diluent bottle so it is in the same ratio as only Oxygen has been added. So He:N2 is 40:42 so add them together is 82 so the Nitrogen is 5.7/82*42 = 2.9 bar and the helium is 5.7/82*40 = 2.8 bar.
We are breathing 1.3 bar of O₂, 2.9 bar of N2 and hence 2.8 bar of helium and a quick add up checks that is 7 bar.
END? Equivalent Nitrogen Depth. How deep would we dive on air to be that narked? 2.9/0.79 = 3.2 bar. 22 meters. Even I still have my head in gear there.

You want to do that one again because you think Oxygen is narcotic? Well you're probably right so:
O2+N2 = 1.3+2.9 = 4.2. Think of it as 4.2bar of air although it's really 31% nitrox. 4.2bar = 32 meters.

Fill Rates
Needle valve I was asked about filling a cylinder. Particularly filling with oxygen. If it is clean why do I fill so slowly?

The short answer is that I'm a coward.
The long answer is time for some maths:

Let's say we're using my old Air compressor. No, not the nice electric one the monster with the big petrol engine that I got rid of. It does a 12L cylinder in about 10 minutes while the electric one takes 25. Let's work on 20bar/minute fill rate and see what's going on. As I can just open the valves between my O₂ bank and a cylinder I could fill faster than this if I don't need the booster.

20 bar per minute into a 12L is 12*20 litres/minute so 4L per second.
Now if you've had a valve out of a tank you'll know what I mean by a 'debris tube'. Basically it is a bit of pipe that feeds the valve that means that if the cylinder is upside down any solid matter does not go into the valve and into your regs and possibly into you.
A debris tube is about 6mm inside diameter so 0.3cm internal radius so, using the pie-r-squared we get about 0.3square cms of pipe. Now 4000 cu cms per second going through 0.3 sq cm must go at about 4000/0.3 about 12000 cms per second.
You're not sure what 12000cms/second is? 12000cm/sec is 120 meters/second so about 270mph.
Now when oxygen is 1.3 gram/litre at air pressure it gets to about 40grams/L at 30 bar. A pretty standard 15 grain airgun pellet is 1 gram and flies about the same speed.

Right so the last part of the equation is oxygen. For something you need to breathe moment by moment every hour of your life oxygen is not your friend.
Do you remember the formula of gun powder? Charcoal, saltpetre and sulphur wasn't it? You've got it. You are adding a good oxygen source (saltpetre) and a catalyst (sulphur) to liberate the oxygen fast. This converts those brick things you can never get to light on your barbie into the stuff Guy Fawkes planned to blow up parliament with (Come back Guy Fawkes - your country needs you).
The final part of the review is using a gas-axe. I love that tool. An Oxy-acetylene flame to heat a metal up, eg: steel, and then squeeze the lever and dump huge amounts of extra oxygen into the flame. Steel burns beautifully in pure oxygen.

Are you getting the idea? We have all the components of a very nasty incident. Oxygen fires do happen on scuba gear, a boat was burnt out to the water line not many years ago and its picture was well aired on the scuba forums at the time. It is not my intention to end up in the obituary notices of the forums I contribute too as burnt to death or blown up. That just lacks finesse.

Current limiting and Oxygen Cells
Don't do this at home kiddies It's old but it give 11mV in air There is an awful lot of rubbish spouted on the internet about the galvanic oxygen measuring cells we use in rebreathers and it makes you want to weep at times. Instrumentation is my business. Making instruments that stay in calibration and knowing when they won't is bread and butter.

OK. All galvanic oxygen cells current limit. Yes all of them. That is how they work. A cell is a battery and a battery's chemistry has a predicted voltage that is derived from the materials used. In theory they should give that voltage from the day they are made except that one chemical that is part of the planned reaction has been left out of the assembly. Oxygen.

Oxygen is one of the fuels of the cell so the more oxygen there is in the equation the more electricity is generated. The chemistry sets the voltage and the fuel, the quantity of oxygen available, sets how much current it can give. If you put a circuit on the cell that draws current you can draw up to this current but ask for more and, like putting too higher power bulb in your torch, you don't get more and the voltage from the cell fades.

cell Not really complicated is it? Look at the graphs. This is for a naked cell without the little circuit board attached. These are V/I plots where volts are in millivolts vertically and current is in micro-Amps horizontally. I am applying a greater and greater load and watching the voltage and the current change. This has to be done slowly as the cell takes time to settle to its new conditions, about six seconds.

The blue line is a cell in air. Notice how it starts at about 660mV when I am applying no significant load. As I try and take current from it at first it drops to its 'working voltage' of about 500mV and then slopes in a way that implies an internal resistance of about 700 ohms. Then it starts to run into its current limit. It is clearly not going to give more than 120uA no matter what I do.

Now the same cell but installed in the system shown on the right with the Oxygen valve open and plotted in magenta. We have the same starting voltage and the same working voltage but now the internal resistance looks like about 300 ohms and the current is limiting point is going to be about 550uA.

To make this work in a practical measuring cell they are wired into a quite low value resistor which will always try to draw more current than the cell can deliver and will drag the output voltage down. This limited current is sampled by a second resistor so a voltage can be read and this has a temperature compensating varistor/thermistor across it to make up for the fact that the uncompensated cell is also quite a good thermometer. I have represented this by the yellow line on the graph which is a V/I plot for a 100ohm resistor. The output voltage readings for a given Oxygen ppO₂ will be based on the point where the V/I plot for the cell intersects that line.

Thermistors in circuits always worry me So in a specific pressure of oxygen the cell gets a certain density of fuel so it can give a certain amount of current which we read as the output. The problem comes when, due to age, the other components of the chemistry start to become exhausted. The battery is going flat. Now a second factor is limiting the current and, regardless of the oxygen, it will not provide more than a new limit on current. There was always a limit like this but before it was the equivalent of several bar of oxygen. To a rebreather user this becomes significant when this new current limit starts to become comparable to the oxygen readings we are expecting. The cell will now read correctly up to a certain ppO₂ and then will give no more current regardless.

So what do we do? We want to be sure the cells are correctly controlling the setpoint. So we need to know that they will detect that we are on or above the maximum ppO₂ and stop the inject. We calibrate them at 1 bar of O₂ or there about and so if we inject oxygen at depth so the displays read 1.4 bar or above we know that they are still working well enough. A cell becomes dangerous when it gives a fictitiously low reading which may cause the system to inject extra oxygen, boosting the loop oxygen fed to the diver higher and higher.

Now this is why cells get 'used up' faster if we leave them in pure oxygen. The other materials in the cell are being eaten up at five times the rate. It is also the reason why I cap off the cells in my oxygen analyser when it is not in use. The cells last longer once they have consumed the oxygen inside the cap and the reaction stops. On the rebreather I just flush it through with DIL (if I remember) as taking things to bits is too much of a chore. Incidentally the electrical load from the rebreather or a nitrox analyser is trivial compared with the internal current so unplugging the cell will not significantly extend its life.

It looks like an epoxy seal The thermistor is pushed right down into the cell You cannot test for 'current limiting' at any value of oxygen less than the normal loop values. The millivolt readings in air or at 1 bar O₂ quantitatively tell you very little. There is a slight clue that a cell that is running out of stuff will respond more slowly to changes in the oxygen but this is a rather ephemeral effect. I have long since decided to change them every 12 months and check I can get them well above the setpoint either, incidentally as they spike on a descent, or by pushing the O₂ inject on a 6 meter stop. If an oxygen flush will go up to 1.5 bar, I can never get 1.6, I know they are safe for the next few dives.

Calibrating Cells
What's all this about? Well the way a galvometric cell is produced means that the output is not predictable so the makers do not sell them with a certification sheet that says something like 50mV = 1 bar ±2% or something as it just wouldn't happen. What they do say is:
2) RANGE - 0-1 ATM PO2, 0-2 ATM PO2
              CALIBRATED WITH 100% OXYGEN.
            ZERO GAS AFTER 36 SECONDS.
8) STORAGE TERMPERATURE - 0-50°C (32 TO 122°F).
                             50% R.H.
11) WEIGHT - 1.2 OZ (32 GRAMS).


©Telidyne 2000
Got that? No? OK let's do it slowly.
Line 1): the output is anywhere between 8 and 13mV (not mega volts) in air. Any value in this range is right. The variation is primarily down to the rate oxygen penetrates the cell and to make it more consistent would make the cells a lot more expensive. Also it changes through the life of the cell. A cell might go up in voltage over its life but more likely will go down.
Calibrating is the business of putting a known level of oxygen on a cell and measuring the voltage. Then you use this remembered number to convert later voltages to the appropriate partial pressures of oxygen.

Right so we expose the cell to air like we do our Nitrox analysers?
So you know how much O₂ there is?
Yes. It's 20.9% I learnt that on my nitrox course.
<sigh>If it's cool and dry yes. If the humidity is 100% and the temperature is 45°C it's actually 18.9%
Now 18.9% to 20.9% isn't a 2% error it is a 10% error.
So if we want it right we need something better than air and in the rebreather business that's pure oxygen. Now the ideal would be to flood the cells with enough oxygen to be certain they are getting absolutely 100% but that's actually quite hard to manage. On the Inspiration the factory's best guess is that you'll get 98% but it is only a guess. However this, and the current barometric pressure gives us a fixed point. The ppO₂ is barometric pressure multiplied by oxygen fraction so if it is 1037mbar and 98% that's 1.016 bar of O₂ so if a cell reads 58mV we then know we are getting 57.08mV per bar O₂ and can calculate the oxygen readings from that cell by dividing by 57.08 so 88.5mV is 1.55 bar and time to do something about it.

Now the cells from the same batch may well be similar but they will not be identical so each one has to have it's own calibration number. You can't use the cells without these numbers as without them all it tells you is more or less minute by minute. Everything depends on these numbers and the biggest bug I see is the section 6) in the specification that say the humidity is NON CONDENSING. It isn't. When I take my scrubber head off it's dripping wet. Actually diving it is borderline non-condensing and good rebreather design puts the cells well inside, where they are kept warm, but moisture on the cell faces reduces the oxygen transfer into the cell and makes them read low.
Now we have hydrophobic (water hating) membranes on the cells to try and reduce this but it usually gets the blame for once cell drifting off on its own.

A final note on accuracy.
We've already wondered what our calibration gas is and accepted the best guess of 98% but what is 3) telling us? Errors are quotes against Full Scale and full scale is 2 ATM (2.0265 bar) so 2% is 0.04 bar. So anywhere on the scale there can be 0.04 error, either way.

An aside to this is checking your nitrox analyser. Calibrating in air as 20.9% is OK but it doesn't test things. Every now and again calibrate on pure O₂ then read air. If the air reads high the cell was limiting at 1 bar of O₂. It was trying to say 80% or such because it couldn't go higher but you turned the knob to make it read 100% so back on air it reads high. If it reads right it's good for quite a while yet testing 36% especially if you cap it off.

BTW: Do you know how much Combro would charge for me doing even a short form analysis like that?

Gills, oxygen and seawater
A little while ago on the UK diving web forums we had some nutter trying to sell his idea for centrifuging the air out of sea water to escape from having to carry cylinders when we dive. He was trying to obtain sponsors to fund this scheme and we all had a good laugh at his expense. I assume he now has sponsors and is living it up on their money.

To debunk this we needed numbers and, at the time, they were a bit hard to obtain so I thought I'd put the figures I found here to save me having to go through the angst of searching them out again and explaining in words of one syllable why this is a stupid idea suitable only for comic books.

We are used to the idea that stuff dissolves in water and we see fish swimming about powered by a metabolism based on dissolved gases so what's the problem?

Section 14 of the Central Rubber Council's Handbook of Chemistry and Physics has a nice section on seawater. The salinity in the ocean varies from about 32 to 37 ppt (parts per thousand by weight) except in the polar regions and near shore where it may be less than 30 ppt. The average salinity of the world's oceans is 35 ppt, which is the same as 35 grams of salts, mostly sodium chloride, in each kilogram of water. Incidentally 30ppt seawater freezes at -1.67°C, 35ppt at -1.92°C and 40ppt at -2.21°C.

O2 against depth Dissolved gases, however, are a harder problem as they vary with depth and temperature. If it was an equilibrium situation where the partial pressure of each atmospheric gas equalled tension of that dissolved gas we would expect 12.5mg/Kg of Nitrogen, 7mg/Kg of oxygen and 0.4mg/Kg of Argon. Colder sea water can hold more gas. We would expect even less CO₂ but this reacts with water and we end up with 90mg/Kg.
The best I can find for oxygen is some research stuff that gave the numbers for the graph opposite. Depth in meters vertically and oxygen concentration in mL/L horizontally measured in a sample brought to the surface so that is mL at 1 bar. They were more interested in the oxygen poor zone extending from 200m to 1000m but I'm more interested in the peak at 40m to about 4.5mL/L. We are not going to get more than 4.5mL/L (incidentally 4.5ml is 5.8mg)

Right what can we do with 4.5mL/L? Well at 20 meters deep on my standard scuba set I use about 20 Litres per minute to breathe, of which 21% is oxygen, multiplied by the 3 bar pressure. Now assuming I can get nitrogen out of seawater in the same ratio as we find it in air that means I need about 3L/minute of oxygen so dividing 3L by 4.5mL and then multiplying by 3 bar gives me about 2000L of water per minute that I must process through the magic box if it runs at 100% efficiency.

This raises three problems in my mind.
Firstly is size. If the magic box is a reasonable size, say a volume of working water is the size of my scuba tank it will have 12L of water to work on at any one moment. So 2000L/minute means that the water must be in, processed and out in 12*60/2000 seconds. That's just over one third of a second to complete the whole process.
Secondly there is the fact that 2000L/minute worries me when I consider that my old 20HP outboard had an 20cm diameter prop on it. To move 2000L/minute it would have to knock the water through at 2000/31.4=63m/s, otherwise 142mph in old numbers. So my second problem is that this is a huge amount of thrust for a diver who does not wish to zoom about like a torpedo and the third problem is that that sort of power isn't going to come from ordinary batteries.

So ordinary Open Circuit scuba is a non-starter but on my rebreather I estimate that I use 1L of oxygen per minute but that's conservative and it's really more like 0.5L/minute unless I'm zooming about or ascending or descending. This would allow us to divide the consumption by eighteen, although it does leave us the problem of separating the recovered gases which might take some creative physics. So then what does this save us? Well it saves us a cylinder 50cms long 10cms diameter and weighing 4Kgs which holds 5 to 10 hours worth of oxygen. I could use one a third the size and get a good few hours diving out of it. This is such a tiny part of the rebreather and is a trivial saving. To do that I still have to process 100L/minute of water so expect a flow rate comparable with 3 meters/sec and that is a lot more than my nice Diver Propulsion 'scooter' does at full power.

OK, the idea is a joke but that leaves us one remaining problem. Fish.

We are all familiar with the picture of the shark endlessly patrolling. Apex predator ready to strike and kill. Where does he get all his energy from?

Well the simple answer is that he doesn't. We have a fantastic adaptation that we are warm blooded and burn a lot of our fuel to maintain that heat. That's where most of the oxygen we need goes. The supposedly cold-blooded land reptiles are amazingly warm blooded when a biologist glues a thermometer to them. They have to bask in the sun to get their bodies up to working temperature and then regulate their bodies with sun, shade and water to stay on the right temperature. The shark can generate a little heat through muscle power but must otherwise stay in a bit of the sea that supplies the temperatures he can work with. The other thing he does is swim about with his mouth open and water streaming through his gills all day every day. Only little fish can sleep. Seawater supplies so little oxygen that to maintain a big body you have to swim until you die and the oxygen you get goes to the muscles with none left over for direct heat production.

When mammals re-entered the sea they could become the biggest creatures ever (eat your heart out 70 ton dinosaurs) and the most efficient predators ever known just because they retained their air breathing, warm blood design. A 4 meter long, 3000Kg Great White may look impressive and a bit frightening but it eats one big kill every couple of months and is not in the same league as a Blue Whale at 30 meters long at 180,000Kgs (that's nearly 200 tons) who is pushing four tons of krill a day into its tummy.

We mammals can't make do on gills. We have evolved a long way beyond that sort of trick. Centrifuging air or oxygen out of seawater just won't work for us.

A note added years after the original text.
We have seen a couple of mouth mounted tube assemblies aimed not at the scuba replacement but the snorkelling arena and working in the 'highly oxygenated' upper couple of meters.
So I looked for any published papers on oxygenation in the surface of the sea and found it is higher. There was a graph showing how it varied by season. The graph maxed out at below 14mL/L. That's three times as much as my numbers above. So for the OC system you only need to process 700L/minute. Hence if your gas separation window is say 25cm by 6cm (based on the advertised device) and again you work at 100% efficiency you need to swim at 46 m/sec to pass 700L through the system. That's 103mph.

Of course this is still all wrong
What we really need to do is to stop all this messing about with disolved gasses, which are hugely variable, and split the exhaled CO₂ back into carbon and oxygen. Then we have the ultimate rebreather and a green human being.
Of course we still need energy for this operation. The raw chemical energy is actually quite high. Based on 33KJ/gm release for combustion of carbon we would need about 88KJ/gm for the oxygen released. 1L of oxygen weighs in at about 1.46gms so that's 128KJ per minute or 2.1Kw. OK this means the batteries are going to be a bit big but draw them small on the prospectus and assume the technology will catch up before we actually have to deliver anything.
Now that I've done the basic plan I will leave coming up with a process for splitting CO₂ electrically to the class as a homework exercise. As a start think about a coal fired power station and run it backwards. Remember we need 100% efficiency or the battery has to get bigger and we don't want that.

Reporting fatal accidents
Diving kills people. OK over the years we have gone from heroics to hobby and the death rate has fallen but every year divers will die.

Now these incidents are often reported in diving forums and the response is always mixed. There are, naturally, expressions of regret from those that knew the person, or knew of them, and those that didn't are saddened and post messages expressing that. However there is often a tendency to treat reporting the actual details of the incident as insensitive and I have seen people who have posted a factual account criticised.

I think this is wrong. Badly wrong. Yes. Respect is important. A dead diver is a dead diver and there, but for the grace of God, go you or I. However a fatality is a wake up call to us all. We need to think these things through and put the skills in place so that if a similar incident starts to unfold for us or our buddy we see it early and resolve it. It is not insensitive to report the facts, as well as possible, without attributing blame.

If the topic is ever Nigel Hewitt has died diving write it up publically, think about it, discuss it and then don't make the same mistake. The only relatives I have that might read a diving forum are my children and I anticipate that they would correct any factual errors for you even if they did feel bad about it. Feel free to speculate if the details are not clear, they rarely are, and consider the 'what if' scenarios of incidents like mine. If the conclusion has to be drawn that I did something silly and won a Darwin Award for it then say so and if somebody realises they can make a change to the way they dive and be safer because of it that can only be good. Respect is remembering my views and considering them, the ultimate disrespect is ignoring my clearly expressed opinions just because you feel uncomfortable about it.

Currently I fear that doing a Hewitt involves launching your breakfast overboard and missing the day's diving but if it becomes the mnemonic for some other daft error that people no-longer make so be it.
Conversely, if all you can deduce is that I just kept diving and diving as I got older and older until one day, finally, I was just too old for it and fell over dead carrying the kit back to the car then you can all remember me, say "It's the way he would have wanted to go," and come to the Wake.

This has to be my specialist subject. Yes. I have a very silly hobby for somebody who gets seasick big time.

So what is seasickness?
Personal observation indicates that it happens when the input to your balance system from your eyes does not match that from the this way is up sensors in your ears.
For example: Ride in a car and drive or watch the road as if you were driving and you are never carsick. Try and read a book and you lose it very quickly.
So what's happening? While things agree there is no conflict. All is as normal. When there is a different input, your ears say you are moving and swaying but your eyes see a nice stable book on your lap, somehow something spills over into other parts of your brain and motion sickness rolls over you.

Now I am told that motion sickness is a poisoning response. Your body assesses the growing balance problem as a bit of bad foraging, you really shouldn't have eaten that dead squirrel you found under a log, and in a desperate attempt to keep you alive it is throwing the rest of the meal overboard. It sounds plausible but who knows?

There is the added point that you can train yourself both not to be motion sick (get your sea-legs) and to be sick. I see a lot of people training themselves to be sick by not taking precautions and choosing the worst, most fatalistic way to face it.

So what are the tricks?
Tablets. Find a brand that suits you. These slightly dehydrate your ears so they don't report the detail so much. This makes you less sensitive. Then buy the cheap generic chemist version of the product. Avoid patches. They come off when you perspire and sitting, kitted up for a deep dive in the full exposure suit for over an hour's deco, I cook.
Some people recommend Ginger and there were some proper studies done. Ginger scored better than a placebo but not as well as tablets. Scopolamine did very well but the side effects are a bit unpredictable and include dizziness often enough to exclude driving from things you should do and I wouldn't dive if I wasn't fit to drive.

Stay away from the exhaust and the fuel. These things predispose you towards feeling queasy and deepen the pit you feel you are going down. They are different inputs and I am not sure they effect seasickness directly but they are pushing the same way. I suspect that with enough diesel and exhaust you'd be sick on a boat sitting on blocks in the boat-yard.

Now visual tricks. Stand up. This forces your feet to work at keeping you from falling over and you can't do this without both inputs agreeing. Also look round so you are seeing the horizon, stationary, not the boat which moves. The alternative is lie down and close your eyes switching off one input. I can sleep through a storm at sea.

Now the point where it always got me was kitting up because this was the point when I had to take my attention from the distant world and point it to the rig I was about to dive. I counter this by setting the system up before we ropes off. On the Rebreather it costs me a bit more in batteries but I get more dives so it is value for money.

Practice. I cox the club RIB and I'm never seasick when I drive. I start the season taking people diving and not getting to dive myself. This teaches my system that going to sea doesn't mean seasick and also puts some brownie points in the bag so that when, later in the season, I get it wrong and just collapse in a miserable heap and don't help pull the kit back in people forgive me.

Finally just learn to be seasick. It isn't a big deal. Throw the breakfast, grab your water bottle and first flush your mouth out and then drink a lot to rehydrate yourself. You feel masses better in 60 seconds and you just bought four or five minutes that can get you kitted up and in the water. When you come down to it sea-sickness is only as debilitating as you let it be. The worse part is the anticipation. Being sick isn't hard just a bit uncomfortable and, provided you watch out for putting the fluid back, it doesn't really affect your diving. I can't make it enjoyable but it's not really that bad.

Garage glove I see too many people struggling to get their nice, tight fit, wet gloves on, especially for the second dive of the day.
Now I like wet gloves. Dry gloves are too much messing about and if they leak they cost you too much warmth as the water gets back up the path that you arranged to equalise the pressure. Also I've never been cold, even ice diving, in 5mm wet gloves. However mine are tight.

So how do you get tight gloves on? This is always the snag. You see divers making a real meal of it. Great gloves when they are on but getting them on...
The answer is Garage gloves. Well mine don't come from a garage but the sort of ultra cheap check your oil and throw away style gloves they give away there in the hope of selling you some oil. They are made from flat sheets of plastic sheet and a hand shape is cut out with a hot blade to seal them. Please notice these are not the nice hospital type which are hand shaped because they fit too well and have some grip. You want the absolute pits of a glove, the cheapest, the nastiest things imaginable.

OK humour me. Pick some up next time your filling up and then take your tightest oldest pair of wet gloves and put one under the tap. Really make it yucky and totally impossible to get on. Then blow a garage glove up like a balloon. Not too hard as they are cheap and nasty and will split on you given half a chance but you want to get the fingers open or it's a pig getting them on. Put them on first and make sure your fingers are right in then slide your hands into both the soggy wet glove and the dry one. Slide.

Hand shaped glove Believe brother. Once you've done it you never want to go back to the squeezing and the pulling and the struggling. You can use vet lube to slide them on but I've tried it and it seems not quite so good. I suspect the plastic crunching up makes for a better insulation budget than a nice even film of relatively conductive gel.

Something else gloves need to be, once they start to get thick, is hand shaped. If they are all floppy they don't have enough cell rigidity to actually provide proper insulation but conversely if your relaxed hand isn't half closed you have to work against the spring in a thick glove to grip anything. It is amazing how many gloves are either too floppy to have any long term warmth or try to spread your hand out flat because it works for thin gloves and it's cheaper to make them that way.

A word on dry gloves. I confess I'm not a fan. I've never been cold diving under ice or in Finland with my nice thick wet gloves/under gloves. So why do dry gloves get used. Well my opinion is that it is inadequate undersuits. If your undersuit is not up to the job you will get cold peripheries. That's cold feet and cold fingers. If your gloves look OK but your fingers are cold beef up the undersuit with an extra layer. Mine is an old M&S Cashmere and Silk pullover bought by a friend as a present. This is thin enough to not hit my weight requirements but still adds some upper body warmth and my hands are warmer. If you don't start with this sort of fix nothing is going to give you warm fingers and your gloves options will get more and more extreme as you try to fix the problem in the wrong place.

I am personally quite receptive to nitrogen rapture. I like it and fear it like doom. It destroys the instinct of life.
Jacques-Yves Cousteau. The Silent World. p43

Narcosis is a worry to me. Sitting under the relative protection of a helium mix I have watched a nitrox diver I both trust and respect take five minutes to inflate and launch a DSMB sliding several meters down the side of the hull of the wreck as he did so. Back on the boat I asked what the problem was. "What problem?" he replied. He was not aware that a routine I would expect him to perform in thirty seconds had taken that long.

Another instance when we were at 50 meters another buddy, who incidentally teaches knot tying to Scouts and Cubs, failed to tie a bowline despite several attempts on a 28m END while I, on a 20m EAD, could.

Perhaps I am fortunate that narcosis makes me nervous and I become a stickler for rules. I watch the slate, I watch the computers and if I don't write 'take photographs' on the slate it doesn't happen. However even then, at 60m with a 36m END when faced with a problem that took me out of the normal I fixated on that and responded to but did not act on the most basic 'UP' sign. I was grabbed and lifted a few meters and normal service was resumed but I can still worry about that one.

So what do I do about Narcosis? Like Cousteau I fear it like doom. I try never to dive with an END (Equivalent Nitrogen Depth ie: the depth you would go to on air to be breathing that level of nitrogen) of 30m or more. I suspect that almost always being sludged up with seasickness remedies doesn't help but even shore-diving taking my brain near 30m slows it down and makes me silly.

I dive helium mixtures shallow, the rebreather makes this inexpensive, but this means I see how much divers are slowed down even at relatively small depths. Never believe you aren't narked at 30 meters. If you think you aren't you are as deluded as the guy that thinks a few pints make him a better driver. Being on the inside of a brain full of nitrogen you feel good, confident, on the ball but these is just the symptoms of the problem. Another good friend tells how the first time he dived to 40m on trimix he was scared. It was the first time he had an accurate perception of just how deep that really was.

I quite like the way that trimix is promoted for shallower diving now. I think the days where 'deep air' was considered acceptable should be considered over. There are courses that fill in the details for using trimix and these are a good thing especially for rebreather users where the solution to a problem may require more thinking about than the OC 'find a better regulator to breath from' solution.

The Bail Out Valve or Demand/Surface Valve is a piece of rebreather equipment that gets a very varied response amongst even experienced rebreather divers. Some think it is vital, a piece of equipment that gives you your life back when you need it and others just can't see the point.

BOV So what does it do?
On a rebreather the gas you exhale is kept, cleaned up, topped up and recycled to become your next breath. This has many advantages but also the snag that the main gas path starting at your mouth and going all the way round through the works and back to your mouth again needs to stay dry. There are all sorts of tricks to try and keep it dry but the obvious first line of defence is a control on the mouthpiece that seals things off if you are going to take it out of your mouth in the water.
Clearly if you are still under the water at this point you need something else to breath from and this is normally a separate scuba regulator. A common trick is to build this regulator into the mouth piece so switching the valve to keep the loop dry also connects you to the regulator and normal breathing is resumed at once. This is the BOV.

Some rebreathers have a BOV as standard, some have it as an option and some do not have one. All rebreather training emphasises that if you suspect a problem switch to ordinary scuba for a couple of breaths and see if this clears things. You cannot taste low or high oxygen nor high carbon dioxide which are the dangers of a rebreather that seems to be breathing correctly.

So why the controversy? Surely divers can take one mouthpiece out of their mouth and put in another? Techies, like rebreather divers, who always seem to be festooned in more tanks than a whole class of beginners, do this all the time?

Here hangs the problem. If your rebreather loop has an excess of carbon dioxide your body cannot get rid of the CO₂ it is producing. The body reacts by increasing the breathing rate as this is what normally works. Now this is a pretty low level design feature designed to stop you dying suddenly and when it takes hold it isn't negotiable. Try holding your breath until you die to experience it. It just doesn't happen.
So our rebreather diver begins to breath deeper and faster. At first they won't notice as this is just automatic processes adjusting as they normally do. It grabs your attention when you can't breath enough though. This is the point when the penny drops, you say CO₂ hit and reach for the bail out reg. Problem. You have to hold your breath while you swap to the bail out gas supply because you are under the water. Stopping breathing has become something you are not allowed to do. You might as well try to stop your heart beating. Divers have died in this situation.

OK I've been there. Your mind is horribly clear but 'breathe' has moved to a 'knee jerk' reflex position. So what did I do?
Well I have an ADV in my loop, a device to automatically top up the gas in the loop on descent etc. I exhaled through my nose rather than my mouth, which was hard enough but at least possible and when I inhaled the ADV refilled the loop and I got fresh gas not the CO₂ laden stuff that was circulating. Don't think of this as a gentle bubble like a mask clear - this was a mask flooding rush. However the desperate 'must breath' lessened and I could pull out the mouthpiece and stuff the bail out into my mouth and then seal the loop. I was still panting but I paused, reseated my mask and cleared it. My buddy's face was in mine. He had hold of my harness. Another couple of seconds and I would have been CBL'ed.

I bought a BOV. I hope I never feel that again but if I do it's a one quarter turn on the big knob and I'm breathing OC gas.

Conservatism on deco and dive computers
I have always been taught on dive courses that conservatism is good.
Now in part I agree. Getting things right to the limit on decompression planning is just looking for trouble. We all know people who have had undeserved deco hits and none of us want to join the list. So for a lot of people this involves setting their dive plans and dive computers to use one or another system to add extra time to the decompression profile so they stay further away from the bent/not bent line.

The problem is that when I'm setting things up I don't know if the dive will go well and I'll be doing a happy ascent reminiscing about what a good time I've had or if it will be a total disaster and I'll want fastest time to surface with enough deco to stop my bad day from getting seriously worse.

So my conservatism comes out of my head and is not pre-planned. Deep stops are easy, you just do some. Giving it another 10 or 20 minutes at 6m, especially if I have got a buddy to annoy is easy too. What I don't want to do is hit the surface with something serious broken and not know if the N minutes of stops on the computer that I just blew away were real or just a precautionary add on.

OK so I set the computer to its lowest conservatism so it is showing the fastest time to surface. Both computers actually. However if I dive this to the letter regularly at my age I'm probably pushing my luck. So my first head conservatism factor is a slow ascent. I will save hurry for the day something breaks. Then add deep stops. I did run with the Richard Pyle trick of taking half the distance to your first mandatory stop and waiting there until the minutes on your computer click twice but now the VR3 wants more than that and more works for me.

A VR3 deep stop is sometimes washed out by my slow ascent but if I've seen a depth listed I like to do a Pyle two ticks on the minute counter so it's between one and two minutes. This takes me slowly up to 6 meters where I settle down for a snooze.

Now I'm diving a rebreather so at 6 meters the 1.3 bar ppO₂ already gives me an 81% Oxygen mix but by pushing some injector buttons I can push it higher. If I know I am nowhere near high CNS levels I will try and flush it to pure Oxygen but that never quite happens as I have 0.0567bar of water in the loop. This is another layer of conservatism because although the Vision electronics in the rebreather knows what I'm up to the VR3 doesn't. Add some extra time and I'm doing what I consider a conservative profile.

Now this might not suit everybody as a non-rebreather diver must watch their gas consumption in a way I don't have to. It's no good doing a slow ascent and then finding that the time left on the computer is getting tight on the SPG hitting zero. I'm fortunate that if I start full and nothing breaks I'm using at tops 60bar from both of my built in 3L cylinders and deco diving I have two 7L cylinders of the same again, with inject whips, and they are not in any plan other than emergency bail out.

This is just the way I do it. I'm not recommending it as it's not an agency view but until I get bent I'll have to say it works for me. Conservatism out of my head gives me the margin I like to stay well away from the edge of the tables while still meaning I have numbers available to me to know how serious a problem is that is likely to force me to the surface.

Of course deco on a computer is a pretty much a heretical idea to some. I should be doing tables and a slate. <shrug> So shot me.

One other thing. It is often said that "We don't really understand deco".
This is rubbish. We have been deco diving on tables for over a hundred years and we can formulate nice predictable results. The problem is not the tables the problem is the divers.

Tables are formulated for a young, fit, healthy, male diver who is eating and drinking properly. The divers who have problems may claim to be most of those things but I just don't see it. We are now aware of the subtle effects of a slight heart defect, the PFO. We are aware of the problems with being dehydrated. We are aware that fit and young are important.
OK, so the next time some obese old buffer says he's 'Dive Fit' to me I will roll on the floor laughing. Darn it I'm over 60 and I take that as a warning to do slow conservative ascents and think twice about doing that second dive, especially if it's not really very interesting. Certainly I don't go out on the lash the night before a deep dive so my body needs to use a lot of its water content to wash all that poisonous ethyl alcohol out of my system, in fact I have equipped myself with a p-valve so I can pour lots of nice water down my neck before I hit the water without any fear of later discomfort. Even the PFO is often signalled by migraines and normally only shows a problem with a Valsalva nose clear so perhaps it's the second dive that caused the problem?

So yes. I would agree there is probably lots still to be learnt about the detail of decompression but let's not rubbish what we know. Ninety nine times out of a hundred that's just an excuse.

Solo Diving
OK so I'm a hypocrite.
I don't approve of solo diving...
        ...but I rather enjoy it.

Yes, that is deep, sea water, wreck penetration, decompression, rebreather, solo diving.

Diving with my club or my friends there is no problem. It is when I travel to somewhere because I've been offered a dive on a specific wreck that I'm interested in that a buddy often doesn't work out. I know I'm a mug for pier pressure so I prefer not to get into that situation if I've taken a day off and driven 200 miles. My deco schedule is based on my mix and, more significantly, my bailout tanks. That alone often makes me and my laissez-fair attitude to dive planning incompatible with other people.

If I turn up on a boat and I don't know anybody I don't push it. I am a quiet, shy, middle-aged geek with a rebreather. I'm easy to ignore. I can solo and nobody notices. When (if?) I become inordinately rich I will always take a buddy along. They will be on the payroll and it will be their job to swim along with me with all the spares I might need. I'm sure getting paid to dive all over the world will ease the burden of having to put up with my dive plan (and carrying my kit) but, until then, forcing my dive on somebody else seems a bit mean. Somebody with the same aspirations and interests is great... but not all that common.

If you dive with somebody they deserve an input into what you do together but that can be at a huge cost. What if they are not equipped or prepared to enter a silty wreck? What if they view fish and weed as important and think that my attacking things with a wire brush to find out what it really underneath is wildlife vandalism? Are they really ready to do the dive in terms of gas and bailout? Have they a clue what they are diving? "It's a wreck? Huh?". Often the restrictions I put on myself solo diving are worth it to get to see what I came to see.

I don't solo all the time, in fact I don't solo by choice as a buddy, a good buddy, is lots of fun and I like sharing a dive with a like minded friend but it does have its place in my system of doing things. I'm not sure I should even recommend it although most of my problems diving have been when I have had a buddy along. Alone I am listening to the rebreather, I am feeling the dive. Alone I am exquisitely sensitive to my kit and my body in a way I never can be with somebody else around. I have stopped a solo dive early just on the suspicion of a fault where with a buddy I would have demanded proof before I cut their dive short. Solo is unsafe in very many ways but in some strange way I think that for me it is often the safer choice.

I quite like not having to take onboard somebody else's model for what the dive should be so the plan becomes more fluid and what I do depends far more on what I find while it's hard to communicate subtle choices with just your hands. The final ascent profile can be very conservative without worrying about somebody else's gas consumption/state of bladder/tea withdrawal symptoms. I know I can be a real pain when I have a camera in my hands and I've gone into wreckie mode and I really love not having to chase somebody who wants to do the whole wreck in the first ten minutes or whose fascination with fish/crustaceans bypasses my fixation on boilers and mechanicals.

For soloing I have developed a somewhat more complex kit setup than normal. The whole dive is planned on the rebreather but I carry enough open circuit bailout to complete the dive on a total failure of my main systems. Most of the essential clutter is duplicated (masks, DSMBs, reels, torches, buoyancy systems, stuff to breath) and if something breaks the dive is over and I am on the way up. Nothing is allowed to stress me as I only have one of me.

This is based on something I posted to the "Change one thing" thread in the BSAC forum.
It tries to describe a problem in the way the British Sub-Aqua Club works in practice.

Now I admit I'm a BSAC fan and I think we do a lot of things well but this is an area where what we used to do needs leaving behind.

I am discriminated against in the BSAC system because I am not a loud person. That is I will not push myself to the front. I certainly will not push myself in front of somebody else.

To paraphrase a quote in a forum thread: you "Hand hold OD trainees, lead SDs, expect DLs to run a bit of their own training but for AD they're on their own".

This selects for a certain personality type in the higher grades and it is a type that thinks it obvious that if you aren't like them you aren't Dive Leader material. To them it is obvious that to be AD or such you need that sort of 'stand up for yourself' aggression and they can't see why it should be otherwise. Think assertive. Think military. Think 'getting things done'.

I have to question this.

This type of personality selection closes the doors to a good percentage of our members and these are the type of person who are choosing to do the new look 'hobby' diving rather than the old 'hero' diving. People aren't coming into scuba now 'to boldly go' but because it looks a lot of fun. These should be the people that will rise up and run our trips and run our branches but the current system tends to drop them at SD so that's where they stick. Divers but not dive organisers.

Many will have good jobs and good organisational skills because diving tends to appeal to that type. They will slide into the non-diving committee positions and we run the risk of having well organised, well financed branches but no ADs, no AIs, no FCDs etc. Our diving will become increasingly segregated into the mundane club dives while the teckies, like myself, are going out and doing specialist stuff outside of BSAC and the training is reduced to just accepting crossovers.

We need to beware of characterising our divers with an out of date model. If you want something done in my office it goes, not to rufty tufty Don with his military 'make things happen' ethos but to sweet little Pauline who finds out, works out, arranges and then makes sure it is happening. How is a diving club and dive planning different from that? The distinction is subtle however because Don will pester me for something he wants but Pauline won't. Similarly I will not force my branch to use my ADI nor to finish my DL. I'm an old enough diver with other qualifications that it doesn't really matter either to them or to me as it doesn't restrict my diving. However what about the new people coming through? We don't have a big pool of ADs to draw from so our DO had better look after himself and not get promoted or offered a big rise to go and work somewhere with less silt in the sea. (I wrote this before we elected a DL to the post in a one candidate election).

I've had problems with DL, the stories I've heard of AD imply it is even harder to get the training and the approvals, the rumours of FCD imply it is almost deliberately made impossible to accomplish even before you pick up a manual or zip up your suit. If that is the policy can it be stated and monitored so everybody gets the approved amount of grief? If not can it be changed?

An Alpinist, in rebreather diving terms, is just somebody who has decided that the problems they get from carrying bail out cylinders are more than the problems they would solve. It is a necessary value judgement. Rebreathers have some failure modes that involve finding something else to breath from but they are not common and carrying 'enough' bail out can also be problematic. For me personally, for a dive with serious decompression time, Alpinist is a step too far. However for no-stop and short deco dives I will happily dive with just the onboard gas supplies as my bail out and when I've used it it has been adequate.

Now a lot of people have taken onboard the idea that rebreathers are dangerous so perhaps we ought to discuss this first.

If you are diving open circuit equipment a fault is hard to miss. You are diving along happily when suddenly you are surrounded by your own personal Jacuzzi of bubbles or suddenly you go to breathe in and you either get nothing or, more distressingly, you get sea water. On a closed circuit system a fault with the oxygen control systems can mean that although you can keep breathing in and out comfortably the oxygen levels go up or down and, if you don't notice, the world just goes painlessly away. The old OC idea that if you could breathe everything was OK has gone and you need to know what you are breathing minute by minute.

This generally means that you need to stay in touch with your gas mix with instrumentation so that, when the evil day comes and something important breaks, you notice and do something about it in a timely fashion. In theory this isn't hard but hour after hour of boringly correct readings can take their toll and we get complacent. If you combine complacent with lucky the system just keeps right on going and you are unworried however, having never won the lottery or such, my luck isn't something I'd trust my life too so a little paranoia is a good thing.

OK so where does this leave us with the Alpinist question? Some people say Alpinists are mad and although, others may say that all rebreather divers are a couple of sandwiches short of the picnic, that is debatable. What you have to do is decide how safe is safe.

The 'dead set' faults on a rebreather are loop floods and scrubber breakthrough. You can manage anything else, stay on the loop and keep using only a small amount of gas. Now both these problems are normally down to bad rebreather care or bad planning. If you are confident that the rebreather is not going to throw either at you the bailout is extra weight, in the way and a total pain moving about out of the water. There must come a point where the complications with managing bailout gas exceeds the dangers of not carrying it for the vanishingly few instances where you will have an undeserved loop flood or and unexpected scrubber break through during a part of the dive where it is irrecoverable.

That is when you become an alpinist.

Specifically getting your weight right after adding stages to your kit.
Cylinder stampings Again, I'm afraid, you get a lot of twaddle passed about by divers so let's do some sums and find out what the real story is.

The first thing we need to know is the volume and 'stamp weight' of our cylinder. Both of these numbers should be punched into the metal. In the illustration you should be able to see 7.0L WT8.8KG. You need to pick it out from all the test marks and the serial numbers and it should be under a nice protective layer of paint.

So this cylinder consists of 8.8Kgs of steel and has a 7 litre hole in the middle. Let us assume that it, my regulators, rigging kit and valve gear are 'typical'. All that changes is the volume, the cylinder weight and the material the cylinder is made of.

Hanging from the scales in water So I put on the regs and stuff and get out the digital scales and dangle the fully rigged stage in the rain water butt. It weighed 3.54Kgs but that was with 210bar of 100% O₂ in it which contributes 2.2Kgs leaving me with 1.34Kgs for the cylinder and all its fittings on empty in fresh water.

Now the first deduction is that if I promise myself the cylinders are never going to be 'staged' or handed off then all I can remove from my weight belt for my two stages is 2 or maybe 2.5Kgs. I can't allow anything for the gas they contain because if I do and then I breathe it all I will become buoyant and do an inadvertent ascent to my well deserved Darwin Award. Since they weigh 13.1Kgs each out of the water this seems a bit mean but that's physics. Obviously if you are planning to leave them somewhere or lend them out you can remove nothing.

Stage with weight belt The second trick is that I can now suggest a simple approximation for the 'weight' of an empty rigged stage in fresh water using the Kg and Litre values.

Steel: weight = 0.7 + stamp_weight*0.87 - vol

Ali: weight = 0.7 + stamp_weight*0.63 - vol

If they come out negative the cylinder is buoyant when empty and most Alis and several modern steels are. If they are buoyant then you need to consider adding lead to your belt so that if you do need to breathe all the gas you don't have to release the now buoyant cylinders before they lift you to the surface prematurely.

For use in the sea subtract 2.7% of total dry weight so 350 grams for mine and, frankly, you'll never notice. OK, if you use chain for your rigging kit it's going to be more and more complex regs will need further changes. However this will only change that 0.7 number at the front.

The other useful formula we can derive from this little experiment concerns the problem of changing your known cylinder for an unknown.
Again use the stamp_weight*constant_for_metal - volume and calculate a value for each cylinder. If it is bigger, ie: 'more positive', then you can lose belt weight and if it is less you need to add weight. If you end up with two negative numbers and you are not quite sure what 'more positive' means get a grown up to help you.

Incidentally: While I had the scales out I did a couple of other tests.
First I weighed a pony (Faber 3L/232/steel) and in the butt it was 1.7Kg empty.
Then I tied my 5mm Hydrotech gloves and my 5mm Otter hood up in a bundle with a belt weight. Out of the water they weighed 2.11Kgs and in the water 0.61Kgs so 1.5Kgs of buoyancy which is, I confess, quite a bit more than I expected.

For limited meanings of 'fresh water'. You really don't want to know what lives in my rain water butt. Darn it I'm not sure I want to know what lives in my rain water butt.

Rebreather types
Let's talk about the types of rebreather out there and, just for now, put the generic, all rebreathers will kill you, scare stories to one side.

There are basically three types of rebreather in common use. The most common in UK scuba circles is the the fully closed type (CCR) in which the breathing gas loop is measured and either automatically or manually topped up with pure oxygen to maintain the desired breathable levels and everything else, well except for the CO₂ which is chemically removed, goes on round and round. The second type is a simplification on that which, by using pure oxygen in the loop, avoids the need for measuring but adds a depth restriction as pure oxygen is not nice stuff to breathe deep. The final type is the semi-closed type (SCR) where the loop is being constantly topped up with a flow of nitrox, oxygen rich air, and that causes it to bubble off an equal volume every breath.

SCRs are somewhat cheaper than CCRs and so often attract the question "Should I get an SCR as a 'starter' rebreather?"
Can I answer that question as clearly as possible. No.

Only buy an SCR if you are planning the sort of diving to which an SCR is suited. What sort of diving is that? Well it's the sort of diving you could do on a single 15L tank with the added fuss of getting the right nitrox, setting the right jet, sourcing the Sofnolime, cleaning the system far more carefully and generally fafing about. Oh and they still bubble.

So I obviously don't like SCRs. Their one advantage is they are cheap to buy initially so if you are wanting a rebreather for ideological reasons they are attractive but I can't see why else...
Let me be emphatic on this one. An SCR is not a rebreather with training wheels that will help you learn some arcane skills that introduce you to the black arts of rebreather diving it is just all the fuss and extra costs for zero payback. They use a clever idea to get round having to measure the Oxygen in the loop but nobody in their right mind dives one without something to do just that because too much or too little oxygen kills you dead.

OK having got that one off my chest let's move on to Oxygen rebreathers. Hans Hass did fantastic work on Oxygen rebreathers but that was back in the days when we weren't all such pussies about oxygen. They are probably perfectly safe if you use them properly but, frankly, we don't have to now as we have the more modern CCRs so why mess about with being able to detect the oxygen twinges and do a rapid ascent before you go into a completely oxygen convulsion. I have an old ex-Russian military Oxygen rebreather. It is light, simple and really good to dive but I don't take it deep because I have better ones for that.

OK so that brings us back to the Blendermatic, the oxygen injecting CCR. These come in three flavours: well two and a half way version.

eCCR The Electronic CCR is basically computer controlled. You normally have three galvanic oxygen cells measuring the partial pressure of oxygen in the breathing loop and a bit of computer stuff working out how long to hold the oxygen valve open to keep that up to the required numbers. The galvanic cells are considered the weak link in the chain, which is why you have three, but you are still dependant on the electronics to do the sums to keep you alive so you are trained to keep checking the displays to see that you like the numbers. I have had the displays freeze up on me and I promptly put the OC backup system in and surfaced. It may have been once in fifteen years but you do need to do those checks.

I have often said that rebreathers favour the paranoid. With OC gear it is easy to spot the fault when it happens. You go to breathe in and get seawater. You will notice that. You only worry about the mix you are breathing when you measure your Nitrox/Trimix before you dive and when you do a gas switch. With a CCR you must have some brain cells permanently on the job of knowing what you are breathing minute by minute as a key life expectancy issue. If you aren't up to this don't dive CCR. It's not common that things fail but they only need to kill you once and all the rest of us safe paranoids get tarred with the 'dangerous divers' brush.

mCCR The Manual CCR takes a different approach to keeping the oxygen in the loop constant. By using a nice trick called a sonic orifice it can arrange a constant gas flow of oxygen (in litres/minute) into the loop regardless of depth and this is set to be at just about life support level. You are then expected to check the cell readings and top it up to the desired numbers by pushing a valve. Now the proponents of manual control say that this is the way to learn 'proper' monitoring and control but to me it always sounds a bit like 'real men dive mCCR'. Frankly you are providing a system that will drift off the number to force you to check it while an eCCR just needs looking at to see that it is still on the job. I rather feel that mCCR is just a crutch for people without the proper paranoia to dive a real eCCR.

hECCR The Hybrid CCR is a bit of both. This is the sitting on the fence rebreather. Let's have the orifice from the mCCR and the electronics from the eCCR but we'll probably set that computer oxygen setpoint low and then manually keep it topped up the the point we want. Basically by buying a Hybrid what you are saying is 'I want a real man rebreather but can mummy come with me in case it plays rough?'. Is a hybrid is an mCCR with the 'real man' button? No. It's an eCCR with a 'poser' button.

Internet Divers
We are internet divers. We may not be good divers but we are enthusiasts to be on a web forum at all and running a website is giving in to it. We have exposure to ideas that most divers don't care about and probably incorporate concepts into our diving that are just not part of most diver's worlds. This has a good side and a bad side.

The whole 'technical diving' thing is our biggest kick. Most divers would think that 40 plus meters down is way too deep and they're probably right. Most divers can't hold a stop and I admit I couldn't until I got used to doing deco with a buddy where we took turns, dive by dive, one to run the blob and the other just to hold station with them.

Doing deco like that forces you to sort out your buoyancy control and forces you to sort out your trim. Real deco, not just a few minutes of stops, forces you to think about 'what do I do if something breaks?' because you are looking up at the route home but it's not safe to go there yet and won't be for another hour.

However just reading about this stuff and becoming familiar with the ideas is the bad side of the internet diver. These are the ones that suffer from the arrogance of knowledge. We seem to have lost the stroke shouting DIR enthusiast these days (aka rec.scuba news group), although his opposite number, the DIR hater, has not relaxed back into obscurity yet. However the newbie diver, who emulates our views without knowing the road we have walked to get there, is still about.

So if you've dived a lot of deco profiles and you see a pattern and you want to just stick to that then do it, but that isn't the same as running a lot of deco programs on a computer. I may do solo deco dives on a rebreather with a green water ascent (nothing within my visual range and no line) but nobody is doing that on my watch as a club Dive Manager.

When you really come down to it there are two sorts of Technical diver:
Those with an abiding interest in something underwater who need the technology to give them the depth and duration to investigate it.
Those who are barking mad.
(NB: There appears to be a considerable overlap between these two groups.)

I talked a while back about narcosis and the way it takes your brain away. So what is our preferred solution?

Well the first thing is to understand the problem so go to the medical text books and look up the Meyer-Overton hypothesis. This correlates lipid solubility of an anaesthetic agent with potency and suggests that onset of anaesthesia occurs when sufficient molecules of the anaesthetic agent have dissolved in the cell's lipid membranes. (my hack on a Wikipedia quote).
This would lead us to believe that nitrogen is indeed an anaesthetic when under pressure but, perplexingly, oxygen is even more so. However it gives helium as a poor dissolver so very good from the 'avoiding narcosis' point of view.
I have some figures that say oxygen is about double the solubility of nitrogen but, since it is consumed in your tissues it will be lessened at the actual point of action while helium should be about 16% of that of nitrogen.

102 meters What we tend to do is treat oxygen and nitrogen as equally narcotic while helium is considered not narcotic at all but, until we have a Narc-o-meter to plug into somebody's head and read out that they are 28% narked, that is good enough as the day to day and person to person differences are bigger variables here.

So. We introduce the idea of END, Equivalent Narcotic Depth, which is the depth you would have to dive on air/nitrox to be that narked.
The sum is very simple. What is the partial pressure of the O₂/N₂ part of your gas? How deep would you have to go to find that as an absolute pressure?

For example diving 18/40 Trimix (18% O₂, 40% He and hence 42% N₂) at 60 meters.
You have 42+18=60% narcotic gases.
The absolute pressure at 60m is 7bar so the partial pressure is 7*0.6 = 4.2bar.
Your Air/Nitrox dive to 4.2bar absolute pressure is 32 meters deep.

So diving 18/40 to 60m is roughly, and I emphasise roughly, the same as diving 32 meters on air from a narcosis point of view. (Please note this is not anything like EAD and decompression. Cutting your gas with O₂ can reduce your deco time, helium will probably increase it.)

That seems good but Helium costs me well over a penny (UK) a litre buying it in bulk so to fill my 10L/300bar twinset with 40% is over £30. Far more if you go to a dive shop and they cost in all the work that I do for myself and the equipment I have. This is on top of any oxygen needed to make up the mix and the usual price for an air fill. A weekend's helium diving can easily cost a hundred quid in store fills alone.

However, cost aside, the real problem with helium is that it is a 'fast' gas.

So what do I mean by fast?

On the Bühlmann decompression model there are 16 'compartments' which represent overlapping tissue types. It isn't that your spleen is a compartment type four and your leg is a compartment type six just that all 16 overlap enough for everybody to get represented. Also when one compartment feeds another it is just represented by a slightly longer compartment. The system dates from Haldane and it's good physics.

For nitrogen they have one set of 'half times' for each compartment representing how long gases take to dissolve and release but for helium they are about a third as long.

This implies that helium dissolves in your tissues at nearly three times the rate of nitrogen so, in very approximate terms, a dive on helium is the equivalent of three times as long. However, as your deco is three times as long too so it evens out and a helium dive isn't all that much different from a nitrox dive.

Until you miss stops. Then those ten minutes you missed on your 40 meter dive is effectively half an hour of missed stops on a dive with three times the bottom time and your problem has moved up an order of magnitude.

Conversely when they haul you messed up carcass into a chamber all those helium bubbles redissolve at three times the rate and you then are back to doing the re-deco at 'normal' rates. Your untreated bend is much worse but, if they get to it in time, the prognosis is much better than for the same level of bent on nitrogen.

One of the consequences of this is that your ascent needs to be spot on. Deep stops can have some compartments off gassing while others are still on gassing so hitting those numbers is much more important. The fact that a only two minute stop is called for doesn't make it trivial (think six minutes). I much prefer diving a computer on helium precisely because in the deep ascent stage of the dive it is recalculating 'on the fly'. If I were diving tables I would have to select a far more conservative algorithm because I know I'm not going to hit my numbers to the second and all my errors are multiplied by three.

To me helium is an essential part of deep diving. Since I use a rebreather some of my diving decision making is more complex than for an open circuit diver so a clear head is a life expectancy issue but happily my main gas tank is three litres not twenty so my helium costs are moderated. I tend to run the rebreather either on 18/40 where Oxygen toxicity is not a problem or 12/65 where it is. I have bailout cylinders with the same mix so I can swap about if I have a problem and my training included switching both my decompression computers back and forth between the rebreather and the bailout.

I even use a tag line on web fora of Helium, because I'm worth it.
So why the picture of Sussex Heights in Brighton? It's 102 meters from the roof to the road. A good trimix dive.

No, not really spanners. I just wanted to list the thread names we use on Scuba gear as I can never remember them.
O-rings are identified by their 'dash number' and ID/CS dimensions. (Internal Diameter/Cross Section)
Notice that the DIN and A-clamp O-rings are listed as different but a DIN ring seems to work in every A-clamp reg I see. This is not true the other way round. They seem to seal OK but fall out at every opportunity. I think everybody uses the DIN size and thinks they are the same.

What I call itWhat it really is  O-ring
Standard low pressure port3/8" UNF-011 .301" .070"
The Apeks bigger version1/2" UNF-013 .426" .070"
Standard high pressure port  7/16" UNF-012 .364" .070"
Standard 'DIN' thread5/8" 14TPI BSPP-112 .487" .103"
New funny EU nitrox thingM26x2Apparantly as DIN
A clamp-014 .489" .070"
HP swivels-003 .056" .060"
2nd stage connection9/16" UNF-010 .239" .070"
So while we are talking about O-rings perhaps we need to explain them a bit:
An O-ring is not just a plug, like the caulking on a boat, that fills up the holes to stop stuff getting in. They are far more sophisticated than that and knowing how they work should control our thinking on how to maintain them.
The first O-ring patent dates from 1896 so it is quite late into the age of machinery. In its simplest form an O-ring is placed in a rigid mounting that applies a uniform load deforming it. The seal is produced because the fluid cannot pass as long as the contact pressure of the O-ring exceeds the applied pressure, ie: the fluid cannot push hard enough to lift the O-ring from the wall it is sealing against. The magic trick is that the pressure of the fluid pushing against the exposed face of the O-ring further deforms it away from the fluid and into the wall increasing the seal pressure so the applied pressure can never 'catch up' with the seal. Basically an O-ring will hold until either the housing fails or the O-ring is physically extruded through the gap it is sealing.
So why do O-rings get greased? Two reasons. Firstly a lot of O-ring materials have plasticisers and other semi-volatile components and greasing stops these escaping but secondly as the O-ring deforms under pressure it must not adhere to the housing wall, even under extreme load but must deform uniformly or the surface tears away from the body and the ring fails as the pressure now rather than increasing the seal penetrates the ring.
The down side of greasing a ring is that it becomes a dirt magnet and an O-ring cannot perform its deforming to seal better and better trick against an uneven surface. The general rule is clean the ring fastidiously and if it is the sort of ring that likes/needs grease apply it very sparingly.

Bailout on a rebreather
There is always a lot of discussion when 'bailout' is mentioned. So we're clear what I'm talking about here I'm using 'bailout' as a noun that describes your provision of gas for a broken rebreather scenario.

Obviously you need something as your CCR 101 class taught you the quick check of a 'sanity breath' to check your body by getting it off the loop for a moment. This is good but how long you can stay off the loop depends on how much gas you have available.

Now on my MOD 1 (basic CCR) I was taught to use the onboard 3L tank of diluent on my Inspiration Classic for my sanity breaths and my quick trip to the surface if all is not well. MOD 1 was a very limited (eg: 5 minutes) deco ticket so a 3L would get you home undrowned.

MOD 2 is Normoxic trimix and you do deco, quite serious deco so we were taught 'team bailout'. We were all carrying 7L stages of trimix that could be breathed at all points of the dive and the drill was that if somebody had an equipment failure they switched to the stage, signalled their team <UP> with urgency and started to ascend. When that cylinder got down to half contents they swapped it with another team member so they had a full one again and the donor had a half full one so they had something to reach for if they had a subsequent problem. The idea is that the team carried enough gas to get one and a half divers safely back from the worst point on the dive and the procedures meant that everybody always has some gas hanging from them.

MOD 3 is hypoxic trimix and moved the emphasis to self sufficiency. You carried enough gas to get yourself out on a total rebreather failure and you only looked at a buddy if you had subsequent failures.

However this is 'worst case' but not all cases are worst. What is realistic?

Well let's start by thinking through what is a 'dead' rebreather. In the basic class you learn to manage the oxygen inject system manually and to set it up properly and that prevents or manages the vast majority of reported problems.

People die on rebreathers but when the results come out they are often the simplest things. The old mantra 'Always know your ppO₂' would have saved most of the lives lost. Diving with the system simply switched off has killed far too many people, I can't save you from that Darwin Award, but what faults are next on the line?

A loop flood. Something unexpectedly broke and suddenly the plumbing is irretrievably full of water. This is not 'it has some water in it' because you should be able to manage that and clear the loop and resume the dive or at least finish the dive on the loop. I am thinking about a breathing hose flapping about in the tide when it should be screwed in behind your head sort of thing.

CO₂ break through. Most of these are operator error, mine was, but recriminations are no good under water so you have to get off the loop (which might be a problem see BOV).

If the loop isn't dead I hold that you should stay on it and use your training to manage the gas in the loop. We didn't have bailout in MOD 1 and it worked but once you commit to the bailout you now have only one path home. There is no bailout from the bailout. The only snag comes if you don't know what the gas in the loop is either because your displays are trashed or because they are all wildly different and you don't know who to trust even after doing the procedures to work out who is telling porkies.
temp profile Temp stick
While I'm on the subject of Rebreathers let's explore the much maligned 'Temp Stick'.

It seems very trendy in trendy circles to be rude about the temp stick but, although it has its limitations, I think it's a good addition to a system.

So what is it? Well it is a rod that passes down the centre, usually the centre, of the rebreather's Carbon Dioxide scrubber with a number of temperature sensors in it. Now the scrubbing reaction is pretty vigorous, exothermic in chemist speak, so as the 'lime reacts with CO₂ it gets hot so we have a tell-tale signature of heat generation.

This means that the heat being generated at the reaction front will progressively warm the bed upstream so we should see a kick where the scrubber is working. The graph should be simple. Right? Explain the graph on the right then. That isn't simple.

This is an hour long December dive in Lake Geneva on a part used scrubber and you can't see a five minute pre-breathe. The sensors in order from the top of the stack are numbered 1 through 8 and the thin line is the depth. The depth scale in meters is correct but I don't believe the temperature scale, in fact I'm not sure they are even zeroed so only look at the up down trends not the absolute values.

So what has happened? Well it has been dived before so we can divide the scrubber into three general areas. At the top (this is a Sentinel and the exhaled gas comes in at the top) is scrubber that is mostly exhausted. It might have some life but it is not going to be making a serious contribution to scrubbing or heat generation. In the middle let's call it 'part used' while at the bottom there is largely unused scrubber material.

The working zone of a scrubber on the surface is surprisingly narrow. When CO₂ hits good lime it gets zapped. Most of this stuff is sold into the very competitive medical market where cost is an issue and not going wrong and getting sued comes a close second so it is very efficient and well tested. If I was just to sit on the shore and breathe the working band would creep down over time leaving dead stuff behind but with untouched stuff below. Getting my fingers on the can and feeling the heat I'm going to guess it's less than an inch wide.

The heat is another matter. The gas coming in is fresh from me so it is at body temperature so my scrubber that has been cooling from the nice warm car as we have changed and been briefed gets warmed up again on the prebreathe. Sensor 1 just reads this body temperature for the whole dive as it is in amongst some 'previously enjoyed' scrubber material so no reaction and hence no extra heat. Remember that a sensor tells you about the reaction in the section of bed before it so when I speak about something happening at sensor 3 I am talking about the section of the scrubber between sensors 2 and 3. I think the prebreathe reaction really happened between sensors 2 and 3 and 2 was heated by conduction sitting waiting to dive. Certainly 2 blips a bit as we start the dive but I suspect is reading pretty much the same as 1. 3 is certainly on the job at the start of the dive and from them on the heat generated here is carried on down the can in the nice warm gas warming the layers below that have yet to be called on to work.

The next thing that happens is a progressive descent to 35 meters. Now this increases the absolute pressure from 1 to 4.5 bar and hence the mass flow goes up by 4.5 times. Now remember that the gas flow in litres per second hasn't changed nor has the velocity of the gas but we are just packing more molecules into the same space. Hence the chance of a CO₂ molecule getting picked up goes down as there is just 4.5 times as much other stuff getting in the way so the working zone becomes wider. The scrubber material is still scrubbing but less efficiently. If this was a well used scrubber and the working zone actually reached all the way to the bottom some CO₂ would make it right through. (We'll discuss this later). What you see is that sensor 4 that wasn't showing much interest in being prebreathed is now getting in on the action and showing heat, then 5 and 6. 4 actually sags through the deepest segment as it is being cooled by the incoming gas more than it is being heated by its reacting with CO₂. I would say that 4 and 5 are doing the work here and progressively heating the gas, and hence themselves. 5 is doing the most work as it is generating a big temperature step while some is getting through to 6 which isn't generating much heat. 7 and 8 are just getting warmed by the nice warm gas coming through.

Then we ascend back to 20 meters and for the rest of the dive you can see 4 dropping out of the game as used up while 5 and later 6 coming back on line. 7 and 8 never seem to get involved and are just heated by the gas flow. That put the scrubber at about three hours so it went in the bin.

This whole business of the working band widening at depth is the reasoning behind the APD mantra:

Be shallower than 50m at all times after 100 minutes of scrubber use
Be shallower than 20m at all times after 140 minutes of scrubber use

temp profile Your worry is that the CO₂ will start to break through as this now puts more into the cycle next time round and even a small fraction of more is even more so it begins to wind up. There is no 'almost OK' about a scrubber but the fix is to get shallow and reduce the depth of the reaction front. The second best fix is a flush and, if you have the gas, a bit of semi-closed on the ascent or the getting out of the wreck phase. For dive planning the image I carry in my head is of the scrubber like an iPhone battery picture. Full is 180 minutes. The working section on the scrubber is about 13 minutes worth of lime on the surface - unlucky 13. Put the pressure up six times and it is now 13*6 so over 72 minutes of scrubber are in use, that has to be subtracted from the battery remaining. When the working section hits the end you are in the kill zone. This works for me but the APD one is easier to remember.

I liked the Sentinel temp stick display because it gave me a little block graph of the sensors (top left in the picture). OK it also gave me a number that is developed on some frightfully clever computer algorithm but it is usually so wrong you want to laugh. I'm looking at the steps. The big step is where it's working. I know how big my working zone is and I use that to warn me to give up early. I don't use it to extend my time as I've had one run in with CO₂ and it really spoils your day.

Attitudes to Oxygen Clean
A story I was once told was about somebody picking up a cylinder from a shop and wiping the face of the (A-clamp) valve off with his finger. The shop assistant berated him that this had compromised the 'Oxygen Clean status' of the tank and it should now be recleaned before it was used for nitrox. I rolled about the place clasping my tummy before I realised that it wasn't being told as a joke.

OK let's put some numbers to 'oxygen clean'. Oxygen clean is different from Oxygen compatible. You must use the right materials for high pressure oxygen environments. This is just the way it is. However we have this routine of cleaning and much of it is just plain wrong.

Oxygen clean is not surgically clean. Surgically clean worries that the one microbe on the one scalpel you failed to autoclave multiplies and multiplies and wipes out half of East Africa. Oxygen clean is just chemistry.

Back in the days of the uk.rec.scuba news group we worked out the energy in scuba tanks based on their pressure and in simple terms:
12L at 230 bar is aprox 1242kJ   This is the working pressure of a tank.
12L at 346 bar is aprox 1868kJ   This is the test pressure of a tank.

So let's get the energy from a fire:
We get 4163kJ/mol burning hexane (eg: petrol)
so (1868-1242)/4163 so 0.15 mols of hexane
at 86gms/mol is 13gms at 0.65gm/cc is 20ccs

This is not half an inch in the bottom but neither is it just *dirty* or *you touched it - it's infected*.

Interestingly a toxicology site gave 20ccs of petrol as a lethal dose if inhaled or injected. So a tank with a deadly level of contamination is not quite an oxygen fire risk?

Bungeed Wings
OK it's funny farm time again as people talk more nonsense about the pros and cons of Bungeed Wings than most subjects in diving. Firstly what are we talking about?

a ten year old picture - anonymous A 'Bungeed Wing' is one that has an elastic thread wrapped round it to collapse it when it is not inflated. The rational is that this offers more streamlining and hence less drag. Now it can be well done and it can be badly done. Lots of tight bungee with the inflated wing bulging out between looking for all the world like some macabre creation of the balloon animal man at a children's party in not just bad but very obviously silly. A bit of discreet bungee arranged to tuck the wing away when it is not in use is more what I'm aiming to discuss.

Let's talk about streamlining because as usual 'no clue' comes from not understanding what is going on. Now my fluid mechanics was a second year physics degree course and for water it is 'incompressible flows'. It wasn't very interesting at the time but it comes back to haunt me as a working industrial physicist far more than the exciting quantum mechanics which has only ever been a hobby interest. If by streamlining you mean drag reduction the rules are quite simple.

Streamlining rule one. Fill in all the holes. Absolutely. If you are moving a pipe through water put a flat end cap on it rather than let the water go down the hole in the middle. You would be horrified to work out how short the pipe has to be for this not to apply any more. It is getting to be a ring not a pipe by then.

Streamlining rule two. Flatten the surfaces. What you are after is not letting energy be dissipated so creating turbulence is your enemy. Any turbulence you leave behind you in the water is energy you had to put into it. A lot of flat surface is much better that a little bit of bumpy. Anything that makes the smooth boundary layer break away from the surface costs you. You want to see the ultimate in streamlining? Look at a Zeppelin. How can a thing that is that big and that light need brakes? But they had real problems with stopping even into wind.

Streamlining rule three. Reduce the surface area. A sphere is very streamlined and a dart isn't.

So let's look at a diver from a streamlining point of view. No! Stop rolling on the floor holding your tummy. I know it's bad but we might at least be able to improve some bits of it. The first entertaining thing we notice is that clipping equipment in close rather than letting it dangle makes absolutely no difference. As the boundary layer is measured in millimetres streamlining is putting it in a pocket or velcroing it hard too you - see rule one. If you want to carry a lot of clutter efficiently buy a bag. That is a bag with a drain at the bottom not a string bag as, from a hydrodynamic point of view, a string bag is just a big pile of clutter dragging through the water next to you with added string.

Now the cylinder and wing. Didn't you have tank wrap on? Oh, you decided it might not be quite so good. Well if it was canvas it wouldn't be a problem but it did have rather a lot of holes to apply rule one too. What about the wing? Well it has X amount of area and that isn't going to change as it is not a balloon, it just unfolds as you inflate it. How you stow it makes virtually no difference, well, unless you ruck it up into lumps and put bungee in the boundary layer to break up that laminar flow.

So that's it. If the wing just collapses bungeeing it can't make it any better and the bungee side effects can only make things worse. My smooth old DiveRite Classic twinset wing might flap a bit when fully deflated but I can't change the exposed surface area unless I roll it up (rule three). Squeezing it down can only start getting me into rule two problems and make matters worse. So can you now see why I'm not a fan of bungeed wings?

So one last question. Does bungee make the wing less likely to snag on things? I'm not sure I can give a definitive answer here as it depends on what sort of snag you are snagging on. However if I am venturing into a place with hookie things that might hook on my bungees I definitely know the answer. If they aren't hookie aren't they just going to slide over my nice smooth wing? When you come down to it I'm not brave enough to dive a bungeed wing.

We were discussing 'being blinded by helmet torches' and I realised that I needed to do a test so I knew what I was talking about. This is also the argument for black kit so you don't reflect my torch back at me and blow my night vision.

The test needs to represent me trying to see with my helmet torch or torches after getting a full frontal exposure from somebody else's lamp and see what the recovery time is. Let's make it a big hit not just a flash too. Basically: is this something I need to worry about having it happen to me or doing it to somebody else? We have been quoted a 30 minute recovery time which sounds bad but I just don't quite believe it.

So I am sat in my 'office', which is really just the more respectable end of the home workshop, and I find my Petzl caving helmet which has a switchable LED/incandescent light on it. I compare the LED side to the SL6 torch on my diving helmet and the SL6 is way brighter. Good I will test on the Petzl because it is so much more comfortable to wear for not being stripped of all its internal padding to fit over a neoprene hood.

I black the place out, turn off the computer screens and hide the laser mice. I do have to leave some stuff on so I have points of light about me but I have a company web server and the mail host running here and they shouldn't go off. I look round on the Petzl light and start waiting to adjust. Ten seconds later I feel very adjusted so I turn the helmet light out to see if that helps. I wait five boring minutes and turn the helmet light back on and I can still see all my tests but nothing more. I ought to be adjusting to the head light but it's more interesting to have it off. Using the helmet light the hardest thing to read is some light blue text on a grey box at about four meters. If I don't have the light pointed just right I can't see it.

Facing a big strip light In the dark so the flash reflects
The hardest thing to see without the light on turns out to be the castored legs of the other office chair just lit by the LEDs on the stereo system (I'm an old buffer who thinks they haven't built proper hi-fi in thirty years so I share my office with a Quad 606 amp and a pair of '57 Electrostatic loud speakers.) Interestingly it takes nearly 25 seconds for the castors to fade back into view after turning the helmet light off. I try this a few times and wait a lot longer but I don't get any more detail. This must be my maximum setting. Remember your eyes have an iris that opens and closes to give a small but fast adjustment that caters for you glancing from light to shade and chemical adjustments for wide range adjustment that takes time but that can adjust from direct sunlight to starlight. Think of it like film speed. It is this chemical adjustment, not the iris, that we are trying to measure here. My iris, see the pictures, contracts to 44% by diameter from dark to light so it factors the light by a mere 5 to 1.

I finish my 30 minutes pre-adjustment period (in case 30 minutes is right) and reach for the Fa-Mi can lamp. 75 watts spread across six LEDs. The helmet light is out but I can run this kit by feel. I start listening to the clock tick in seconds, push the button on the stop watch and turn the lamp on. It's in my lap pointing at my face. Sheesh that is bright. Don't cheat, look at the bulbs. -8-9-10 turn it off. OK I can't see anything, well other than this purple blob with the tracks of the six LED head in it. The LEDs on the stereo come back but, interestingly, they are orange not green. I try the helmet lamp. Hey I can see. The centre of my vision field is a bit fuzzy but it works. 35 seconds on the stop watch - yes I can read the stop watch. I can read the text on the box OK.

That was the test, remember the number 25 seconds from when the big lamp went out until I read the watch. It might have been less but I'm not doing it again to find out. However there is one more thing to try. Turn the helmet light out again and try for the ultimate readjustment to max sensitivity. My orange lights are greener. Actually it's the slightly luminous blob that is making them orange. They go green. The castors of the chair finally fade back in. Check the watch. 3 minutes and a few seconds so less than three minutes from when the light went out.

Conclusion? Dazzle isn't a disaster and certainly it isn't 'blinded'. This gets the 'Nigel Rating' of 'inconvenient' (the next level up is 'annoying'). I really don't expect to have your torch in my face for ten seconds without me closing my eyes and turning my head away but even if I did twenty five seconds of glowy blobs isn't a great hardship to get my 'torch vision' back.

Reverse Profiles
One of the constant subjects that comes up in diving forums is that of Reverse Profiles.
So what is a reverse profile? It is simply when you do two or more dives in a day and you don't do the deepest dive first.
This seems to come up in a lot of diving instruction that this is wrong and is repeated as doctrine but is it justified?

Firstly from a deco point of view deepest dive first is a good idea. If you are going into a deep dive already with a considerable dissolved gas loading it will increase your decompression time considerably. So if the second dive is the deepest dive then you will end up spending more time that day dangling on a string than if you were able to do them the other way round.

Conversely if the extra time isn't a problem that's all it is: extra deco time. If, on the other hand, the first dive is to check your equipment and procedures are all in place that might make the second deeper dive much safer and the extra time is more than worth it. Also the nonsense about setting a limit on the second holiday dive because it 'mustn't be deeper' is best laughed at and somebody who seriously suggested doing a quick touch-and-go to 40 meters on the first dive so as 'not to restrict' the second dive was to my mind detracting from safety rather than adding to it.

However in 2000 the American Academy of Underwater Sciences ran a Workshop on reverse profiles where a lot of learned people came and presented papers and the procedings are available and the Abstract says:
1. Historically neither the U.S. Navy nor the commercial sector have prohibited reverse dive profiles.
2. Reverse dive profiles are being performed in recreational, scientific, commercial, and military diving.
3. The prohibition of reverse dive profiles by recreational training organizations cannot be traced to any definite diving experience that indicates and increased risk of DCS.
4. No convincing evidence was presented that reverse dive profiles within the no-decompression limits lead to a measurable increase in the risk of DCS.
We find no reason for the diving communities to prohibit reverse dive profiles for no-decompression dives less than 40 msw (130 fsw) and depth differentials less than 12 msw (40 fsw).

I was horrified.
Given findings 1, 2, 3 and 4 the conclusion just boggles the brain.
The conclusion should have a full stop after the word 'profiles', end of paragraph, end of document, go home pleased with a job well done.
They should be ashamed of themselves.

Gas boosting
Specifically how much drive gas does my booster use?

I'm not going to do an exact model because the important bits get lost in all the adjustments but I'll admit to the approximations as we go. Consider the diagram as they say:
Two chambers both with a piston in them. The pistons are connected by a solid connecting rod. We blow low pressure drive gas into the left hand side pushing on the large piston so we get a force on the rod that is basically the drive pressure Pa multiplied by the piston area. This acts on the smaller piston so if the reverse force Pb multiplied by the smaller pistons area is less than this force the piston will move to the right compressing the gas.

The ratio of piston areas, which is naturally the same number as the ratio of the swept volumes, is called the drive ratio of the booster N. It sets the theoretical maximum pressure output as being N times the available drive pressure.

Let us consider one cycle of the booster:
We connect it to a supply tank that is at pressure Pin through a one way valve so that means that any time Pb falls to Pin the valve will open and allow gas in so Pb never falls below Pin. We also connect it to the target tank which is at pressure Pout through another one way valve so that means that any time Pb rises to Pout the valve will open and allow gas out so Pb will never go above Pout.
Here is my first approximation. One way valves don't open magically as soon as the pressure is 'equal'. They need a certain pressure difference to compress their springs. This is usually at least one bar and if you want small and such it can be more. I'm going to ignore that loss for now or everywhere I want to write Pin I'll need to write Pin-Pvalve loss etc.

We are going to feed drive gas into the left hand side and I'm not really bothered about the pressure available there provided it is enough, we'll discuss 'enough' later. We are going to let the gas in gently so our expensive pistons don't bang about and hence the pressure rises smoothly.

So we start with the left hand side pressure, Pa, at ambient so no force on the piston from that side but the right hand side has pressure Pb = Pin so its piston is pushed hard to the left. We start to let in some drive gas and the pressure Pa starts to rise. The first key event is when the drive gas exerts a force on its piston that equals the force from the other side. From then on the gas we let is is going to start to move the piston to the right. This will happen when the force on one piston is equal to the force on the other piston so Pa*N is equal to Pin.

Now things are moving. As drive gas comes in the piston moves progressively towards the right and the pressure in the output cylinder rises closing the inlet one way valve. Since the piston is only moving comparatively slowly we can estimate that it is in balance so Pa*N will be equal to Pb so both pressures climb in step.

Our next key event is when the pressure in the output cylinder reaches Pout and the output valve opens. Now the piston's movement just pushes gas out so the pressure doesn't rise any more. This is the point when Pa*N equals Pout and from now on the pressure will not change on either side significantly as the booster is now moving gas. Finally the pistons reach their maximum rightwards travel and they trip the reset.
Let's take stock.

We started with a quantity of gas in the output cylinder Vb at pressure Pin. Using the 'surface litres' trick that is just Voutput = Vb*Pin litres of gas.
The drive side has swept the same distance as the output but at N times the area so Va = Vb*N.
The final drive pressure was still governed by Pa*N equals Pout so we can rearrange it as Pa = Pout/N.
The volume of drive gas we have used, in 'surface litres' is Va*Pa which is, using the two previous bits
Vdrive = Vb*N * Pout/N
The N cancels out and we get
Vdrive = Vb * Pout

Before we go any further let's admit to more approximations. The first is that no pump is perfect. The pistons can't get quite flat up against the one-way valves so not all the gas gets pumped. Now this is an important matter to pump makers so they do their best to make this volume small. Looking at the output side we have some 'dead' volume so the gas we put in was really (Vb + Vdead) * Pin and the gas we got out was that less Vdead*Pout left inside. To give you a feel for the problem if the dead volume was 5% of the swept volume and we were multiplying the pressure by 3 we would get 9% less gas per stroke than we expected.
The second approximation is more subtle. We are using Boyle's law and we don't really have the 'fixed temperature' it assumes. Actually the instantaneous heating helps us at the right hand end to get more of that 'dead space' gas out but the overall heating of the whole unit reduces the throughput. Conversely the drive gas expanding and hence cooling means we need a bit more of it. However it is in the single figures of a percent and, like the rest of the laws of physics, it's unavoidable.

Let's pick the two results up and think them through.
Voutput = Vb * Pin
Vdrive = Vb * Pout

So the first one tells us that regardless of how much gas we are pumping the drive gas used per stroke depends entirely on the output pressure.
The second one tells us that the amount of gas transferred per stroke depends entirely on the input pressure.
To me the most delightful point is that the ratio of the pump just cancels out. This becomes more funny when people start to compare boosters. I remember reading a comparative review and to me the most obvious point was that the reviewer was maxing out his little 'hobby paint sprayer' drive pump. So, as you will now understand, several very different boosters all came in at virtually a dead heat in the cylinder filling time test. Yes the ratio does matter because it sets the maximum pressure you can boost to for a given drive gas and a higher ratio pump will do more cycles and hence more wear to move the same volume of gas. Mine is a 75 and, frankly knowing that I can boost to 745 bar and destroy virtually anything I own does me no good but I have it and it works and, frankly, pumping 3L rebreather cylinders I really don't care.

Let's just mention 'enough' pressure. Well obviously you have to have 'enough' drive pressure to push against Pout. If your booster is limited to 10 bar of drive, as most are, that means a 35 ratio unit will pump to well over 300 bar but if your ratio is only 20 you'll be struggling to get close to to 200bar.

And the last point. How much drive gas was that to fill a cylinder?
Well fortunately everything is roughly linear so we can play averages rather than integrals. Let's say we are filling a 10L of O2 from a 50L J with 120bar in it and we want 220bar final pressure.

Well the first bit is free. We connect up the empty and open the valves and the gas just runs straight through our one way valves until it all evens up. That is it goes from 50L * 120bar into 60L * 100bar.
However we want a total of 10L * 220bar so 2200L which is 44bar out of our J. So we are going to pump our input pressure from 100 down to 120-44=76bar. Average input is 88bar. Average output pressure goes 100 to 220 so 160bar average.
Now something tells me that the number of strokes of the output side will equal the number of strokes of the input side so I can just assume that just cancels out and I can eliminate it and Vb by dividing my two equations giving me:
Vdrive/Voutput = Pout/Pin otherwise Vdrive = Pout/Pin * Voutput
Bang in the numbers and it's Vdrive = 160/88*1200 = 2182

Not bad for the big chunky air tools type compressor pushing out 400L/min (14CFM) that I have but not quite so happy on an airbrush driver rated at 25L/min. Still, provided the compressor doesn't die horribly after being on solidly for over an hour, it will be a nice cool fill.

Gas planning
OK I'd like to whinge about how some people want to make a big issue out of something simple but take that as read and we'll do the sums.

Well there are two sorts of gas plan. There is 'I'm going on a normal dive, how long have I got?' and there is 'I'm going deco diving, I want to live'. The sums are the same but the imperatives are less on the first as if you keep watch on the SPG discovering you have done your sums wrong is merely annoying.

OK, so we all know we draw more from the tank at depth and let's just check we agree on why. Your body is dumping Carbon Dioxide into your blood as a waste product and this makes your blood more acidic. Acidic blood triggers the little 'I must breathe more' automatic system so you push more nice CO₂ free air through your lungs and wash it out. However getting the nasty stuff out of all those little alveoli takes a predictable volume of gas and if you are deeper there are more grams of gas in a lungful but you still need the requisite number of lungfuls a minute going through to stay nice and acid free. You can't cheat the system because its design assumes that breathing more is safe so breathe long and slow so the flushing works efficiently to use the least gas to keep it all happy. The fact that you push considerable amounts of oxygen through does you no extra good as if you have enough that's enough.

Right so the first thing we need to do any meaningful sums is that required number of lungfuls or, more usefully, litres a minute. I'm sorry but you have to do this test in the water. All those mammalian things about water kick in and the numbers, if measured just sitting in front of your computer of an evening, will be so wildly out it's funny. Now some people like to talk about average depth and stuff but averages are a pig to work out unless you do square dives so take a square section of a dive. One of those boring flat bottom ones will do nicely. You know the type, second dive of the day and it's on some silty 'reef ' (pile of rocks) and it's covered in 'life' (weed) but you paid for a day's diving so you're darn well doing it.

Get to the bottom and pull out your wet notes and note the depth, time and SPG reading. Mooch about until the boredom begins to hurt and if you've done enough time to be respectable log those same numbers again, blow the DSMB and get back on the boat for some tea.

So what do we discover? We were at 18 meters for 25 minutes and we used 125bar. Roughly. The other thing we need is the tank size and mine's a 12 litre.
OK 125bar at 12L is 1500L of gas breathed in 25 minutes to 60L/minute.
However we were at 18 meters so that's 2.8 bar deep (see depth in bar if this sounds strange) and we want to factor out the density change so divide by 2.8 and get 21.4L/minute.

This is your SAC ie your Surface Air Consumption. It's the number we can multiply by our depth in bar to tell us what's coming out of our tank minute by minute. You don't need it accurately. I'd put this down as 22.

Unfortunately your SAC isn't something fixed for you like your birthday it's a best guess, if you're lucky, on a good day so we need some margin on this but we'll talk about that later. Also remember that we picked a reasonably benign dive so you weren't going up and down and hitting the drysuit and BCD buttons. What we are going to do is to work out how much gas you use on a simple dive and then you can decide how much gas over the 'what you should use' you are going to take.

Simple dive: We descend to 30 meters in three minutes, we swim about for twenty minutes and then we come up (gropes for tables) no stop. OK we'll bung in a three minute safety at 6 meters.
Descent: Average depth 15m for 3 minutes, the ascent will be the same just with a break at six in the middle. So 15m is 2.5bar deep so 22SAC * 2.5bar * 3 minutes is 165L out of the tank.
Bottom time: 30m is 4 bar so 22 * 4 * 20 is 1760L
Ascent: another 165L plus a stop at 6 so 22 * 1.6 * 3 = 106
Add it all up and get 2196L. Divide by 12L and get 183bar out of the tank.

Well I don't know about you but I think that sucks. We all know that our nice warm fill was 230 bar in the shop, 215 by the time we had got home and let it cool down and is going to be more like 200 when we check the gauge at the top of the shot rope as the tank cools some more to sea temperature (if a fullish tank loses 10°C it goes down by about 7bar). A plan that means we get out with 17 bar has not got any built in life expectancy. Quick! Recut that bottom time or borrow a 15L.
Back to our 2196L and on a 15 that's 146bar. That's more like it. It might not be a third but it's a plan to get out of the water with more than 50 bar. Watch the SPG and the magic number to store in your head is the ascent usage of 165+106 = 271L is 18bar so add in a 50bar margin for error and you need to be on your way up by the time you hit 68. That's on your way not just reaching for the DSMB.

A deco plan is pretty much the same tricks to work out the volumes but now you really need to think about the 'what if I break something?' question. 'Can I do the whole dive on back-gas?' is a good thing to ask. Frankly accelerated deco gets you out sooner and feeling better but you don't need much gas hanging on a string at 6m so a fall back plan renders breaking your deco reg merely annoying. Also, if your buddy has a gas problem, it means you can stay with them which isn't a bad move.

Let's plan a deco dive. 45 meters for 25 minutes and let's plan it for air.
My planner gives me 2mins at 9m and 22mins at 6m and I'm going to ignore the descent time so it doesn't matter how fast we get down there.
45m is 5.5bar deep so 22*5.5*25 = 3025L for the bottom part.
Ascend to 9m (avg=27m=3.7bar) in about 4 mins = 22*3.7*4 = 326L
9m stop 22*1.9*2 = 84L
Ascend to 6 22*1.65*0.5=18L
6m stop 22*1.6*22=775L
Nice slow final ascent 22*1.3*2=57L
Add it all up and we have 4285L so on my twin 10s that's a notional 214bar. Good thing they are 10L/300bar twins.
I'm not hanging about at 6m for 22 minutes! That means the total ascent is longer than the bottom time. I've got a 3L of 80% and that knocks it down to 10 minutes. Well it does if I can find some more 80% for my buddy. What will I use?
22*1.6*10=352L so 117 bar.

I could now do something horribly complicated using trimix with multiple deco gases and several bailout scenarios but the sums are just the same. SAC * depth_in_bar * time, add the bits up then divide by the cylinder volume to get bar. Then look at the numbers and say 'but is it safe?'. Now I'm not going to tell you you should have a third left at the end of the dive although I think that a good plan but I do think you should start the ascent with at least double what you require and then some so if you suddenly have to stick your secondary/octo in your buddy's mouth it all still gets you both out. If you're the type that gets a bit puffy under stress allow for that as there is nothing quite like a mouth full of seawater and having to do an abrupt swim to your buddy on empty lungs when you really want to cough the stuff out to get your buddy's octo to blow your cool. As an experiment I have tried and I managed to hit 100L/min SAC as a sustained breathing rate and I don't think I could physically hoover more gas down my neck but that would make a pretty big hole in a gas plan pretty quickly. You have to make a personal call on how much extra you need but comparing your pre-dive plan with the post dive facts is a good start. If it's coming out wrong you might need to adjust that SAC rate up.

Fair and Foul All dive gear needs routine cleaning or you are going to regret it. A lot of it is just a quick hose down with tap water but some things need more attention. Compare the two DIN threads on the right. As you can guess one does up easily and the other grinds and grates.
There are two main deposits you get on dive gear: salt and chalk. Salt goes on fast as seawater deposits it and salt crystals are a nasty abrasive but a good rinse après dive sorts it out but I live in a 'hard water' area so I also get lime scale in my home plumbing and on my nicely rinsed dive gear.
So what gets off lime scale? An acid. It doesn't even need to be strong so you could use vinegar or even lemon juice but I already have some stuff on the bathroom cleaning shelf to do the inevitable streakies on my big glass shower door. This has the advantage that it is cut with a line of detergent so it rinses off well plus, I am assured, it doesn't smell like a chip shop on a Friday night (OK. I have little sense of smell so I worry about things like that). An old toothbrush works as an applicator but some little brushes sold to apply plumbers flux seem to be my tool of choice although next time I have a plumbing job to do about the house and they are all in the diving tool box I shall be annoyed.
Fair and Foul My preferred rinse after cleaning on dive gear is 'deionised water'. Look for the label for 'Car Batteries and Steam Irons'. This does not deposit a film of salts or chalk as it dries so at least things leave the cleaning bench pristine and beautiful. If I am putting them away for a bit or taking them on holiday this seems a good idea. I know it only lasts until the next time I hose things down but it's the thought that counts.
One point to note when doing kit like this is to orient things so the cleaning compounds do not go inside the thing being cleaned while you work. The DIN valve was cleaned pointing down and three, no effort applications and a rinse later it was a lot better. The picture on the left is the same fitting.

Things that go in my mouth used to get done in washing up liquid (if it works for a plate I eat my dinner from...) but these days I have stuff to clean rebreathers so I use that. I use it neat. It gets squirted on, toothbrushed or massaged about then rinsed off. I avoid the stuff that is sold to clean baby bottles and stuff like that because it may do a good job on glass and rubber but it doesn't have a good reputation on metal. If I'm worried that there is really nasty stuff there, eg: phlegm or vomit, I reach for the bleach. Bleach makes a serious mess of anything biological but then washes off nicely after it has killed everything in sight. Also, if you fail to rinse it out well you can taste that there is a problem at once.

Crack bottles
Powder I was talking on a forum, as I do, and disagreeing with people, which I do far too often, and the subject of crack bottles and water ingress came up. Now somehow the idea that shaking a thick walled aluminium vessel would let you hear all the water that had got into it sloshing about seemed pretty dubious to me but the only way to be sure was to try it and see.
This one would be easy. Take my crack bottle, pop off the end, put in various amounts of water and give it a shake, listen to it and record the results. Easy.

Pop the end off. OK when the heavy industrial strap wrench gave up on it as no contest maybe not so easy. The final solution was clamping the cylinder in a big machine vice using balsa wood padding to stop it leaving teeth marks in the paint and using a 300mm long adjustable wrench on flats of the valve. I really don't take no for an answer from things.
It had powder in it. Stuffing my endoscope into it revealed more deposits inside but it looked totally dry. Time to detour into working out what is going on here. Is it salt? Am I wrong about water not getting in? Or is it Aluminium oxide (Al₂O₃)? Is my 2005/04 stamped crack bottle eroded and corroded?
It is stamped 0.21Kgms but the scales tell me it is 0.2152 so nothing much to deduce there. So I tasted the powder and it isn't salt. I'm going with the Aluminium Oxide theory for now so it's clean time then do the water thing.

Water? Well I have good old deionised so I won't leave deposits behind when I dry it out for a reassemble. Right so 20ccs of water goes in. Screw in the valve. Shake it and you can hear it. I was wrong. Back on the forum to confess.

One last observation. If you have water in an older crack bottle like mine there is no debris tube and the threaded part of the valve does not protrude into the body of the cylinder. Fill it, point it straight down and open the valve. Gone.
I am informed that later ones, no I don't know the date, do have a debris tube so that trick won't work. However, after being stripped, cleaned and lubed as I reassembled it it works so much more smoothly.

Oh well. It needed to come to bits so I could work over the push button as it was getting stiff. I guess I shall continue to think about sending it up with the valve open but I'm going to keep doing it.

Scrubber duration
Inscribed on tablets of stone is the Mantra
  Thou hast 180 minutes
  Thou shalt be shallower 50m after 100 minutes of use
  Thou shalt be shallower than 20m after 140 minutes of use.

Well that's how it is for my Inspo. Perhaps we need to discuss it because it seems to worry some people.

Point one: I am going to assume you really do have CO₂ production rate that really does max out a scrubber in exactly 180 minutes. I know I can do longer but I would never plan on it. From the point of view of this discussion at 181 minutes you start to feel your breathing rate rise and you then proceed to die horribly and painfully. Nobody is in the slightest bit sympathetic as they all know that doing 181 minutes on a scrubber is a well deserved Darwin Award.
Point two: I am going to assume you metabolic rate is constant so your CO₂ production rate is constant. Yes, we all know that the CO₂ exhaled is dependant on your level of exertion and how cold you are but we can't factor in everything to make a general rule.

It is an approximation to say that a working scrubber can be divided into three parts, the used zone, the working zone and the unused zone. I spent quite a long time playing with the statistical mechanics and it's not quite true but it actually works well enough. I am also going to assume that I can ignore the fact that top of the working zone cannot really move back as I get shallower. I can justify this assumption mathematically provided it never reaches the top but not here.

It is a further approximation to say that the length of the used zone depends on your CO₂ production so, to all intents and purposes, it depends on your time in the water. Take this in the sense of the Point one and Point two cop-outs that I started with. In my scrubber the used area grows by one unit every minute.

The width of the working zone however depends on you depth. Why the size of the working zone depends on your depth was so wonderfully explained by Gordon Henderson in a message from the Planet Zorg that I'm not going to try and improve on that. I am going to give you the rule of thumb that the width of the working zone is the equivalent of 13 minutes of use multiplied by the ambient pressure in bar. This one is not an approximation, this one is wrong. However it is right enough to help me explain things.

Time to draw some scrubbers

So, here is a scrubber after 20 minutes at 20 meters (3bar). I've drawn it 220 units wide to represent 220 minutes of time. We'll come back to why later.
Exhaled gas comes in at the left and after 20 minutes we have used the red bit of the scrubber. Now I've drawn it flat but it will be slightly conical although scrubber designers work hard to get it as flat as possible.
The red bit is used scrubber. The pink bit is working scrubber. There is CO₂ in the gas as it enters the pink and it is progressively scrubbed and by the time you get to the white, that's the unused, pristine material, there is effectively no CO₂ left.
What is going to happen is that as your dive progresses the red bit will steadily lengthen and the width of the pink bit will expand as you go deeper and contract again as you ascend.

So what's the plan? Well obviously we want to keep the pink from reaching the top because when that happens some of the CO₂ is getting through un-scrubbed. This is called 'break through' and is universally considered a bad move. If just a small fraction of the CO₂ comes round again there is more next time round the loop to not get scrubbed and so on.

Let's consider some significant 'break through' moments

So here is a scrubber after 180 minutes total use and currently at 20 meters (3bar ambient).
The working zone is 3*13 so 40 minutes wide and is touching the top. You do not want to be here. If I am climbing the ladder and I am going to sit on my bench puffing a last hit of nearly pure O₂ I can stretch it out another few minutes but realistically you want to be out of the water. Remember that even breathing it sitting on deck the width of the pink does not go to zero.

This sets the limit for the 0 to 20 meter zone of 180 minutes.
Here is a scrubber after 140 minutes currently at 50 meters (6bar ambient).
It's the same sort of thing. The working zone is 6*13 so 80 minutes and again it is touching the top. You need to already be shallower.

This sets the limit of 140 minutes for the 20 to 50 meter zone.

Right. Last one. 80 meters (9bar ambient) and 100 minutes. Again it's on the limit.

This sets the limit of 100 minutes for the sub 50 meter zone.

Now we could draw up a big table with lots of values and even derive a formula but when you start thinking about a deco schedule then the three rules at the top are enough. Yes, if you are for example shallower than 20 meters there is a bit more life but this is not an accurate enough model to make it worth writing down the extra rules. If I said that you need to be shallower than 110 meters after 60 minutes of scrubber time it would never limit your diving because the others would limit you by insisting you get your deco into 180 minutes.

Right. Since I have written this HTML scrubber gadget let's do a 100 meter 25 minute dive. That's 183 minutes run time on 25/90 GF.
Leaving the bottom after 25mins
Arriving at the 39m stop
Arriving at the 6m last stop
Leaving the 6m stop
And after all that the limiting feature is the 180 minute scrubber.

These rules don't actually have a huge impact on a new scrubber on a single dive. Where they do kick in is where you want to do two dives on the same fill. Don't pull a 60 meter dive with 110 minutes run time, including setting up and prebreathing the thing, in the morning. Then have lunch and expect to break 50 meters in the afternoon on the same fill just because you reckon you can do it in an 60 minute run time and "that's less than three hours innit?" By the time you've set it up and prebreathed it again you're at 120 minutes and you shouldn't even be thinking about 20 minutes below 20 meters. The pink will get you.

The naming of parts

That is trim as in being horizontal in the water.
OK nobody is going to argue that holding your body dead horizontal looks cool and we can all see that it suits diving as we want to look forward and down and go forward so it is practical too. However it doesn't come naturally at first. Most people I see, the moment they stop finning, flop into head up feet down. Definitely not cool and actually making diving harder work and less comfortable.

The first snag with looking cool is that it feels very funny. You've spent your whole active life feet down head up and suddenly you are not only lying flat on your face but with your knees bent your feet are actually above your head. You feel like your feet are floating and you imagine yourself being dragged to the surface fins first.

Key Point: trim should not be hard work. No bending, twisting or working at it. This is the most comfortable way to dive so go with comfort.
My preference is to set things up in the pool:
The magic trick is a couple of quids worth of cheap supermarket polyprop washing line and your shears (a dive knife sucks at cutting rope). You want something chunky enough to tie and untie knots in with wet fingers. Get in the shallow end with some weights on the side and the full kit on, drysuit and undersuit, I tend to put the hood on then push it down round my neck so I'm not deaf and since my gloves have Velcro straps I Velcro them to my wrists. Since my gloves and my hood contribute 1.5Kgs of buoyancy this is a significant part of the deal. It's a good idea to have a couple of weights that you've measured on the kitchen scales so you know what they are. Have a big one and a little one if you have choice. Wetnotes round the deal off or, by the time you get home, you forget pertinent details.

So take a bit of washing line and tie your measured weights to your belt. First trim your weight so you are neutral with the suit inflated then allowed to dump and with nothing in the BCD/Wing. Just spend some time getting this just right.
Now try and hover horizontally just off the bottom with your knees bent so your fins are horizontal. Freeze, think cool and just hold the right amount to breath to stay there. It probably won't work. Your knees or your nose will hit the tiles. I'll put money on it being the knees. If it's your nose you need ankle weights. If you don't have any move one of your measured weight to an ankle and tie it on, and try again. If it's you knees take one or both of your measured weights in your hands and hold them to your tummy, your chest and your shoulders. There should be a place where you stay horizontal.
Now there won't be a place that makes you go horizontal because this is balance. What you are looking for is the weight distribution that means you just stay where you put yourself. Once you have this clasp the one or two weights to yourself and remember the position. Now stand up and work out where on your rig you can attach them. Get out. Rebuild things. Fudge it with bits of washing line for now and then get back in the pool. Still works? Great.

Now trundle off down the deep end. I want you to try a fin-pivot and fail. When you breathe in you discover you just rise evenly from the bottom. No pivot. Now you can play. Head stands, back floats, lie on your side, practice the old drysuit summersault where you tuck, fin and when you've gone all the way round to heads up spread out in a big star. It's all easy. Feel smug. You deserve it.

Right then. It's real wild water time. You splash into the sea, swim to the shot line, agree <down> <OK> with your buddy and suddenly you feel horribly foot light. No. This is balance. You can swim nose first down the shot and when you level out at the bottom you just stay horizontally.

OK what can go wrong? Well the biggest problem is diving overweighed. You have to bang lots of air somewhere and you put it in your suit it rushes to your boots and makes you foot light. Cure one is don't dive over weight. Cure two is put the extra buoyancy in your BCD/Wing. Now some times you have to dive a bit overweight because your twinset has 6Kgs of gas in it at the start of the dive. So don't try and do it all on the suit - do some and some with the BCD.

If you do get it wrong always remember that the short way to 'suit dump value at the top' is not forcing your head up, that won't happen, it is tuck and fin - the old dry suit somersault you learnt back in drysuit class.

PS. Stuff a couple of two foot bits of washing line in the bottom of your drysuit pockets. You will be amazed how useful a bit of 'throw away' rope can be.

Pressure and height
I was discussing pressure and height with a friend and he was having a problem because our standard scuba model of gases didn't seem to explain pressure falling with altitude. We postulated a 5km tall tank. Would the pressure be the same at the top as the bottom? I quite liked the reasoning I came up with so I thought I'd save it here so I can use it again.

OK, stop thinking about a gas as a thing in itself and start thinking the particles involved.
They zoom about at a thousand miles an hour or so, doing high speed billiard ball impressions. The 'pressure' is the force exerted on the walls of the chamber as they bounce off.

Now for a small chamber the average velocities near all the walls are as good as equal but the best way to think of a big chamber is to think about it having just one molecule in it, all on its own. It bounces off the bottom and just happens to go straight up (it could rattle off the sides but the effect is the same) it is going up with an energy of ½mv², that's the usual kinetic energy formula half mass times velocity squared. However on the way up gravity is dragging it down so by the time it gets to the top it has lost mgh, that is mass times gravity times height, worth of energy. This has to come off the term so it dings into the top with less v so less pressure.

Put more molecules in and they just bang off one another but the effect is just like the one molecule (look at the Wiki page for Newton's cradle to get an idea how motion travels 'through' bulk free particles). So a tall chamber has less pressure at the top than the bottom and, what is more, slower molecules equates to less temperature, so it's colder at the top too.

This happens whether the chamber is sealed or not. Remember molecules are dumb. They don't know you closed the lid.

If you really want the sums the average kinetic energy of a gas molecule is 3/2*kT where k is Boltzmann's constant and T is the absolute temperature but the drop in pressure comes out pretty much like the atmosphere.

Oh and useless facts, well I like them, to get down to half a bar pressure you need to ascend to 5500 meters, that's 18000ft in aeronautical speak. The highest town in the world is La Rinconada in the Peruvian Andes at 5100m so you can apparently live there and mine gold. The pressure at the top of Mount Everest at 8848 meters is 0.337 bar total of which 0.07bar is oxygen.

We tend to assume that the minimum safe ppO2 when diving is 0.16 bar but you get down to that at a mere 2500 meters. The capital cities of Ecuador, Bhutan and Colombia are higher than that and La Paz, the capital of Bolivia, is at 3640m so you're breathing less than 0.14 bar.

Let's talk about breathing because it's a problem to divers. We'd all like to breathe as little as possible from our tanks to make them last longer but most tricks we try have horrible side effects that can quite ruin our day. We need to understand what is going on and why.

Breathing does two good things for us. It puts Oxygen into our blood so it is available to all the organs as they all need it and it removes Carbon Dioxide, the waste product of being alive. While we only have to eat on a daily basis these two gases need management on a minute by minute schedule.

Let's start with oxygen. As I have said elsewhere oxygen is a nasty reactive gas and, despite the fact that we need it moment by moment to live, it is quite dangerous stuff. It is a good source of oxygen that converts those charcoal briquettes that you can never make burn on your barbie into gunpowder, the stuff Guy Fawkes tried to blow up parliament with. Our bodies need quite a bit of oxygen and have developed some quite sophisticated tricks to manage it.

Our lungs are a mass of tubes dividing and subdividing until they end up at little 'balloons' called alveoli. This is a trick to get more area because the gas transfer mechanism needs a lot of space to work. To put some numbers to this an alveoli is about 0.2mm diameter relaxed and gets bigger when it inflates (as you breathe in) and you have about 700 million of the things. This adds up to 100 to 200 square meters of gas transfer area for two lungs, about thirteen parking spaces.

Now the rules for gas transfer are simple: gases like to pass through thin membranes so there are gas molecules going both ways. However there are more molecules on the side with more gas so the average trend will be from the richer side of the membrane to the poorer side. So when we inhale a lung full of nice fresh air and on the other side of the alveoli wall is blood rather short of oxygen it starts to move across dissolving in the blood plasma. However we have a bonus trick in our Red Blood Cells in that they can hold oxygen bound to a complex molecule called haemoglobin. So as the oxygen goes into solution in the plasma the red cells promptly soak it up and the blood stays oxygen poor until they are satisfied keeping up the inwards flow. This means that blood can carry lots more oxygen than if it were just dissolved.

Blood full of oxygen, now bright red, is pumped off to service its customers and as they pull the oxygen held in solution out the plasma the red cells start dumping their load and the blood keeps all the organs working just fine. As most cells consume oxygen to metabolise sugars and such so they convert their oxygen and sugars into water and carbon-dioxide and release the energy they need. Most of this unwanted carbon-dioxide is actually reacted with water to become bicarbonate ions ⁻HCO₃ by an enzyme in the red cells, a little more is carried in solution and yet more is carried in the red cells binding to the haemoglobin. This tends to make the blood darker, often said to be blue but if it's blue you're dying, and you can buy little toys, I have one, that use LEDs to read the colour of the blood going through a fingertip and tell you how well oxygenated it is. I use mine more as a pulse checker as it only ever says 98%. Perhaps I just have OK lungs.

Now this oxygen poor carbon-dioxide rich blood is pumped back to the lungs where the CO₂ transfers out and more oxygen transfers in.

Now bicarbonate ions look like carbonic acid and the pH (the acidity measure) of your blood when it is full of carbon dioxide swings into the acid zone. We don't actually have a set of reflexes wired to blood oxygen so if you try and breathe, let us say, pure nitrogen the world just drifts quietly away with the minimum of discomfort but we do have sensors for blood acidity.

As I understand it we have a breathing reflex that is designed to keep us on track by monitoring blood pH and running our breathing rate to suit. This does rather make the assumption that if we are breathing enough to remove the CO₂ we are taking in enough oxygen but that has worked since mammals evolved so if we mess up it's our Darwin award. However since conscious control of breathing is useful, so you can blow on things, this is left to tick over in back ground and we can kid ourselves we are in control. The system is actually pretty good. Without us having to think about it exhaled 'air' normally contains a remarkably constant 5 to 6% CO₂ with our respiration rate going up and down to keep it there. Of course if the 'air' we inhale is richer in CO₂ than the expected 0.035% then the transfer isn't going to work so well and the increasing blood acidity will call for more flow to solve the problem because that is all it knows to do.

In the extreme if the blood pH starts to drift too far from acceptable limits the conscious control is clearly not working and it gets turned off. Now that breathing has become a matter of life or death it has to become non-optional. This is the CO₂ hit of the rebreather dive with a scrubber problem. The scrubber isn't taking out the CO₂ and the breathing rate as quietly increased to compensate but now the loop gets more and more CO₂ rich the CO₂ isn't leaving the blood and so the override kicks in. Breathing has become mandatory, which is a pig if you gave a reg of nice clean air in your hand but you can't stop the laboured pant-pant-pant to pull the loop out of your mouth and get in the bailout without risking a lung full of seawater. One run in with the much feared CO₂ and I decided I needed a bailout valve because I never want to do that again. It is still a problem with an OC diver. If you don't breathe enough to clear the CO₂ the reflex will strike. This is where you hear about silly breathing rates. Now the diver is locked into a fast pant that is not clearing the lungs, is going through their gas like mad and getting no benefit from it.

So what does this all tell us? Well being out of control on a dive is about as bad as it can get and being out of control of your breathing is the worst of the worst. I suppose it sums up as "don't mess with CO₂" so don't try tricks to reduce your breathing rate artificially. Do the relax, chill, slow down by all means so you produce less but don't skip breathe. Yes, we know that at depth you have vast amounts of oxygen in every breath so you don't need to breathe as fast for that but as you descend the air becomes more viscous so you aren't clearing your lungs so well so you probably need to breathe a bit more to keep the dreaded CO₂ troll under its bridge. If you want to make the most efficient use of your gas go for a good flush so breathe deep, long and slow. Get every last little bit of that evil CO₂ out. Most of all be aware of how you are breathing so you stay in control.

Incidentally one of the most perceptive comments on the subject I have read was to "let your breathing rate control how much effort you exert not the other way round." Basically stay in control.

Fins are another of those things where divers and diver's attitudes make me want to bang my head against the wall. There is no one right and true fin. No fin and no fin kick is ordained and sanctified from on high.

Theory check. The object of a fin is to push water in one direction so it pushes you in the opposite direction. However the idea fin is impossible so like a fishes tail, a ships screw or an oar we need something that at least propels some water one way while staying with you.

Let's look at some fin types:
Solid fins. Rigid, probably negatively buoyant, black. Having little or no flex these rely on the ankle acting as a trailing hinge when up down kicking so not very efficient. These come into their own if you frog kick and being rigid are the fin of choice for tricks like helicopter turns and fining backwards. Jetfins are the classic model of this type. This is the fin of choice for the fining guru and I admit some of them are very good.
Flexy fins. Not rigid so in an up down kick they bend producing a lagging blade so they push a good deal of water backwards. This more emulates fish and whales. A good fin for covering distance in but apt to kick up the silt. The long free diving fins are the ultimate form of this fin offering enormous fining power provided you can provide the leg power to drive them.
Split fins. Normally a variant of a flexy but with a slit up the centre of the blade. Again the intention is to make the shape of the fin in an up down stroke more apt to push the water backwards. Efficient but very hard to use the more complex tricks of reverse fining and tight turns. Frowned on by the purist for dumbing down fining which is probably true but not everybody needs smart tricks for a few simple dives.
Force fins and the like Hugely flexible and very small. The fin bends a lot and is designed that when fluttered up and down at the ankle they produce a very efficient propulsion. They are an acquired taste.

As I said at the beginning fins divide divers. Little will cause another diver to despise you before you open your mouth than the fins you use. Split fins label you as a tourist and merely by bringing Force fins onto the boat you have diss'ed their Jetfins. I moved from the Jetfin camp through flexies to Force fins and I now have some of each. They suit the different types of diving I do. (So now everybody can hate me).

Fin kicks:
How you kick really depends on your fins.
If you have big solids then frog kicking is really good and you can feel the punch forward as they come together. That doesn't work with flexies, especially split flexies, as they spill the big push. Flexies are designed for an up down kick and split fins only work that way.
I either use my big Gara free dive fins that want a huge scissor kick so they flex a lot and provide enormous power but they trash your legs quite rapidly or my little stubby Force Fins that want a funny little flutter kick but then motor you along with great efficiency and very little effort. If it's a camera dive I'll probably take the solids as they are the only ones I can go backwards in to frame the shot with two hands still on the camera.

As with much of diving there is no one 'right all the time' way.

What is technical diving?
Many years ago the infamous John Bantin, I think it was, published a nice little piece in Diver magazine entitled something like 'We are all Techies now'. I've searched the web but the copy has been removed or maybe I remembered the name poorly.

What this did was to ask the question 'where does recreational diving change into technical diving?' Since I find the distinction pretty arbitrary I was amused by the article but as my archive of old copies of Dive has gone the way of all recyclables I can no longer call on his headings so here are mine roughly based on my recollection but coloured by my personal prejudices. The list isn't quite ordered as that would imply that each case included all the cases above it but it's not far off.

  • Nitrox divers: Those that deviate from simple air in their cylinders. They now have to make a calculated decision as to what mix to use for a specific dive, what advantages they will gain and what risks they will take.
  • Twinset divers: Those that have more than one cylinder. I'm not really thinking of a simple pony bottle here as that is a problem management tool.
  • Decompression divers: Mandatory stops. When the 'glass ceiling' closes over your head and you can no longer just quit the dive and go for the surface then we have moved up another step.
  • Accelerated Decompression divers: Add other gases to your set so you change the mix on the fly to one more suitable for decompression. This mix may be very dangarous if you breathe it at depth.
  • Overhead environment divers: I am lumping cave and wreck penetration together here although specific cave courses are usually more detailed. 'Up' is no longer the way out so a 'silt-out' goes from annoying to deadly if you don't have a plan for it.
  • Rebreather divers: Now move to the state where you are blending the gas you breathe as you dive and you must monitor and control the process. This has big advantages but it brings new problems.
  • Trimix divers: Add Helium to reduce narcosis to allow extra depth. Factor this into your decompressuion plans.
  • Hypoxic Trimix divers: Increase the depth to the point that a safe mix at the bottom is not a safe mix on the last stages of your ascent. Now you have a gas that can kill you at all points of the dive.
  • Hypoxic Trimix Rebreather divers: Do hypoxic on your blendermatic. Never forget the a rebreather diver has to have all the open circuit techniques at his finger tips as this is his 'get out of jail free' card for when he has to bail out of a broken unit.

    So where do you draw the line? Who is a techie? Who is not? All of us or none of us? I don't see a point where the line can be drawn so the term becomes meaningless.

    Deep air
    Deep air diving has been one of those macho things in diving. It sorted, so said the self professed 'men', the men from the boys. Now I never considered that things with that sort of reputation in any field were a good idea as they all eventually caught you out and the men hospitalised themselves or worse. So why don't I like deep air?

    Well the first problem is narcosis. We are getting into the very narked region, see my other item on trimix for more details. Forty meters was my limit on OC when the nitrox stayed home and I wanted helium in the mix. Certainly even diving really deep I would not want the effective nitrogen depth much into the thirties or I'd forget to take photographs or something. This is stupid mistakes territory.

    However there is more. Professor Simon Mitchell, one of the good guys in diving, did a talk at the 2015 BSAC Diving Conference where he put up some soon to be published work by Gavin Anthony on dive outcomes plotted against breathing gas density. They were doing a study on gas viscosity and Carbon Dioxide and it makes interesting reading. Without going into details the data plotted a graph of dive objective failure that starts low sloping up gently as gas density increases, much as we would expect, and then suddenly turned up at about 6gm/litre into the 'objective mostly goes fails' region. This was plotted on experienced professionals not us amateurs. What is 6gm/L on air, and nitrox for that matter? About 40 meters.

    Strokes and Strokery
    This is getting far too serious so I feel I need to finish with description of the fine art of diving strokery. Now having been called a stroke for diving my inverted twins, for using helmet torches and for diving the 'should be sold with a shovel' Inspiration Rebreather I feel I am an authority on this.

    Stroke buoyancy and trim consists of doing an hour of stops flat on your back (trim) holding a reel and blob (buoyancy), reading a book by alternate pages as they tend to disintegrate as you turn them and looking at your computers every ten minutes or so to see how the deco's going (dive planning).

    For the working stage of the dive buoyancy and trim consists of hanging onto the wreck like grim death because we're on springs, slack lasted about four minutes but we've a new shot of 'lime in the can so it must be good for four hours. Hence we're trying to memorise a rivet pattern by feel because it might just help id this pile of scrap despite the zero vis.

    Probably solo because we lost them before we were half way down the shot in all this yoghurt.

    If you need something take two - two different brands of dive computer, one on each wrist.
    Shutdown drill - turning the DIL off when the AutoAir freeflows.
    Force Fins - your flexible friends.
    Steel Stages - to stay below your and not bob about like some demented puppy.
    Variable kit configuration - stops people messing with my gear in the water.
    Boots on cylinders - better the no rust you can't see than the obvious rust from dragging them along the floor.
    Boots on gauges - keep the grit out.
    Standard mixes - 18/40, nuff said.
    Helmet torches - don't attract my attention, I'm not interested.

    Oh well. That should have stomped on enough sensitive areas to get me lynched by virtually anybody who reads this.

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    Who am I?
    Please excuse the web tagged intro but I'm playing with Google 'Authorship' so it can look me up as genuine:
    Well my name is Nigel Hewitt, but I tend to skulk on the net under the pseudonym of nigelH.
    My personal web stuff is mostly in but you'll also find me rambling in some of the better diving web forums.
    I live at Bighton in Sussex, UK and I earn a crust working as a Designer at Combro Ltd.. We make instrumentation systems with a nice line in ballistics.
    I am to be found diving around much of the UK, mostly deep and dark but often out with my local BSAC club from the Marina here.
    I've been at this diving lark for quite a few years now and, as a working physicist, I confess that a lot of the nonsense I hear on the web gets right on my nerves. This web page is the result.