Nigel's twinset rig

AJ on conventional BCD Yes I do have a standard BCD (Scubapro Club) which carries a single 12L 232bar tank equipped with a Scubapro Mk20 and M380 regulator/octopus combination. I like it and for a lot of the diving that I do, eg: warm water holidays, it is good competent kit. The equipment set-up I describe here is my twinset rig with all sorts of personal reasons for things.

I don't use it with PADI students when I'm out with my local club as they don't need their lives complicated when first learning things. They see me just as they are, doing just what they are doing and trying, I hope, to do it better so I am a good example (Role Model in PADIspeak but anybody who uses me for a role model is in serious trouble). They might get to see this kit later if we do a nice wreck on a spring day in the cold, old English Channel but that's another matter.

Diving is a personal thing and there is a vast range of expertise to draw from to get things right for you. Now I am not going to say there is one right way as some might because the only real rule of scuba must be "Everybody comes back OK". How you realise that rule is not a matter of hard and fast application of holy writ, more of matching the equipment to the task in hand and maybe matching the task in hand to the equipment available. Even if everything goes pear shaped the object is to place you and your buddy back on land saying "That was a cr*p dive".

This is my synergy of what I have learned. I put my money out to build a scuba rig that suited my diving and, as I have been asked to write it up, this is the write up.

(Brief explanation of why twinsets.)


Safety stop If the dive isn't safe don't dive.
If the dive isn't fun don't dive.

I am a recreational diver and I'm not going to waste my money and time on a dive that is not going to give me value for money in the entertainment stakes. I would far rather sit on the boat and watch the clouds roll by than do a dive that I did not consider safe or was not fun for me. So what are my rules?

Keep warm.
Is this an odd one to put first? I don't think so. I dive mostly in the English Channel for the simple reason it starts about a quarter of a mile south of where I live. This is never warm water and I rapidly loose interest, enjoyment and, more seriously, concentration when I get cold. If I am not going to stay warm the dive is a non-starter. If my concentration goes then all my other rules will go with it so stay warm has got to be first.
I had a Hydrotech custom membrane dry suit and I wear a lot of good insulation under it, 100gm insulating suit and now I have a made to measure Otter Britanic. If I get cold, if I get flooded, we call the dive.

Know your buddy
Well perhaps trust your buddy would be a better phrase. If the dive starts with an introduction that means either I am the student or they are the student and we both know what that means. Dive with people you know (as divers) and trust (as divers). I have better things to do with my life than throw it away. Everybody I dive with seems quite happy that either of us can call *END* at any time for NO reason. If you want to call the dive that is quite all right by me. I don't ask or need to know why. You don't need an excuse. On your signal I will go into up and out mode. Please give me the same courtesy.

Version one Nürburgring on the road bike OK admit it. This was the bit you really wanted wasn't it? What's he got? Well the first thing he's got is a history of serial motorcycle crashing. My arms don't quite bend the way they should but if you want to track race motorcycles for years that's just one of those things you have to get used too. I don't do it now as the damage finally added up too far. This is my problem so I cater for it. Oh, and before you ask, yes, it was worth it.

I have to add to that a few other medical problems but I tend to assume that as long as a proper hyperbaric doctor will sign me a UK sports diver medical form and I can still climb the ladder back onto the boat with a twinset and stages in a rough sea then I'm going diving - although I admit that the two heart attacks did worry me a bit at the time.

Briefly the rig is based on a manifolded, isolatable, twin 10 litre, oxygen clean, 300 bar, DIN valved steel tank set. This is mounted inverted on a custom back-plate with a Dive-Rite Classic wing and two independent Apeks TX50 regulator sets.
The key word is inverted. The tanks are set valves downward - look at the picture and understand. The problem is that valves behind my shoulders are inaccessible to me (well without contortions I think they are inaccessible to most people) but behind my bottom I can reach them just fine.

Royal Engineers at Horsea Island I am told the military used to use this position - the picture probably dates from the late nineties. They took it further than I would though. As I understand it the drill ran like this: No contents gauge but start with both tanks full and one isolated. Breathe from one tank until the increasing resistance tells you it is empty. Open the valve and equalise the two tanks then shut the value off again. If you started at 200bar you are now at 100bar in both. Again breathe it out and finally open the valve and this time leave it open and ascend. You have 50bar across both the tanks, one quarter of the initial supply. Why do this? Because in zero vis you can't see a gauge and zero vis is not a reason not to dive. Could they do this if the valves were behind their shoulder-blades? Some people might be able to but in the inverted configuration I can reach both the valves that feed the regulators and right round to the isolator in the middle if I need too.
These days, I am advised, the military use surface supply, ropes and comms. I like surface supply, ropes and comms but I can't afford to pay for the team to go diving with me.

UKRS backplate OK what tricks are needed for this system? The back plate is laser cut marine grade stainless steel. It is my modification of a drawing supplied to me by Chris Stephenson via the uk.rec.scuba newsgroup and it was originally drawn by Roger Lacasse whom I have never met but thank. I changed the shape and the hole configuration and added slots so I can add cam bands for single tanks or at least my Autocad literate brother did. Somehow I seemed to end up copying it for the rest of the group. If you want the drawings they are here.
The tanks are on an MDE isolatable live manifolded and banded with bolts that pass through the wing and wing-nut to the backplate. The backplate is threaded with a Hogarthian style harness. There is a Custom Divers dual valve protector over the first stages. The rig can stand on this without weight going onto the valve set. Sit, straps, stand. Frankly it is easier than the usual lift and wiggle with the arms that a BCD normally requires.

Is it easy to set up? Look at the pictures. Lay the tanks down with the bolts pointing up. The picture shows the Custom Divers tank protectors (Bull bars) on which the rig stands while you put it on. I also use screw in DIN valve protectors to keep the threads clean but I remove them totally and put them in the toolbox once the regs are fitted as they will either get lost, get entangled in something or, more likely, end up under the Bull Bars when I sit back down and get trashed. 1
Fit the regs and route the hoses. On the left (near side) is the primary reg, a longer than normal wing inflator running up between the tanks and a Scubapro SPG and depth gauge console. On the right the secondary reg feeds up between the tanks while the drysuit inflator and a second SPG are clipped together. The two SPGs end up clipped to the chest D-rings so they are quick and easy to see but the drysuit hose is only there to stop it getting lost and is unclipped as I put the rig on and I fit it to the Apeks swivelling injector. 2
Lay on the wing ensuring the two hoses are safely in the gap between the tanks so the backplate will trap them without pinching them. The DiveRite Classic, purchased without the harness, is very simple and tough as old boots. It has a fabric panel virtually the width of the backplate so it is unsuitable for a single cylinder rig but pretty much ideal for the twinset. I have kept the pull to dump that is hated by the purists as it is, I feel, a useful feature. The inflator will be passed through a bungie loop on the left chest D-ring to keep it in an obvious grab zone. 3
Then the backplate is fitted which is always rigged with the harness. Again I depart from the pure Hogarthian rig by fitting a pinch clip to the crotch strap. This makes dekitting to get back into a RIB much easier as I can let go the clip and hand off my weight belt more easily. Finally fit and gently hand tighten the wing nuts and stand the set up on the valve protector and put it on. I can now turn on the gas, check the isolator is open about three quarters of a turn. 4
Since some of these pictures were taken I have changed the isolator valve to point straight back away from me. It is easier to reach the slightly longer distance round the pillars than to slide a gloved hand with a computer on the wrist etc. between my back and the first stages.

Conventional twinsets Both the regs have longish (1.5m) hoses. The primary one loops forward from the first stage behind my left hip, up my left side, back over my left shoulder, round the back of my neck and over my right shoulder to my mouth. This is copied from the Hogarthian system and this is the reg to donate to an OOA (out of air) buddy. It is not as long as the classic 7 foot DIR hose which, starting behind your shoulder, allows a buddy to precede you through confined tunnel to do a shared air exit but I am not building a cave rig here. Wrecks may be constricted but it is by bulkheads, the passageways are not the problem. In fact in the few wrecks that are intact enough to enter the problem is normally visibility not confined space and, anyway, the wrecks that really interest me are not intact enough to enter in that sense. At five feet it is enough to save you from completely being tied to one another's face as a normal octopus does so you can deploy the blob and get up and out conveniently.

You could fabricate a cave rig with a nine foot (3m) hose to use with inverted tanks but that would rapidly become an entanglement hazard. Since I am not setting up for a passage-way exit I follow the maxim of *if you don't need it then don't take it*.
I point out in my buddy check that in an out-of-air situation they should just grab the reg in my mouth and that I read an open hand coming towards my face as a clear I am OOA signal. This gives us a nice long hose between us which could be handy if we need to deploy a dSMB or get ourselves out of somewhere that suddenly seems a bit tighter now we have to be tied together.

The secondary reg feeds from the right hip first stage up between the tanks and over my right shoulder to hang on a necklace and act as a reserve to me. The wing inflates from the same first stage as the primary and its custom length hose runs up between the tanks to the normal Dive-Rite inflator. The dry suit hose comes up round my right hip to an Apeks swivelable valve on the Hydrotech dry suit. Add a small Scubapro 400bar contents gauge on the right and a combined depth and contents gauge to the left and you have my breathing gas system.

Conventional twinsets The harness and the backplate are, frankly, a straight copy of the Hogarthian system including the crotch strap except that I added a a pinch clip in the crotch strap to try and make the dekit operation for RIB diving more direct. To make things clear the weight belt is in black and the harness in yellow and red stripes so a buddy can tell them apart. Also one clip is left handed and the other is right handed so I can tell them apart blind.
The valves are now 8 inches back from my hips. The isolator is half way between them. Put your hands behind your back and imagine them. Easy isn't it?

I like to run Nitrox at 30 to 40% because I am not a deep deco diver. I do not find scuba in the dark fun unless we have deliberately planned a night dive to see the night life on a tropical reef. The max depth penalty does not trouble me but my boredom threshold means that any dive with a lot of deco might have been fun for a while but ends up ruined.

The rest of the kit is pretty standard. I carry a Buddy self-inflating dSMB more as a surface location device than a deco stop tool but always keep the small Mallon reel clipped to it anyhow. I have a real bull-horn of a whistle on the wing inflator blown by the back gas not my little lungs. Torch, back-up torch, optional strobe and the Sea and Sea MX10 camera/YS70A strobe. Oh yes, and the ubiquitous O-Three p-valve.

There are times I wonder how I can walk in this lot let alone climb back into a hard boat but it works well. Remember the tanks alone and empty are 33Kg and in full kit, but without the weight belt, I weigh in at 125Kgs of which only 72Kgs is me. It's not much fun in a RIB. Not surprisingly people seem to leave it in the water while I get into the boat so I can help drag it aboard.

What's it like?
Off Weymouth, with deco mix and DSMB It's heavy. Maybe the 300 bar 10's were a mistake as I have 33Kgs of steel on my back to give me the gas supply of twin 12s at 230bar. (Remember that due to the Van de Waals forces you only get 20% more gas going to 300 bar from 230 rather than the 30% you expected on the ideal gas laws.) However what it does do is reduce my weight belt so overall I am lighter than twin 12s as I displace 4 litres less water. Good news on a hard boat climbing the ladder but harder on a RIB as you hand off less weight belt.

The wing is very comfortable and I much prefer not to be squeezed when I increase the buoyancy. It offers 22Kgs of lift to cope with a dry suit flood where I would loose about 6Kgs of lift.

The Hogarthian harness is first rate. The backplate sits low so the straps rise several inches up your back before going over your shoulders so when the crotch strap is not fastened the shoulder straps are large lose loops and are easy to get into (remember I have dodgy shoulders) but once the crotch strap is engaged the whole rig is firm without being tight and the fit is totally independent of the amount of buoyancy I am carrying.

I weight myself to be neutral just below the surface virtually on empty as I can breathe the twins down to 30 bar and still know that I have 20 minutes at 5 meters reserve. That works out as 3Kgs in fresh water and 6Kgs in the sea in a pocketed belt that I know I can remove weights from in the pool while wearing it although I have never tried this in the open sea.

This means at the start of the dive, loaded with 6.5Kgs of gas, I carry air in the wing to maintain neutral buoyancy. I do not overfill the drysuit as a buoyancy control. It may work for some people but I prefer that my suit inflation is about warmth and squeeze and only if I have a problem is it a buoyancy control. The wing is first rate and the air bubble does not migrate to my feet if I am nose down with my camera lens in a hole leaving me hanging upside down while everybody else struggles to keep their reg in their mouth laughing at me.

Easy... ...Peasy The long hose rig and donating the primary is simplicity itself. An OOA buddy grabs and even when they jump me on a drill I've been diving long enough to be annoyed rather than panicked. You just duck and the hose goes free and they can breathe and get their act back together on a reg that they know is working while I have the secondary just below my chin with the adjustments wound up tight to stop it free flowing and I tested it in my pre-dive checks. If it has a problem I have not just done an OOA fast swim so I can take time to fix it or at least survive while my buddy gets enough of my gas into him to be able to buddy breathe with some self control.

A key trick I learned racing a motorbike was to have preplanned responses to problems so you just do a right thing at once rather than running out of time deciding between two. It saved me a lot of pain and, according to reports of the last accident that finished my racing, saved my life. I apply this to my diving and practice practice practice my shutdown and air sharing drills - it passes a boring deco stop. Also if you can do them and hold stop depth shallow then you will have no problem at depth.

Finally the point of it all. The valve assemblies are wonderfully accessible. I can reach the isolation valve and close it in ten seconds worst case and even with a drill sprung on me by a buddy I was looking at through the camera view-finder I have actually never done it in less than five. With the manifold closed and half my back gas safe regardless of the fault I can close the appropriate side pillar valve and start to try and figure the problem and, hopefully, if it is a first or second stage problem salvage a lot of the gas on the side with a problem.

This rig is not for everybody as it addresses my sort of diving and my sort of problems but dive gear should be thought through. If I was doing it again I'd probably try twin 12s at 230 bar although the added length might move the valves down further and offset the gain in having less weight.

The final finally point has to be the fact that I have now progressed another stage further and been seduced by the yellow side. Yes. I confess to an Inspiration rebreather and it has progressively taken over all my diving. So the inverted 10s have now become inverted 3s.

Keith Lawrence (BSAC DL etc.) for originally explaining the concepts of wings and isolatable systems to me,
Margarett Stally at Newhaven Scuba Center for guidance and help and for locating the equipment I needed,
Robin Hewitt for custom metalwork,
Adrian Hewitt for putting up with all the development work and
the whole uk.rec.scuba crew for suggestions and advice.


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