Nigel's quick guide to Rebreather theory
Apology
Right, I know there are lots of internet guides that will make you into an
instant rebreather guru and many of them are actually quite good. However there
are some points that I feel I need to emphasise about real world rebreathers
that aren't in them. So, here is my 'Yet another rebreather theory web page'.
I'm sorry but I just had to do it.
Well what's the problem with ordinary Scuba anyway?
The problem is that people weren't really designed to go under water for long.
This was supposed to be the preserve of the whales and the fish. However lack
of design features never stopped me doing the silly things that I wanted to do
(read up on hang gliding) so let's go for it.
Right we need to breathe and we need to do it quite often. Much as I enjoy an
occasional foray into the world of breath hold diving most of the time I want
to go down and stay down. Let's put some numbers to this.
We consume about a litre of Oxygen every minute. Well most of the time it's a
bit less but that's a good number to work with. OK if we go down under the
water it gets compressed so it's less than a litre there but I'm talking about
the same amount of oxygen the whole time. Think of it as a head count of the
number of oxygen molecules our metabolism needs to keep firing on all
cylinders.
We use this oxygen in our cells and that produces many waste products but one
is a gas, Carbon Dioxide (CO₂). We need to get rid of this. We
actually produce a bit under a litre of this stuff every minute too.
Now, with gazillions of years of natural selection, we and our predecessors
have fine tuned our handling of gases so it all works pretty smoothly. The air
we breathe contains a very predictable 20.95% Oxygen and very little CO₂
like about 0.04%. We breathe in and out and our lungs exchange the gases in the
air with the blood. The blood going into the lungs is used blood so it is poor
in Oxygen and rich in CO₂ and the lungs even things out. The 'air' we
exhale has less Oxygen and lots of CO₂ while the blood leaving our lungs
has plenty of oxygen and very little CO₂. Actually we easily get enough
Oxygen and most of our breathing is to flush the CO₂ out of our lungs. If
you hold your breath that urgent 'must breathe' sensation is caused by
CO₂ building up not lack of Oxygen. If you clear the CO₂ but don't
get enough oxygen the world will just fade away quite painlessly. To remove the
CO₂ you need to breathe in and out about 15 to 25 litres per minute.
Think of it like using a big bucket of water to rinse away a bit of soap.
OK now consider a diver diving. The big snag here is that water is a lot
heavier than air and once you have descended 10 meters into the sea the weight
of the water above you is already equal to the weight of all the air above that
so, adding them together, you are now under twice the pressure. Twice the
pressure compresses the air you are breathing into half the volume. This is at
10 meters and that is just starting on a good scuba dive.
So what does that do to your lungs from a gas exchange point of view? Well The
Oxygen percentage hasn't changed so you now have twice the number of oxygen
molecules available in your lungs per breath, so that's cool, but to get out the
CO₂ you still need those 15 to 25 litres per minute going through and you
get no credit for it being more compressed as this is just here to flush stuff
out. This means we take twice as much gas from the tank to get the same volume
so we run through the gas in our cylinder faster as we go deeper.
The progression with depth just carries on. At 20 meters you have three times
the pressure and at 30 meters four times as much and so on. If you want to dive
deep you need big cylinders and what is it going on? Flushing out CO₂.
Yes it's got lots and lots of extra Oxygen in it but you still only use one
litre a minute regardless of how much is available. On standard scuba you
breathe in from the tank and when you breathe out it just bubbles away. If you
just want to dive a shallow dive or a short dive this is no problem however
divers soon begin to discover that the size of the tank they carry limits their
diving.
The Rebreather solution
So if we just need to move something through our lungs let's be a bit green
about this and recycle things. If, rather than just throwing away the exhaled
gas as bubbles, we save it in a bag and breathe it again next time. How about
that?
Well this idea has a snag. After the first few inhales and exhales the
CO₂ level in the bag is starting to rise and the CO₂ only comes out
of the blood if there is less CO₂ in the lungs. Right then filter it. Do
some smart chemistry and take out the CO₂. It was, in my school chemistry
days, 'lime water'. If you breathed out into it with a straw it went milky. Get
the stuff that reacts with the CO₂ and package it so what we exhale goes
through it so the gas we inhale is what we exhaled less the CO₂. Well
'lime water' is a saturated solution of calcium hydroxide and we can improve on
that by packing it into granules. I won't bore you with the details of the
chemistry as we just buy it in tubs. The stuff I use is sold under the brand
name Sofnolime.
Problem two is Oxygen. For something we need minute by minute to live Oxygen
actually really nasty stuff. OK you can't overdose on oxygen at normal
pressures but if you start breathing too much of it under pressure, just like a
diver could, it can mess you about a lot. However breathing a calculated high
level of oxygen is conversely quite good for divers as it helps protect us from
decompression problems. About the lowest oxygen level we want to risk is about
the equivalent of 16% oxygen in air (0.16bar), below that we risk suffocating,
and the highest oxygen exposure you want to reach is the equivalent of
breathing pure oxygen at 1.6bar (1 bar is just about the atmospheric pressure
at sea level). To try and cope with this we measure the oxygen level in the gas
are re-breathing and top it up to some preset number but no further.
Now we have our rebreather design. We make the system into a loop with flapper
valves so as we breathe the gas goes round the loop and part way round we put
it through a block of CO₂ removing stuff and then we measure the Oxygen
and top it up as necessary. Let's put it together using the APD Inspiration as
an example. I have two other rebreathers but this one is the clearest.
A Rebreather teardown
Right let's start with a mouthpiece. Pretty obviously an ordinary scuba
mouthpiece but notice that AP have picked one with a rather large 'bite' so
your teeth can't close to far. This is down to one of the snags with a
rebreather that it is driven by your inhale exhale cycle as the pump to keep it
going. Now although we can blow pretty well we are not designed to work any
continuing load with our lungs and 'suck' is something we are actually very bad
at. A standard scuba regulator has to pass tests to make sure it has a low
'work of breathing' to avoid problems from this and it has a major power source
of compressed gas to help it. Here we are just down to good design and that
means no obstructions. Getting those teeth a bit further out of the path is a
first move.
On either side of the mouthpiece, inside where you can't see them, there are
flapper valves to restrict inhale to come from the left and exhale to go to the
right. Also notice that the tube we breathe through is large again to keep any
resistance to an easy gas flow down.
One other thing we need is the ability to close off the mouthpiece if we need
to take it out of our mouth even for a moment. The last thing you need is to
let all the pipe work flood with water before you put it back in your mouth.
That, as you can imagine, is considered a bad move as the Sofnolime doesn't
like it, the electronics doesn't like it, the oxygen cells really don't like it
and when it comes back into your mouth from the inhale side you won't like it
either.
You will remember that in my first example I suggested breathing in and out of
a bag? Well naturally something has to expand and contract as you breathe in
and out so rebreathers either have one or two of them. They are termed
'counter lungs' as they inflate and deflate opposite to your lungs. Here you
can see the loop hose passing over where the diver's shoulder would be and with
a side branch on a T shaped pipe connecting into a bag all wrapped up in BCD
style material that goes behind your shoulder blades.
The counterlungs also perform the additional function of acting as water traps
for anything that gets into the loop. You can see that the flow would be
downhill into the counterlung so if you start getting gurgling noises from the
loop you roll yourself upright, close the mouthpiece off, pull it out of your
mouth and stretching it above your head give it a good shake. Most of the water
then ends up in the counterlung and you can put the mouthpiece back in your
mouth and continue the dive. You will notice on the front of the counterlung at
the bottom there is a BCD style pull-to-dump valve. If you deliberately
overfill the loop you can make this dump and get rid of any excess.
Of course even if you don't have an accidental loop flood the counterlungs trap
all the condensation, saliva and other 'stuff' and they need to be cleaned and
rinsed out with the rest of the hose work. We use good disinfectants and,
personally, I take out the scrubber and sensitive electronics and stand the
rest of the rebreather in the bath and give it a good dose of cleaner and then
hose it out with the shower head. After a week's holiday it is not a pretty
sight.
One effect of having counterlungs is that the usual diver trick of trimming
their buoyancy by controlling how much gas they hold in their lungs just
doesn't work any more. We all learn this in Scuba 101 as fin pivots and then
hovers and practice it as a rite-of passage. I sadly recall my first real water
training dive on a rebreather where, on the descent, I saw the bottom coming up
and automatically took a deep breath to bring myself to a smooth halt about a
meter above. Of course that made no difference what so ever and I smacked into
the silt in a big cloud of embarrassment. <sigh> You have to admit
that when you start rebreather training you are a beginner again. All that
super cool technical diver stuff you were so proud of has to be relearned.
To maintain reliable buoyancy control the recommendation is to run the system
at 'minimum loop volume'. This means your counterlungs are pretty much empty
when you inhale. This gives you a nice predictable point to base you weighting
and trim on and it also means that you will notice if the system starts to dump
excess gas into the loop for some reason. Now I am prepared to admit to being
naughty and fine trimming my buoyancy by slightly over inhaling and getting a
puff of extra gas from the automatic injection valve and by exhaling through my
nose to bubble off just a fraction but this works a treat when I've got a
camera occupying my hands.
Now we come to the 'Scrubber', the chemical CO₂ absorber. There are super
expensive, super efficient ones, like those used in a space suit (yes, a space
suit is a rebreather), but this stuff that we use that is manufactured
primarily for medical anaesthesiology (yup, that's a rebreather too) and is a
bit cheaper.
The scrubber material is supplied as slightly porous granules and we put it in
a container between two pieces of mesh, known as scrims, that let the gases
through but keep the granules and any dust in. My ex-military unit uses a metal
mesh but most others use a gauze like scrims supported by a grid. The commonly
accepted wisdom is that 2.5Kgs of scrubber granules will last 3 hours. Under
most circumstances you can go quite a lot longer but, as death by CO₂
poisening is a pretty horrible way to go, we tend to assume that three hours is
the top limit and I'd only go past that if I have a buddy on emergency OC
bailout who needs monitoring.
Scrubber design? There is a lot of voodoo talked about this. Obviously you
don't want a design where there are a lot of different path lengths through the
material because once the shortest path has started to leak CO₂ through
then its time is over and anything left is waste. Hence the two main players
are a radial design, where gas goes in in the middle of a cylinder of granules
and comes out at the periphery or an axial design where it goes in at one end
and comes out at the other. I can't see much to choose between them although
I'd personally go with an axial because I prefer the scrim design. The
Inspiration is an axial with the exhaled gas fed down to the bottom of the
canister so it then goes up through the granules into the head.
One of the important considerations however is heat. Scrubbers generate heat as
they absorb CO₂ so they run hot and they run more efficiently when hot.
Hence one of the design features of a good scrubber is that it is insulated.
The APD one in the pictures is insulated by a narrow air gap just like in your
double glazing. The inner canister, shown on the right is inside the outer
canister shown on the left. If the scrubber material is allowed to run cold it
will be less efficient and hence the distance gas moves through the scrubber
before it is fully scrubbed will increase. This means that this zone will reach
the top of the scrubber before that in a well insulated scrubber so you get
less life and the life is less predictable.
You notice the cable coming off the top of the scrubber canister in the
picture? This is a trick to use a set of temperature sensors down the middle of
the scrubber to detect the heat generated by the reaction to sense where the
reaction is taking place. As this point moves through the scrubber body it
gives you a feel for how much of the scrubber has been consumed. This doesn't
tell you much other than where the scrubber is working but, although it's not
perfect, it's a good trick so many rebreathers have them.
You can also get gauges that measure the CO₂ level in the head but this
seems a bit pointless to me. Yes, they can tell you that the dreaded 'break
through' has occurred but it's too late by then. Now if I had lots of them
distributed through the scrubber so I got some advanced warning that would be
much better. Having had one run in with breakthrough and CO₂ I have added
it to my paranoia list and when I check the handset to see if the oxygen level
is correct, see the next section, I also check my breathing rate. That is my
CO₂ sensor.
So now to the head. Now you don't have to design a rebreather with a specific
head area but the Inspiration provides me with a single item to cover the last
parts of the generalised design. The head here is the lid of the scrubber
canister and I have removed the protective surround to make the works more
visible.
The large pipe in the middle is the next section of the loop that goes, via the
inhale counterlung connection, to the inhale side of the mouthpiece. Around it
are positioned the three galvanic oxygen cells with their inputs facing into
the inhale pipe work. The box to the bottom right is the sealed battery
cases, sealed as batteries can emit interesting gases. Right of center is the
solenoid operated oxygen injection valve.
Not shown anywhere are manual valves to inject oxygen and 'the other' gas known
as Diluent and some sort of control and display pad.
I ought to put in a word here about the galvanic cells here. Basically they are
a battery with one of the chemicals needed to be an effective battery left out.
That missing chemical is oxygen. When we apply a gas containing oxygen to the
open end of the cell it starts to generate a current. The battery is wired into
what is virtually a short circuit so we can just measure the current produced
by converting it into a low voltage and then more oxygen, either a higher
percentage or a higher pressure, means more current means more voltage. Notice
however that we are not measuring the voltage generated by the cell, that is
about 2 volts and is set by its chemistry. What we are interested in is the
maximum current that the cell can give which depends on how much fuel there is.
Well for new cells that's it. More oxygen is more current so more voltage at
the terminals. However as the cell begins to age some of the other chemicals in
it begin to run out and the limiting factor ceases to be oxygen but them. This
means the cell under-reads at high levels. The manufacturer sells them to last
for a year. At the end of the year they should still be working just fine but
nobody wants to guess how much longer they will go on for. I change the cells
in my rebreather every year and the cells in my oxygen analyser when they fail.
So what happens? Well the main purpose of this bit is to maintain the desired
oxygen levels for continued diver life expectancy. We tend to set the Oxygen
level to something like 1.3bar so at 20 meters (3bar presure) we are breathing
43% oxygen and at 50 meters (6 bar) 22% so Nitrox divers can immediately see we
are pretty much on the optimum gas for depth but, as we change depth, it
reoptimises. Naturally if we are at shallow depths or on the surface 1.3bar is
not possible so we can reduce it. Most system I have seen have two values,
called set-points, and you can toggle manually or automatically between them. I
use 0.7bar and 1.3bar and I think that's pretty common.
So what's the diving procedure? Well at some point you need to calibrate the
cells. This can be done with air, 20.95% less water vapour, or pure oxygen.
When you calibrate is a bit system dependant. It might be part of your routine
start-up sequence or part of a maintenance procedure. We need it because the
voltage a cell produces for a given pressure of oxygen should be consistant for
that cell but may vary from cell to cell.
Then, when you switch on and start to breathe the system, the it adds oxygen
to bring things up to the lower set point. As the diver continues to breathe
the gas circulates and becomes more even and the scrubber starts to scrub and
heat up. Please notice that the scrubber does not have to be hot to work just
to work efficiently. It will not leak CO₂ just because it is cold just
that CO₂ will get further through the 'lime before it is scrubbed. This
is only a problem when the scrubber is nearly exhausted and 'break through'
occurs earlier than would be expected.
The diver now enters the water and starts to descend. As the gas in the system
compresses the counterlungs become too small to sustain breathing so the diver
either manually or automatically adds more gas from the diluent supply to the
loop and the control system injects more oxygen to keep on target. At some
point they will switch from the low setpoint to the high setpoint. I tend to do
this at depth because otherwise I might find myself breathing too high a
percentage oxygen to descend.
"Always know your ppO2" is the magic incantation. Look at that display panel at
least every minute. Be good and paranoid, it will serve you well. On a fully
electronic CCR you don't have to do anything until it breaks but you need to
know it broke. The one rebreather fatality I knew personally could have been
saved by this simple rule. He re-entered the water to do a simple piece of kit
recovery after he has switched the rebreather off. If he had just looked at the
blank handset once he would be here today. My current rebreather will auto
switch on and go beeping mad if I try that.
The dive progresses and when the diver comes to ascend they just ascend and
make any required decompression stops. If the stops are calculated for a CCR
the times will take account of the rising oxygen levels as they get shallower
and shorten appropriately.
Well that's it. That's my take on rebreathers. Now I'll be the first to admit
that is is not a complete document by any means. It is not a 'teach yourself
rebreather diving' guide as there are masses of things you need to do on a CCR
course that I haven't even touched on here but this should give you a pretty
full featured overview of what a rebreather is, how it works and, more
particularly, what some of the pitfalls are.
