Inspiration Rebreather Training
This is a rework into HTML of an article I wrote for the
Sussex Dive Club
newsletter.
I am getting old and getting lazy too, so hauling a twinset around the
country was beginning to wear heavily on the poor old back, the damaged hip and
the broken shoulder. Then adding the problem of getting blown out on the dive I
planned and discovering the Nitrox mix I had in the tanks was then exactly
wrong for the dive now on offer settled it. I was going rebreather.
A normal scuba rig, referred to as open circuit, takes gas from the tank on
your back, feeds it to your lungs and then bubbles off the exhaled gas as
waste. It is tried and tested kit and it has converted diving from heroics to
hobby by its safety record.
However your body only requires about one gram of oxygen a minute to keep
the fires of life burning and at the surface that is about one litre.
Unfortunately we also need to flush out carbon dioxide and that takes ten to
twenty litres a minute of gas going through our lungs. That is real litres so
as we descent it is more and more gas from our tanks. At 30 meters, hence 4 bar
of pressure, we are taking forty to eighty 'surface' litres, including eight to
sixteen surface litres of oxygen if this is air, from the tank every minute
mostly to blow away a waste gas.
The rebreather, closed circuit, takes this exhaled air, sends it round a
loop and through a chemical 'scrubber'. This takes out the unwanted carbon
dioxide and lets you rebreathe the gas. Since it goes round and round you can
breathe as deeply as you want with no loss of gas and the oxygen you do not use
this time just comes round again. OK so we need to top up the oxygen we consume
or the world fades gently away but that is one surface litre a minute finning
hard.
Now add the Nitrox bonus. Since we are adding oxygen we get to decide how
much oxygen to add. We can always blend the optimum Nitrox for the depth and
for the deco so we get out of the water sooner with the best no-deco-times
available.
But you never get something for nothing so what are the snags? Well, aside
from the steep price tag, the key bug is that if the oxygen inject doesn't
happen you are in serious trouble. All dive gear needs setting up and
monitoring but knowing what you are breathing on open circuit gear involves
measuring your own Nitrox mix before you dive or just using good old
predictable air. Once you have that you are OK till something breaks. When O/C
gear breaks you know about it. The reg goes hard in your mouth or suddenly you
are surrounded by a cloud of bubbles. A rebreather fault can be far more
insidious.
There is only one answer to concerns like that and that is to do some
training. Then you know what you are talking about and you can make a more
informed judgement so I booked the five day course with International Training
in Plymouth run by Dave Crockford, a BSAC NI and, at the time, a member of the
BSAC Rebreather Working Group plus the head of TDI UK.
It didn't happen. I ended up in hospital and when I came out the course I
had booked was over and it took a while to get my UK Sports Diver Medical
back.
Then came the call that Dave was running the first two days of the course in
London for a school there and would I like to come? One day class and pool then
the next day class and a dive at Wraysbury. Would I.....?
The class stuff takes you back over diving theory and Nitrox theory and
checks out that you can put numbers to this. It also covers the history of
rebreathers which predate this new-fangled open circuit stuff by a long way.
Then we let lunch go down and headed off to the pool.
Now I first met Dave when he worked at the DDRC, the Diving Diseases
Research Centre in Plymouth, (the Plymouth pot) so I have extreme confidence in
him as a diver and he explained why the course is structured as it is. It used
to be quite short when the Inspiration was new but people did the course and
almost immediately stuffed the thing full of helium and dived to 100 meters and
did not have the experience of the rebreather to manage any problem at that
sort of depth. Hence the drills and skills are now reinforced by a requirement
for five hours in the water under instruction just rebreathering it. So this is
to be the first splash of a long run together.
We students had already set up the three units we were diving so we were
familiar with the plumbing. There is the twin hose mouthpiece with valves to
ensure the circulation is one way. The 'scrubber' containing the Sofnolime to
remove the carbon dioxide you exhale on your back and a 3 Litre cylinder of
diluent (air to us beginners) on one side and another of pure oxygen on the
other. So you can actually breathe in and out some part of the loop has to be
flexible so coming over each shoulder you have the counterlung bags.
Now the counterlungs are the things that make rebreathing so different from
normal diving. As you breathe in and out the counterlungs naturally expand and
collapse in exact opposition to your real lungs. Hence you get absolutely zero
change in buoyancy as you breathe. Now I have a quite generous 5 litres of
lung volume so I can normally be quite sloppy with the BCD/drysuit and still
do crisp depth holding, all that was about to change.
My turn came and it was into the pool. First just bounce around in the
shallows to get the feel of it and then trundle down to the deep end. As you
descend you press the injector on the left counterlung to add diluent (air) so
you can compensate for the compression of the gas in the loop. Tweak on the dry
suit and you're neutral. But you're not because your brain is still trying to
compensate by breathing in and out to stay level. It's frustrating. Trim, trim,
trim. This used to be easy!
Drills. We will do a mask clear. A mask clear? I think I can manage that. Ho
Ho. OK then. Clear the mask, start to sink, inject air to replace the loop
volume lost. Not as easy as I expected. Darn it I injected air and now the
electronics is adding oxygen to get the mix back up to the selected partial
pressure. I'm on my way up! Dump! Dump! Rats. I'm sitting on the bottom. It
looked so easy when you did it Dave.
Close the mouthpiece so you can take it out of your mouth without letting
water run into the loop and flood things. Put it back in your mouth, clear it
and re-open it. Yes and now top up the loop to replace the volume lost blowing
the water out. "Don't worry" says Dave. "It's manic at first. By the end of the
course you won't even notice you're doing it."
"Really?" asks poor little Nigel, now quite disheartened.
Then on to the real drills. There are seven ways an Inspiration can fail.
You need to understand and demonstrate managing each of them. More importantly
you need to demonstrate monitoring the equipment to know it is still working.
The problem is that with open circuit kit you know when it's gone wrong. Either
you stop breathing or you are suddenly surrounded by a cloud of bubbles. You
tend to notice things like that. A rebreather is more insidious. Most of the
ways it fails leave you still breathing. If the oxygen stops injecting then
you can swim on for five minutes before the levels in the loop are even down to
surface air levels. To know what you are breathing you must check the handsets.
The rule is check every minute. If the computers are working then you never
have to do anything but who wants to trust their life to a computer? Even if it
has a back up ready to take over?
You learn to manage failures. You learn to drive the rebreather manually if
the oxygen valves have failed and to use it as a semi-closed rebreather if the
oxygen sensors have failed. And you do it again and again because you've got
five hours to get through.
Actually this, the rebreathers greatest weakness, is its greatest strength.
If you are monitoring and something does fail you have plenty of time to
sort out a solution to the problem. You have 3L of air that you hardly use
during a dive you can call on and in semi-closed mode that goes five times as
far as open circuit and 3L of oxygen that makes a magic safety stop even if you
have some deco pending.
So what's it like? Well I went from feeling I had buoyancy totally under
control to feeling like a useless clot in the first ten minutes. Yes I ended up
where I wanted to be but with a lot of messing about. As the course progressed
I can't claim I seemed to be doing much better but I used a whole lot less gas
getting it wrong so something must have improved. The idea of doing a hours
dive and using just 30 bar from each of the two 3 litre cylinders pretty much
regardless of depth still amuses me so I suppose I must still be a rebreather
newby.
The other thing is that it is so silent. You never realise the noise you get
from an open circuit rig until you hear one chuffing up behind you. At
Wraysbury, diving with one of the DMs, he disturbed a big pike. It turned away
from him to move out into less busy waters and over three feet of fish glided
past me, two feet from my face, as if oblivious to my presence.
I'm looking forward to getting it out with my club as it is lighter and
smaller than my twinset so it will go in the RIB better. I really am resolved
to do more diving with the club as this is my third year of membership and I
always seem to be rushing off to Weymouth and the like to dive with the
uk.rec.scuba crowd and never diving
with the guys on my doorstep.
BSAC do a Rebreather
Awareness Course where you get to try both a Dräger SCR and the
Inspiration CCR. This is a recommended thing for diving with or marshalling
rebreather divers as it gives you hands on and a run down on the problems and
benefits. I can only say *DON'T DO IT*. It is far too addictive.
Back to the main Rebreather page
by Nigel Hewitt