(by Arnold Talats)
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here
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As
a result of a phone call a few weeks ago I am at Stoney Cove on a sunny
Wednesday morning to have a supervised dive with an Inspiration rebreather unit.
Clive Bowen arrives with Dave
‘Rebreather’ Simmons in a Suzuki van, which is well loaded with all manner
of dive equipment and an Inspiration rebreather unit.
He manages to park right behind
the ‘Bus Stop’ and after we exchange greetings Dave gives me the news that I
will be the first one into the water today with the rebreather and Clive second.
Dave pulls the rebreather to the back of the van and immediately launches into
how to switch on the two handsets, which display what you are breathing as a
PPO² figure from three independent oxygen sensors in the CO² scrubber unit
whilst you are diving.
One handset is the master and
the other is a slave, which takes over if the master handset malfunctions, when
you are ready to dive they display the O² content of what you are breathing as
three sets of numbers per handset.
The readings drop as you breathe
and increase to the set value of 0.7 bar when O² is injected automatically into
the breathing loop, these should be checked every minute or so when you are
diving and all the readings should be constantly changing but remaining near a
0.7bar setting.
The Inspiration can have two PPO²
settings, 0.7bar or 1.3bar with the former being used at a relatively shallow
depth and the latter setting being switched to at a greater depth and back again
as you ascend to a shallow depth, as today is a gentle introduction to the art
of rebreather diving we shall be using only the 0.7bar setting for relatively
shallow diving.
Unlike Nitrox the O² content is
optimized by the rebreather according to the depth you are at, so at three
metres depth you are breathing 53% oxygen, 43% at six metres and 36% O² at nine
metres on the 0.7bar setting.
Enough of the PPO’s, part of
the startup procedure involved calibrating the three O² sensors with pure
oxygen from a three litre cylinder on the right hand side of the back mounted
scrubber unit, this is added automatically by the computer as part of the start
up routine.
The handsets display all the
procedures you are to follow and they are very clear and logical and even prompt
you to turn on the correct cylinder valves in sequence.
I still read everything very
carefully twice before I push any switches. On the left hand side of the
scrubber unit is another three litre cylinder which is filled with air as a
diluent, both of these cylinder contents can be manually injected into the
breathing loop by a button on the two counter lung bags which are mounted on
straps on either side of your chest.
These bags are where your breath
goes after being exhaled, into the right hand one first and after it has been
through the scrubber on your back into the left hand one.
The air injection button is on
the left hand bag and O² injection button is on the right hand bag, but I am to
use only the air injection button on my dive. This is to avoid any chance of
filling the breathing loop with pure O², which even at a relatively shallow
depth would lead to convulsions and unconsciousness underwater or even worse.
I am also shown that in the
unlikely event of if it all going pear shaped how to close the Inspiration
mouthpiece and go over to a conventional open circuit bail out regulator using
the Buddy Auto Air 100 wing inflator on my left hand side.
I normally dive twin 12 litre
cylinders and need ten kilos around my waist for correct buoyancy control,
because a rebreather is quite a bit lighter and carries three kiIos of lead
itself I have added another couple of kilos to my weight harness.
I’m all ready to go now and
with a final adjustment of the harness we all make our way to the jetty with
some inquisitive looks from the other divers. Dave goes in first and I follow on
his command, I check the handset readings on the surface and as all is well I
attempt to descend and then discover that I need more weight to go down.
Dave loads me with another two
kilos of lead and I slowly descend to the bottom of the six-metre level.
I try to take a breath but I am
stopped halfway because there is not enough air in the counter lungs for a full
breath, a couple of short puffs of air into the counter lung and I am breathing
to full capacity. Strangely you have to put quite some more effort into
breathing than on open circuit but this might be due to the hurricane
on demand from my open circuit Poseidon Jetstream regulators.
A few puffs of air are added to my drysuit and I get almost neutrally buoyant and I swim off without any regulator noise, just a quiet whooshing sound as I breathe. My mask has a small amount of water in it so I clear it as always by exhaling slightly through my nose, as the water is forced out with a few bubbles following I lose buoyancy in a big way and drop heavily onto the bottom. From habit I try fully inflating my lungs to lift off the bottom but nothing happens. To achieve neutral buoyancy I have to replace the air lost into the counter lung and breathing loop using the left hand inflator button. Too much is added and I begin to rise, from habit I exhale fully but I’m still going up, to descend and regain neutral buoyancy I have to take another breath and exhale some of the excess air in the breathing loop out through the mask through my nose. I then remember Dave’s advice that the volume in the breathing loop has no affect on buoyancy so long as none escapes or you change depth, it does not matter if you hold your breath or exhale fully. If the volume in the loop reduces you will sink, if it increases you will rise. Add a suit inflator and dump valve as well as a wing inflator and dump valve into this equation and you should begin to appreciate some of the task loading involved, it is like you are starting to learn to dive all over again. There then follows a bit of a minor ‘Yo-Yo’ scenario until I get things under a semblance of control. Dave waits patiently for me to get it together and we then practice the swapping of the rebreather mouthpiece for the open circuit one on my left. I shut the rebreather mouthpiece and switch to the backup regulator smoothly, unfortunately I did not fully close the rebreather mouthpiece and some water has entered the breathing loop. An Inspiration rebreather can tolerate a large amount of water in it and still function safely so I swap back to the rebreather mouthpiece, apart from a slight gurgle in the hose all is well with the unit and the handset readings. We then have a nearly silent tour (only slight gurgling) of the six-metre ledge and then Dave signals to head down the road. As we descend pressure increases on the counter lung reducing volume and buoyancy in the breathing loop and I can no longer take a full breath, suit volume and buoyancy has also reduced and as I am reaching for the suit inflator button on my chest as I lose buoyancy again and drop onto the road surface. Air is added to the breathing loop first and then to the suit and buoyancy is regained once more, this needs more thought than normal open circuit but I seem to be getting the hang of it now. We turn left at twelve metres and instead of going around a rock outcrop I head over it, this results in having to vent from the loop above the outcrop to prevent a rapid ascent and then having to replace what I have vented when I go down the other side. The golden rule here is if you can swim round it rather than over it you will conserve your gas and your task loading. There is an awful lot to think about on your first rebreather dive. We continue along the wall at twelve metres and I am now really enjoying the sensation of a throat that is not dry or cold and the quiet. I relax my grip on the mouthpiece a little too much at this point and you’ve guessed it some bubbles escape past my lips, I’m heading down slowly but I replace it with a push my counter lung button without too much thought now and I’m back to were I was. Dave signals to head back and we start to ascend up the road my suit starts to expand at the same time as the counter lungs. So I’m doing a teapot impression with my left arm to vent the suit from the shoulder dump and breathing out through my nose on almost every breath to vent the excess from the counter lungs to control the ascent rate. I find that if I take an almost full breath and then can inhale a little more I have too much in the counter lungs so I exhale that breath through my nose entirely. You repeat the process if you can do the same, if not breathe normally for a breath or two and check again, I’m also checking the ascent rate on my dive computer and keeping an eye where Dave is. Yes, it can get very busy ascending on a rebreather. Back at the six-metre level we level out and mooch (a technical term for a gentle exploratory dive) around enjoying the soft gurgling sound, watching and trying not to get in the way of the many trainees present. Before I know it my thirty-minute dive is over and we climb out at the slipway to more inquisitive looks from other divers. The weight difference between a twin 12 litre set up and rebreather becomes very apparent on the exit with the rebreather feeling possibly only half the weight. I would say to anyone who has enough experience to try this piece of advanced diving equipment out and enjoy the quiet and the new skills you have to master. I enjoyed it immensely and I have learnt a little of what it takes to dive with a rebreather. After a generous surface interval I dived on my twin 12’s on open circuit Nitrox 30 and I must say they completely fill the bill for me at this present time. But who knows what the future might hold.
Arnold