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What is 'Closed Circuit Rebreather' diving? |
When AP Valves' Buddy Inspiration closed-circuit rebreather (CCR) was
introduced commercially to the dive community in 1997, many
professionals within the industry hailed it as possibly the greatest
advance in scuba-diving since Jacques Cousteau introduced the aqualung.
When AP Valves' Buddy Inspiration closed-circuit rebreather (CCR) was
introduced commercially to the dive community in 1997, many
professionals within the industry hailed it as possibly the greatest
advance in scuba-diving since Jacques Cousteau introduced the aqualung.
Since then, a number of people have suffered fatal accidents while
diving with rebreathers, including the Inspiration, the hi-tech CIS
Lunar and even relatively simple semi-closed circuit rebreathers. While
we know of no accident that has been attributed directly to the
equipment itself - as opposed to way in which it was used - its
presence does raise a serious question: "Are you right for it?"
The CCR is not merely for a small minority who want to go deep. The
greatest advantages are to be seen in the normal European recreational
diving range of 20-30m/60-130ft.
The device varies and delivers to the diver the optimum mix of nitrox
at any given depth. This means that the diver absorbs the minimum
amount of nitrogen and breathes reduced amounts of the gas on ascent.
This reduces dramatically the decompression requirements over normal
open-circuit (OC) scuba or semi-closed circuit rebreathers (SCR).
No-stop time at 18m/60ft for a diver using a CCR (with an oxygen
pressure set constant at typically 1.3 bar) is more than three hours.
No-stop time at 18m/60ft for an OC diver using air is 55 minutes,
according to traditional air tables and most modern dive computers
.
At 52m/170ft, the CCR diver breathes the equivalent of air, but this
mix progressively richens as he comes shallower, speeding
decompression. OC scuba divers sometimes have trouble getting
their minds around the fact that CCR divers use less oxygen as they go
deeper. In fact gas supplies cease to be a problem. The limiting factor
is CNS oxygen toxicity and CO2 scrubber life - effectively around three
hours. Those who use OC scuba can have useful dives to
50m/165ft if they take enough breathing gas with them, and can
shorten decompression penalties by breathing progressively richer mixes
as they ascend. For example, they might take nitrox 21 (air) for use at
the greatest depths, plus additional tanks of, say, nitrox 32 for use
between 18m/60ft and 33m/110ft and nitrox 50 to use shallower than
18m. This, however, means taking extra cylinders and different
regulators from which to breathe. There is always the risk that if a
regulator is not clearly distinguishable, the wrong one might be used
at too great a depth - with possibly fatal results. And there is
always the possibility of running out of gas, something that should
never happen with a CCR.
The Inspiration user's supply at 30m/100ft is equivalent to that
of an OC diver with more than twenty 12 litre cylinders. So if he
starts with 200 bar of O2 in his 3 litre supply, he has more than
enough for a few days continuous diving!
SCRs, such as the Drager units, use a constant flow of a fixed nitrox
mix into the breathing loop. They offer a slight saving in gas
requirements over an OC rig (reflected in the smaller nitrox cylinder
needed) but offer no decompression advantage. The flow-rate is
the same regardless of work-rate and excess gas is allowed to bubble
off, although in reduced amounts compared to OC scuba. Some would
say that SCRs offer no advantage over a suitably large OC twinset
filled with the same nitrox mix. And it is simpler to put the right OC
regulator in your mouth at the right time than to grasp the theory
needed to cope with the dangers of rebreathing CO2-scrubbed gas.
With OC scuba, the diver inhales breathing gas and exhales it into the
water. Rebreathing is like being locked in an airtight trunk, relying
on the technology within it to keep you from asphyxiating. More
specifically it's like breathing from a plastic bag, except that the
scrubber unit removes the waste carbon dioxide, the build-up of which
normally signals to our brains that the air supply is inadequate, and
makes us breathe harder. Without this signal, we can continue to
breathe normally until there is inadequate oxygen left to support life.
The first sign of hypoxia is unconsciousness, quickly followed by death
if the diver is under water.
The CCR automatically tops up the breathing-loop or "polythene bag"
with the required amount of oxygen. The diver uses his instruments to
tell him what he is breathing , like a pilot who has to rely on his
control panel to gauge his height. An SCR tops up constantly with
pre-mixed nitrox whether it is needed or not, as long as there is no
obstruction to the flow, so it can be regarded as a good first step on
the way to using a CCR.
The advantages of bubble-free operation are often emphasised as an
argument for CCR. This is exaggerated; CCR divers are not invisible,
and wildlife is still nervous of any big animal that suddenly appears
with a box on its back. What CCR does give you is time for creatures to
get used to you before you need to head back to the surface.
Rigged for diving to 50m/165ft, a CCR such as the Inspiration costs
nearly $8000. For open-circuit scuba, four cylinders, four
high-performance regulators and a technical BC with redundant bladder
costs around $6000, many advanced OC divers feel that four cylinders
are essential.
The simplicity of open circuit seems attractive until you address the
problem of entering the water while wearing a twinset on your back and
possibly two other cylinders hooked on to your BC harness. There is the
weightbelt needed to get it all under water (especially when using
aluminium cylinders) and all the plumbing. Divers with multiple
cylinders have jumped into the water before now without their tanks
turned on and, unable to inflate their BCs, disappeared to great depth
under the weight of them, drowning on the way. The CCR diver is
far less encumbered, but has to pay even more attention to equipment
maintenance and essential pre-dive checks. Jumping in without doing
these could be equally fatal.
If an OC set-up doesn't work properly, you don't need to be a
rocket-scientist to spot it. But if your CCR is not working properly
or, indeed, if it has not been switched on, you can still breathe -
until the point at which you become hypoxic and pass out. There are no
symptoms, so it's the perfect way to commit suicide. You must
rely on instrumentation and your ability to correctly read it.
SCRs are not immune from this effect, either. Their simplicity is
seductive but the flow-rate of nitrox into them must be rigorously
checked, and it seems hazardous to use one without some method of
constantly monitoring the oxygen partial pressure in the counter-lung,
although many users have no such device.
Alas, mistakes are made. A CCR diver died from hypoxia while breathing
from it swimming head-down on the surface. It is said that he did not
have it turned on. The extra complication and unfamiliarity can be
distracting. Two CCR divers have died from embolisms connected with
making very rapid ascents; one, it appears, when the direct-feed to his
suit jammed on.
If you choose to take the CCR route, remember this: you might jump into
your car and drive off without making any checks, but you wouldn't do
that with an aircraft. In the same way, you can never afford to be
cavalier with a CCR. But prepare to put your open-circuit training
aside and be ready to start again from scratch!!!!
.
Parts reproduced from DIVER magazine - October 2000 |
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