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The Basics
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Gas Choice
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Decompression
Gas Choice
How do you choose your decompression mixes ?
Its easy to calculated a best bottom mix by
throwing a PPO2 and END into an equation (for bottom mix we adopt a MAX PO2 of 1.4 and an
END of 30M). But what about decompression gases? - every one agrees it makes good sense to
adopt a set of standard mixes, but what rules should we use in picking them ?
For decompression it makes great sense to use
100 %O2 from 6m. Not using 80/20 for as the final decompression gas has a number of
very good reasons. The rest of the choices are dependent upon
the dive and whether you are doing two decompression mixes or just one (excluding the O2).
For the bottom mix, intermediate mix(es) and O2
dives, most of us do (i.e. not 100m for 5 + hours as completed by the WKPP), we would in
general only have to pick the intermediate gas - Our choice of this intermediate mix is
driven by three primary points.
The max PO2 you can experience safely (so the
deepest you use it) is dictated by the 1.6 rule (unless its a deep long dive, when you may
not want to use 1.6 as a max at the intermediate stops)
The narcotic effect of the gas, may drive us
to use a trimix as a decomprtession gas
The shallowest you can use it, which is
dictated by two things (1) the next mixes MOD and (2) the PO2 this mix has dropped to due
to decreasing ambient pressure - if the PO2 drops too low the deco mix is not being
efficient anymore, if you can't get to the next mix in your plan you need to put in
another mix
So what is the lowest we want to let a mix drop
to before we consider a change, experiance shows that a deco gas should never drop below
about a PO2 of 1bar - this drives us to choose 50/50 as the travel/deco mix
(it has a PO2 of 0.95 at 9m), used before the Oxygen - this can be started at 21m - if we
compare this to the other "common" choice which is 35% - this can indeed be
started at 36m - but the difference of 0.95 PO2 of the 50/50 against 0.67 of the 35% at
the 9m stop which is probably as long (in time) as the stops between 36 m and 21 m added
together would mean that we have utilised the O2 to the maximum we could.
Generally for deeper stuff, where you are
needing O2 and two other decompression mixes (i.e. a 4 mix dive - possibly more) - the
current thoughts are to NOT consider Air as any of the choices. The WKPP believe
that the mere presence of helium in a mix alters the way nitrogen effects the rigidity of
red blood cells, and reduces or eliminates the microcirculatory damaged associated
therewith. As such they have adopted a 21/21 mix (also playing with other similar
mixes instead of the air they previously used) this they use up to 50 % nitrox which is
used up to the 100% O2 - also they do not spike the oxygen after a long bottom exposure,
so move the gas switches up a deco stop over what is normal (i.e. drop the 1.6)
When you get to the O2 remember to taking back
gas breaks at a regular interval, including these breaks will greatly reduce the risk of
seizure on the one hand, and greatly reduce the pulmonary damage on the other. We break
every twenty five mins for five mins.
What redundancy should you carry for
decompression mix
Take the example of two divers - each with
three mixes Back, nitrox and O2 - they each have the same back and nitrox and same deco so
they are running the same plans - diver A looses nitrox - he will travel to deco gas on
his back gas - this means he needs to plan for this within his back gas requirements, once
at deco he is on that contingency plan - diver B will be clear before him but of course
will stay with him at each deco stop - the worst he does a bit more deco than he needs to.
If Diver A lost deco gas (i.e. O2) he'd deco out on the nitrox and then swap to Divers B's
deco gas when Diver B had finished, again needed a rule of halves on the deco gas - OR
Diver A and Diver B can have spare deco gas in water.
First stage failure - use the valve to control
the breathing - i.e. turn it on / off as required or consider swapping the whole reg
assembly completely with your deco gas reg - then swap them back when you need the deco
gas
Free flow - won't be catastrophic as the stages
are turned off when not being used so all you do is flood the reg
Second stage failure either fix it if you can
(i.e. get inside the reg ......ahh see a trend here) or swap it with your deco gas reg
Hose failure - swap regs between deco and
nitrox
The only issue we are concerned about is a
smashed the valve knob and now can't turn the stage on - here you fall back to contingency
plan as discussed above.
Why we do not use 80/20 -
Adopted from The Bakers Dozen
100% oxygen is often perceived as a buoyancy
control risk at 6m feet and 80/20 is seen as a method to overcome a divers inability to
control their buoyancy in open water, if this is the case then why is the same ppo2 (
intended) not a risk at 9m ? In reality even a heavy sea is not a problem for a deco stop
if it is not posing a lung-loading problem. Look at your depth gauge in a heavy sea and
"see" for yourself what the changes are - insignificant, and if they are not,
you should either not have been diving or incurring a decompression liability of this
magnitude in the first place. In the event of a change in conditions during the dive, see
below where the 80/20 becomes a liability rather than an asset.
If the decompression is extended, you do not
want to heighten your ppo2 at deep depths (9m) while still being faced with a long
decompression at shallower depths (6m). This subjects you to a higher risk of tox in the
final deco steps. Tox you do not get out of.
The 80/20 mix is in fact totally useless and
contraindicated as a deco gas. At 9m it is only a 1.52 ppo2 ( the real 1.6 ppo2 gas
would be 84/16) and as such does not either provide the right oxygen window, nor does it
does it work as well as pure oxygen without an inert gas at any depth. The gas mixing in
your lungs has already lowered the effective ppo2 enough to prevent spiking at 6m anyway
with the use of pure oxygen
Any perceived decompression benefit of using
a higher ppo2 at 9m with 80/20 is then given back by the lowered ppo2 at 6m feet, not to
mention the fact that the presence of the inert gas in the breathing mixture defeats the
purpose of using oxygen in the first place ( see the Physiology and Medicine of Diving) .
The ppo2 of 80/20 at 6m is 1.28, not much of an oxygen window, and at 3m it is 1.04
- useless for deco. To make matters worse, you can not get out from your 9m stop in
an emergency ( not doing the other stops) on the 80/20 mix without really risking a type 2
hit.
The 20-30% longer 9m foot time on the lower
ppo2 is not only overcome on the pure oxygen at the next stops, the breaks do not come
into play until the initial good dose of pure oxygen has been absorbed, since you are not
spiking from a high pervious dose without a break that is effectively achieved on the
previous gas.
In an emergency situation, getting onto the
pure O2 for 20 minutes or so (for long dives something approximating the bottom time or
any decent interval) would give you a real good shot at getting out of the water having
missed the rest of your deco and living through it with pain hits only.
If there is some problem with your deco or
you otherwise develop symptoms and need oxygen on the surface, it is silly to have not had
it there all along. 80/20 is a joke for that purpose. This is probably one of the
best reason for carrying pure oxygen as a deco gas in that it will be immediately
available in an emergency. The administration of pure oxygen is standard practice in the
aftermath of almost all forms of diving related accidents.
All the agencies require that scuba cylinders
which carry above 40% mixtures need to be "oxygen cleaned" and we
constantly hear about concerns about these clean cylinders been filled with "dirty
air". If you are concerned about the quality of some of the air fills you receive a
pure oxygen cylinder, only ever gets filled with oxygen from oxygen tanks, not from any
old dive shop compressor.
Copyright © 2001 [Gas - Diving].
All rights reserved.
Revised:2 July, 2001
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