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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.

  • However why would you loose the nitrox ?

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

  • Why would you loose deco gas ?

    Same as above.

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