O2 at 80/20 or 100?

[TheDarknessLord]

Hi!

I was reading some article that says that 80/20 is not good enough to deco stops.
Could you tell me please more about it? or when I can find a worth article...

 

[Dryglove]

http://www.baue.org/library/bakers8020.html


http://www.baue.org/library/irvine_baue_talk.html

 

[Waterlover]

The best reasons I have found to use 100%.There is no mixing just filling it. For an increase in 20% of O2 you get a 29% increase in off gassing effiency......

 

[100days-a-year]

There is no significant change in runtimes on any program I've ever run.In some cases the 32/80 runtimes were shorter on both M-value and RGBM programs.But you have to ask yourself why are you including a gas in your mix if you are trying to get that gas out of your system.

 

[Cave Diver]

Waterlover once bubbled...
For an increase in 20% of O2 you get a 29% increase in off gassing effiency......

Where did you get that figure?

I personally agree with the 29% figure, I was just wondering if we got it from the same source.

 

[100days-a-year]

I understand what you're saying but no program has ever givin me more than or less than a 10% variance either way.How do you substantiate your 29% claims.Just askin'.VPM even gives you shorter runtimes in some cases with 80/20 if there is a way to get out earlier I'd love to know about it.

 

[Ontario Diver]

There are a couple of advantages I see to using an 80/20 (or even a 70/30) mix over a 100 mix.

The first one is that I can get on the mix at a lower depth. I can use an 80 mix at 33 feet and still be at my 1.6 PPO2 versus 20 feet for 100% O2. That means I can do my 30ft deco stop on deco gas. I'm not sure how this will be reflected in the algorithm.

The second is that if the chop is very bad, my 20 foot stop can be a 18-24 foot stop. I feel for myself (before I'm told that if my boyancy was perfect it wouldn't be a problem) that softening the mix gives me a little margin of error on my stops.

 

[Waterlover]

Came from a friend familiar with deco tables( I've been using O2vs 80/20 since '95 from his recommendation) and if you want to read about the figures I believe the GUE tech manual has the figures in. As of right now the tables get us out about as quick as we can. There is ongoing research, but I personnaly think they about as good as we can guess right now.... something else to think about.... Why would I want to breathe a deco gas that has the gas I am trying to get rid of in the mix If I can breathe a gas with none of it in?.....

 

[Cave Diver]

100days-a-year once bubbled...
I understand what you're saying but no program has ever givin me more than or less than a 10% variance either way.How do you substantiate your 29% claims.Just askin'.VPM even gives you shorter runtimes in some cases with 80/20 if there is a way to get out earlier I'd love to know about it.

The GUE Tech manual has a section on it. It's detailed in Section 9.4 Deep Stop Advantages. Brian Hills derived the formula for calculating the relationship for the nitrogen gradient inside to outside of a bubble as:

PN2 = P(1-x) + 47mmHg(x) - 133mmHg


It states that P represents ambient pressure and x represent the fraction of nitrogen in the gas mixture.

Using an 80% deco mix:

PN2 = 760mmHG (1-.20) + 47mmHg(.20) - 133mmHg
PN2 = 760*.80 + 47*.20 - 133
PN2 = 608 + 9.4 - 133
PN2 = 617.4 - 133
PN2 = 484.4 mmHg

100% deco mix:

PN2 = 760mmHg (1-0) + 47mmHg(0.0) - 133mmHg
PN2 = 760*1 + 0 - 133
PN2 = 760 - 133
PN2 = 627 mmHg

Given these parameters, it shows that (if the equation is correct) that doing a deco on 100% O2 results in a 29% faster reduction in the nitrogen gradient even though it is only contains 20% more O2.

And just for the record, if you attempt to duplicate the examples in the GUE Tech book you will find that they printed the wrong fraction of nitrogen to arrive at their answers in the book. They printed a .78 percentage of nitrogen, but the answers given are actually requiring a .79 percentage. This leads to a 30mmHg difference in the 4 ATA examples and a 15 mmHg difference in the 2 ATA example.

 

[100days-a-year]

Nice math.And I still wonder why deco tables Haldane,Buhlman and RGBM still don't give a 29% reduction in time spent at those stops.I wonder if a tissue group offgassing is the limiting factor and has a finite limit that would negate any differences.The reason I ask all this is I believe 100% is better but the logical engineer in me sees no mathematical proof even in all the ways you can express the differences.A Deco program particularly RGBM takes all these factors into consideration yet still doesn't in effect any numerical advantage in deco time.Is there a parameter involved that negates any advantages,perhaps the 10' deeper stop allows for more deltaP earlier and the 29% doesn't equate to any time advantage.I ran a lot of profiles trying to convince a buddy and we called a draw.