O2 at 80/20 or 100?

[Big-t-2538]

First off...I know very little, but what I have looked at, and what I understand this "off-gassing" to be is something like this....

The 29% is an effiency based on the difference in pressure inside and outside the nitrogen bubbles.

this is not something that is time dependent, once you have the pressuge gradient, then that is used to determine the time requred to "off-gas"

I'm just guessing, but I can see that the time and the pressure gradient are not 100% directly related...there is a return on investment so to speak, but the rate changes as the pressure differential changes.

 

[Dr Deco]

Dear Readers:

29% Faster

I looked at the equation posted for the elimination of nitrogen from a gas bubble and it appears to me that something is amiss.

First, the equation applies to the exchange of bubble gas into a medium that directly reflects the nitrogen tension of the breathing gas. The only medium in which this occurs is arterial blood. The bubbles must be in the arterial blood also. However, the bubbles responsible for joint-pain DCS are in tissues and not in the arterial blood. In all other cases, the dissolved inert gas tension is eliminated in a time dependent fashion; the exchange rate is dependent on the compartment half time.

Second, I do not see a term for the surface tension of the gas bubble. Gas exchange as described here does not appear to be the correct use of this equation. I do not recognize it and thus do not know the purpose for which it was originally derived.

Dr Deco :doctor:

Please note the next class in Decompression Physiology
http://wrigley.usc.edu/hyperbaric/advdeco.htm

 

[Charlie99]

100days-a-year once bubbled...
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.

Nice math that CaveDiver quoted from Gue's manual, but what does GUE's Decoplanner say?

What counts is the deco time. I don't have a copy of Decoplanner handy, but I doubt that O2 gets you out of the water 29% faster, even with the Haldane based deco program supplied by GUE.

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There may very well be other reasons to prefer O2 over EAN80, this "29% increase in offgassing efficiency" seems to be bogus.

Charlie

 

[Waterlover]

Charlie99 once bubbled...
Nice math that CaveDiver quoted from Gue's manual, but what does GUE's Decoplanner say?

What counts is the deco time. I don't have a copy of Decoplanner handy, but I doubt that O2 gets you out of the water 29% faster, even with the Haldane based deco program supplied by GUE.

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There may very well be other reasons to prefer O2 over EAN80, this "29% increase in offgassing efficiency" seems to be bogus.

Charlie

Tables will not show a 29% time difference. The point as was explained to me is in the efficiency( I didn't say time) of the offgassing as there is no nitrogen in the pure O2. Whether you agree with the 29% figure or not I'll keep using my O2....

 

[Cave Diver]

Dr Deco once bubbled...
Second, I do not see a term for the surface tension of the gas bubble. Gas exchange as described here does not appear to be the correct use of this equation. I do not recognize it and thus do not know the purpose for which it was originally derived.

To clear up some of the numbers used in the derivation of this formula

Quoting from the GUE Tech manual:

"Based on experimental measurements, gas in bubble form is composed of saturated water vapor (47 mm Hg) and elevated partial pressures of oygen and carbon dioxide in the tissues; which can be approximated by the use of venous blood values. Inert gas, nitrogen in this case, will diffuse into the bubble to occupy the remainder of the pressure, up to ambient pressure minus surface tension and the effects of tissue confinement. Based on experimental measurements, Hill approximated the surface tension and effects of tissue confinement as 133 mmHg (1ATA = 760 mm Hg = 33 fsw)."

By reducing the amount of nitrogen in the mixture, the oxygen window is increased, which results in an increased gradient for nitrogen removal.

All of the technical jargon and mathematics aside, it just makes sense to me that if you are trying to offgas nitrogen, then the best way to do that effectively is to breathe a mix that contains no nitrogen.

 

[BIGJC]

the depth you can hit the mix is irrelevant as far as efficiency of the gas goes.

O2 at 20 feet has a higher partial pressure (all that matters) than 80/20 at 30 feet so the pressure gradient between the gas you inhale and the gas leaving your lungs (nitrogen) is the greatest on O2 at 20 feet... ergo you will be decompressing faster, a simple matter of physics! As for the percentage faster that is a whole other matter, I am not sure it can be calculated to such an exact number. I know the hang times from most dive plan software show about a 25-30% difference, but you would lose some of the efficiency due to the need for air breaks... I use it because O2 is SAFER than 80/20, not due to the speed!


But hey, who wants to get out of the water anyway???

 

[Dr Deco]

Dear Readers:

Oxygen and Efficiency

As I mentioned, I doubt that the figure (29%) can be given with any accuracy as this depends on the biophysical conditions in the body and these cannot be well defined for a large diving population.

Certainly 100% oxygen will be more efficient than a mixture with nitrogen added. It is important to maintain some degree of activity during the deco stop. In the 1970s, some researchers found that decompression with oxygen required more time than using air. They attributed this to vasoconstriction. Their divers, however, were told to remain quite and essentially motionless to preclude CNS oxygen toxicity. This reduced the blood flow produced by the mechanism of the muscle pump.

Dr Deco :doctor:

Readers, please note the next class in Decompression Physiology :grad:
http://wrigley.usc.edu/hyperbaric/advdeco.htm

 

[100days-a-year]

VPM desn't get you out even 5% faster,nor GAP,Mplan,or Proplanner.In fact in some dives 80/20 will get you out faster.I mentioned that earlier that I tried using that argument on my buddies to get them to use 100%.They asked me to run these and prove it.Most dives I could get out 2-5 minutes earlier on VPM with 100% but the funny thing was that 50% alone came up only a few % slower than either 32/80 or 50/100 .Doc has already mentioned the tissues are the limiting factors on offgassing.I don't use
GAP for diving ,but it's graphic on M-value is priceless to start to understand how tissue absorbtion and saturation act as a function of the deco equation.Sort of like a graphing calculator for a new calc. student.

 

[Rick Murchison]

Hmmm... looks like the principle objection to 80/20 is that somebody thinks it's a sop to poor buoyancy control.
That's BS.
It's money.
My LDS has EAN 32 banked.
I can get a bit over 2100 psi into an 80 off a fresh O2 bottle, or about 54.1 CF of deco gas at 100% O2. Or I can top it up with EAN 32 and get 3000 psi of 80/20, or 77.4 CF of deco gas. (or go higher for a leaner mix in an HP tank which may be even better under some circumstances).
The 80/20 bottle will give me more flexibility and successfully deco "bigger" dives (or more dives) than the 100% bottle.
All of us don't have easy access to an O2 Haskel.
Hell, some of us have been known to decompress on (gasp!) AIR!
Love,
Rick

 

[BIGJC]

you could switch to a LP steel stage like the OMS 45's, that would solve most of your fill problems. if you always have a full O2 bottle on hand you should be able to get close to that 2400 psi mark.

but your right for the average joe 80/20 is easier to mix.... I still think O2 is just safer.