Oxygen Window: Explanation and Purpose?

[texdiveguy]

At this time of the year I try my best to keep my 'window' closed as the O2 coming in has so much pollen from the early spring blooming of plants and grasses mixed in with the O2 and inert gases.

 

[limeyx]

At this time of the year I try my best to keep my 'window' closed as the O2 coming in has so much pollen from the early spring blooming of plants and grasses mixed in with the O2 and inert gases.

My window is open & closed as it seems a combination of summer but high-winds have hit, so I have to open it to cool down but then I start sneezing
Maybe I need to apply an S-curve

 

[texdiveguy]

Maybe I need to apply an S-curve

.............

 

[UWSojourner]

As far as I am aware [for the past several decades], the oxygen window [OW] and decompression applies ONLY to the gaseous phase and bubbles. It does not deal with some “magical” ability to effect a rapid release of dissolved nitrogen or helium.

The OW will help with the shrinkage of bubbles in tissues and the venous system [because of surface tension effects].

Really?! I'd love to see a reference that relates the "OW" to something as specific as the surface tension of bubbles.

As far as I can tell, the O2 window just means that even at the surface as you simply breathe air you still offgas. Why? Because part of the gas breathed is metabilized (O2). So you can open the "O2 window" further (i.e. offgas faster) by breathing more O2 at any depth ... either 70ft, or 20ft or at the surface. It seems to me this is basic nitrox --- if you still are an air diver, a better window is there for you to open .

 

[Charlie99]

As far as I can tell, the O2 window just means that even at the surface as you simply breathe air you still offgas. Why? Because part of the gas breathed is metabilized (O2).

This is a common misconception.

It's not the presence of O2 in the breathing gas that counts, it's the absence (or reduction) of the inert gas that you want to offgas out of your body that enhances offgassing.


You offgas when the partial pressure of a particular inert gas in the breathing mixture is less than the partial pressure of that particular inert gas dissolved in the body tissues. There have been several studies that show that all that counts for dissolved gases is the relative partial pressures of that particular inert gas. Metabolism of O2 by the body does indeed create a "partial pressure vacancy". However, it has been shown through various stuides and experiments, that the partial pressure vacancy does NOT speed up offgassing.

Although having a partial pressure vacancy of O2 "sucking out the inert gas" is intuitively appealling, it doesn't happen. The partial pressures of other inert gases, and the partial pressure of O2 have no effect on the offgassing of an inert gas dissolved in the body. The partial pressure of O2 is only relevant so far as you have reduced the partial pressure of a particular gas by replacing it with O2.


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The partial pressure of O2 in the breathing gas may have some subtle effects on bubble growth, but I have yet to see any evidence of that.

 

[lamont]

I'm an experimentalist and not a theoretician at heart, so I find it more useful to break down what the "oxygen window" means by those who talk about it not by the theory behind it, but by how they change their deco. There are two changes:

1. lengthening gas switches at 1.6 ppO2 and shortening low ppO2 stops right before the gas switch

2. spending the entire time on 100% at 20 fsw at 1.6 ppO2 and not moving up to a 10 fsw stop after your ceiling has gone up enough.

The latter one is probably the starkest difference. If you plug into v-planner or any other software that I have seen a last stop depth of 10 feet or 20 feet on pure O2 it will make no difference to the overall runtime. So, clearly whatever it is that Irvine and GUE are talking about with "oxygen window", none of these algorithms implement it. Also its pretty clear that diving into all of the physiology around the "oxygen window" that nothing there predicts more efficient deco in the way that Irvine and GUE suggest.

So, what you're left with is the possibility that Irvine and GUE are wrong. Or that they're describing some "oxygen voodoo effect" which is yet to be explained and are attempting to ascribe it incorrectly to the published theories on the oxygen window.

Trying to tackle this problem from the theoretical perspective will result in assuming that the deco practices of Irvine and GUE are simply incorrect. However, trying to tackle Pyle stops from a theoretical approach back when decompression theory was all dissolved-gas would have resulted in assuming Pyle was simply incorrect -- but with free-phase gas models now we know he was actually on to something.

 

[Charlie99]

In your analysis, what is your conclusion if the current models, without any "oxygen voodoo effect" agree with the Irvine/GUE diving profiles and results?

In the case of Pyle stops/deep stops, the results from pure dissolved gas models differed from the real life results when doing deep stops.

Both of the Irvine/GUE items you mention are consistent with the results of bubble models such as VPM or RGBM. Why invent a nebulous oxygen window effect when the experimental results and theoretical calculations already agree???

Trying to tackle this problem from the theoretical perspective will result in assuming that the deco practices of Irvine and GUE are simply incorrect.

Please give an example. Your last stop 10' or 20' is not such a case. If you look closely at VPlanner or bubble model results, you will see that staying for entire shallow stop time at 20' will indeed slightly reduce the calculated free phase volume. If you choose to accept additional CNS oxtox risk for this small reduction in free phase volume, then go ahead -- but it is not correct to say that this is a discrepancy in the model.

As far as your item #1 about shortening low ppO2 stop just before gas switch, you can easily verify that this is a natural result from VPM by just see what happens when you do a lost deco gas run. The iterative solutions of VPM will indeed shorten the stop just before a gas switch. Then do the same profile but with a missing deco gas, and notice how the time for the stop just deeper than your normal gas switch is longer than for the run with a deco gas. Why come up with a voodoo effect when none is needed?

 

[wedivebc]

However, trying to tackle Pyle stops from a theoretical approach back when decompression theory was all dissolved-gas would have resulted in assuming Pyle was simply incorrect -- but with free-phase gas models now we know he was actually on to something.

That is not true. Pyle only put into practice what had already been theorized.

As a scientist by profession, I feel a need to understand mechanisms underlying observed phenomena. Consequently, I was always bothered by the apparent paradox of my decompression profiles. Then I saw a presentation by Dr. David Yount at the 1989 meeting of the American Academy of Underwater Sciences (AAUS). For those of you who don't know who he is, Dr. Yount is a professor of physics at the University of Hawaii, and one of the creators of the "Varying-Permeability Model" (VPM) of decompression calculation. This model takes into account the presence of "micronuclei" (gas-phase bubbles in blood and tissues) and factors that cause these bubbles to grow or shrink during decompression. The upshot is that the VPM calls for initial decompression stops that are much deeper than those suggested by neo-Haldanian (i.e., "compartment-based") decompression models. It finally started to make sense to me.

 

[UWSojourner]

It's not the presence of O2 in the breathing gas that counts, it's the absence (or reduction) of the inert gas that you want to offgas out of your body that enhances offgassing.

Hmmm ... the presence of O2 is what causes the absence (or reduction) in inert gases. If you increase metabilized gases you reduce inert gases (by definition).

 

[Charlie99]

That is not true. Pyle only put into practice what had already been theorized.

The sequence was more along the line of:

Pearl shell divers in Torres Strait figured out deep stops through the school of hard knocks and getting bent repeatedly. B.A. Hills published a paper on this in 1965. Interestingly, in this paper, he discusses some 1909 caisson work by Japp in which the decompression profile from 3 hours at 40psi included an intermediate stop of 10 minutes at 29psi (5 min ascent from 40psi to 29psi) before doing a 6 minute ascent to at 10 minute stop at 12.5psi preceding a 17 minutes ascent to the surface.
Japp 1909 profile converted to depths:
All on air.
3 hours at 90'
5 minute ascent to 65'
10 at 65'
6 min ascent to 28'
10 minute at 28'
17 min ascent to surface.

Richard Pyle later rediscovers the same effect, but doesn't see any theoretical support for it. At this point, there have been various speculations and hypothesis about nuclei, bubble formation, etc., but had not been developed or tested.

Later, in 1989, he see the first theoretical confirmation.

All throughout this period though, decompression researchers were very aware of discrepancies between some experimental results and the models.

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Deep stops is a good example where actual observed results differs from a particular model -- specifically the dissolved gas only models, aka Haldanian, neo-Haldanian, or Bulhmann. The dissolved gas models assumed that there were no adverse effects from being shallower, as long as you were below the ceiling, but did see the adverse effects of ongassing during the deep stops.

This is some evidence that many of the dissolved gas modelers, even back to Haldane recognized that this "no bubbling at all until above a specific ceiling" assumption was an oversimplification.