Oxygen window misunderstanding

[dmaziuk]

What is this study you're referring to ? Could you give me the reference, it seems very interesting !

There's 2 posts in Robert's blog that are I think a better abstract: Search Results for “fraedrich” – The Theoretical Diver

 

[Duke Dive Medicine]

No, this:



For illustration, say our equation is empirically calibrated to give correct results with the fraction of inert gas of 1 and the magic value of 2:1. If we reduce the fraction of inert gas without changing anything else (by upping "non-inert" fraction i.e. nitrox dive) we'd expect its calculated gas loading to be lower, i.e. resulting deco schedules to be more "aggressive". Conversely if we up the fraction of inert gas we expect the schedules to be more "conservative".

So one question is how "off" does the formula get as move towards either extreme. The other: if more conservative is "safer", is the formula "safer" for hypoxic dives than for EAN ones. As you up the fraction of oxygen that extra gas is not accounted for, for a numbers geek it does not compute. And so on.

I think I get what you're saying, but are we making a distinction between supersaturation, and bubble formation on ascent?

Best regards,
DDM

 

[dmaziuk]

I think I get what you're saying, but are we making a distinction between supersaturation, and bubble formation on ascent?

I tried reading up on VPM but my eyes glaze over every time so don't ask me about their bubble formation. In the base dissolved gas model, safe ascent rate takes care of them, and it is entirely external to gas loading equations.

 

[Duke Dive Medicine]

I tried reading up on VPM but my eyes glaze over every time so don't ask me about their bubble formation. In the base dissolved gas model, safe ascent rate takes care of them, and it is entirely external to gas loading equations.

Haha, understand. I do think it's worthwhile in the context of the present conversation to make the distinction though, since dissolved O2 becomes part of total tissue gas tension, though a very small part.

Best regards,
DDM

 

[dmaziuk]

Haha, understand. I do think it's worthwhile in the context of the present conversation to make the distinction though, since dissolved O2 becomes part of total tissue gas tension, though a very small part.

That's exactly the disconnect: when you look at the actual equations: in Baker, in Morrison's "DIY decompression", in subsurface, anywhere, you'll see that dissolved O2 is not part of the math.

 

[Duke Dive Medicine]

That's exactly the disconnect: when you look at the actual equations: in Baker, in Morrison's "DIY decompression", in subsurface, anywhere, you'll see that dissolved O2 is not part of the math.

Probably because the effect is negligible.

Best regards,
DDM

 

[BubbleJet]

Probably because the effect is negligible.

If the effect is negligible, then it should also not be considered for this oxygen window principle. One cannot state that dissolved O2 has no effect on total gas tensions leading to supersaturation, but that it does "leave some space" for Nitrogen or Helium as stated in the original DAN quote when consumed by metabolism.

Only way to combine those statements is to say that two different oxygen windows are discussed:
- the quicker inert off-gasing is just linked to a gas mix breathed in with a higher ppO2 leading to a lower ppInert for the same total pressure and thus the same supersaturation state
- the lowered supersaturation risk is linked to an oxygen window where O2 is part of bubble formation just like any other inert gas : the partial pressure vacancy leads to a lower total gas tension in the tissues and thus a greater margin to the supersaturation.

 

[dmaziuk]

If the effect is negligible, then it should also not be considered for this oxygen window principle. One cannot state that dissolved O2 has no effect on total gas tensions leading to supersaturation, but that it does "leave some space" for Nitrogen or Helium as stated in the original DAN quote when consumed by metabolism.

That's the problem with biologists and computer scientists: for us either 2+2 is 4 or it isn't. It can't be "4-ish but the -ish doesn't count except when it's oxygen in which case there's a window open. But mostly it's 4. Usually".

 

[Jack Hammer]

That's the problem with biologists and computer scientists: for us either 2+2 is 4 or it isn't. It can't be "4-ish but the -ish doesn't count except when it's oxygen in which case there's a window open. But mostly it's 4. Usually".

Welcome to science of things not fully yet understood.

 

[Duke Dive Medicine]

If the effect is negligible, then it should also not be considered for this oxygen window principle. One cannot state that dissolved O2 has no effect on total gas tensions leading to supersaturation, but that it does "leave some space" for Nitrogen or Helium as stated in the original DAN quote when consumed by metabolism.

Only way to combine those statements is to say that two different oxygen windows are discussed:
- the quicker inert off-gasing is just linked to a gas mix breathed in with a higher ppO2 leading to a lower ppInert for the same total pressure and thus the same supersaturation state
- the lowered supersaturation risk is linked to an oxygen window where O2 is part of bubble formation just like any other inert gas : the partial pressure vacancy leads to a lower total gas tension in the tissues and thus a greater margin to the supersaturation.

Maybe a better way of saying this would be to state that the relatively small variations in inspired pO2 while ascending/decompressing on bottom mix make the effect fairly consistent across gas mixes and diving techniques, which I think speaks to your second point. Switching to a higher-pO2 deco mix speaks to your first.

Best regards,
DDM