I agree putting O2 aside seems to have been empirically validated for shallow EAN dives, but maybe it is not that sounded for deeper trimix tek dive, and could be one of the reasons why most divers have to use a 20% gf... What is the relevance of a model when you have to apply an arbitrary cut that important on the results it gives ?
Oxygen window misunderstanding
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I agree putting O2 aside seems to have been empirically validated for shallow EAN dives, but maybe it is not that sounded for deeper trimix tek dive, and could be one of the reasons why most divers have to use a 20% gf... What is the relevance of a model when you have to apply an arbitrary cut that important on the results it gives ?
Depth and diluent are indirect influences on oxygen dissolution. Ultimately it's the partial pressure of oxygen in the breathing mix determines the amount of dissolved O2. If your inspired pO2 is 1.2 ATA then it doesn't matter whether you're relatively shallow on N2O2 or deep on trimix.
Best regards,
DDM
On a deep dive you'll be wanting a hypoxic mix anyway.
The disconnect in the model as is, excess oxygen is simply not part of the equation. On nitrox dives it just lowers % of inert gas and extra O2 "vanishes", as far as the numbers are concerned. There's no "extra O2" in a hypoxic mix, that problem doesn't exist there.
Sure but the fraction of inert gas is higher than .79; it's the opposite of nitrox where it's less and the extra gas "just vanishes". Here it is present and accounted for.
You shouldn't: Fraedrich study shows that the statistics agree between different models only in GF70..85 range. At GF20 all probabilties are off.
As for hypoxic mixes, you could just as easily argue (I wouldn't) that as the relative fraction of "disappearing O2" goes down, calculated gas loading goes correspondingly "too much" up. I.e. on those deep trimix dives the model in fact errs in the other direction and produces unnecessarily long deco and you want GF120 instead.
Protondecay123
Contributor
I'm not so sure about most divers using a 20% gf. I have heard of some using 99/99 Lo/HI. Here's an article by someone who uses 70/85 Lo/Hi. Gradient Factors in a Post-Deep Stops World
I'm not sure what the point or agenda is here other than a possible misstatement on a web page? The modern Neo-Haldanian models have served relatively well. I think that the criticisms of Dr. Buhlmann's work regarding bubble formation and being bent forget one thing. That he used empirical data to build his tables and built the theory and math from there. My disagreement is that he predicted exponential on and off gassing. The starting point for the gas molecules are very different on gassing and off gassing. Diffusion into the tissues is a very different beast than diffusion from the tissues to the blood stream. Also if the diver experiences pulmonary edema during the deeper parts of the dive the gradient across the alveoli could be very different on gassing than off gassing. The Thalmann algorithm actually uses linear rather than exponential off gassing with apparently better out comes than the Bubble Permeabilty Models according to NEDU 2008 experiment. https://apps.dtic.mil/dtic/tr/fulltext/u2/a561618.pdf
Doug Fraedrich's Study is here Validation of algorithms used in commercial off-the-shelf dive computer
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 and "unsafer" 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.
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