Underwater off-gassing equivalent to a surface interval on air

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boulderjohn

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Back in my DIR days, it was hammered into me that nitrogen leaves the body fastest at the highest PPO2s because of their idea of the oxygen window. That is why both the GUE and UTD dive profiles had the S-curve, where in the middle portion of the ascent, you actually did the stops at 70 feet and 60 feet (after the switch to 50%) longer than the stops at 50 feet and 40 feet, the opposite of what is done with normal decompression profiles.

This made no sense to me, and when I argued it, they pointed me to a paper that said exactly that. I read it with my lack of science training, and it seemed to me that the conclusion on that point leaped out of nowhere. I could not identify anything in the paper's data the led to it. It talked about the fact that oxygen being metabolized created an "oxygen vacancy" that left more room for the nitrogen to leave the tissues, just as 5 people leaving a room creates more space for 5 others to enter it. That made no sense to me--gases are not like solids. I asked for an explanation, and I never got it. Then Mark Powell came out with the first edition of Deco for Divers, and in it he emphatically said that this idea was false, because it violated Dalton's Law. My pointing to that did not make me popular.

I was still fighting with my DIR agency (UTD) about it, and I was put in touch with Jarrod Jablonski of GUE. In our discussion, he said that at GUE, they had realized that the science behind that version of the term "oxygen window" (the term is defined differently elsewhere) was not valid, but they had been doing it that way for a long time and were hesitant to change. They did change shortly after that. UTD did not change until their version of Ratio Deco came out poorly in the Spisni study.

As for me, I set my last stop to 10 feet, but I don't go there. As soon as I am finished with the 20 foot stop, I go up to maybe 15 feet or so. That way I can do mild exercises to increase perfusion and relax without having to be too anal about my depth.
 

lowwall

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As for me, I set my last stop to 10 feet, but I don't go there. As soon as I am finished with the 20 foot stop, I go up to maybe 15 feet or so. That way I can do mild exercises to increase perfusion and relax without having to be too anal about my depth.
That only works because you use imperial units. You can't expect the rest of the world to hold a stop that ends in .5.

:)
 

dmaziuk

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That only works because you use imperial units. You can't expect the rest of the world to hold a stop that ends in .5.

:)

As this thread's resident nit picker I feel obligated to point out that it actually ends in .4559274326701081, give or take a few decimal digits and assuming the water is salty.
 

dm9876

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As this thread's resident nit picker I feel obligated to point out that it actually ends in .4559274326701081, give or take a few decimal digits and assuming the water is salty.
Regardless of the fraction I think this principle is rather practical and I do the same. Clear the 6m o2 stop but only ascend to 5m (16.404 ft) 😀
 

dm9876

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Regardless of the fraction I think this principle is rather practical and I do the same. Clear the 6m o2 stop but only ascend to 5m (16.404 ft) 😀
Could also watch the "ceiling" on the computer and move up earlier once ceiling is less than 5m but I've never bothered and just wait the extra few minutes for the 3m stop to be available. I'm not sure if the computer would consider that a missed stop despite not violating the algorithm (though that's not really a big deal on a shearwater but could still be annoying)
 

dmaziuk

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Could also watch the "ceiling" on the computer and move up earlier once ceiling is less than 5m but I've never bothered and just wait the extra few minutes for the 3m stop to be available. I'm not sure if the computer would consider that a missed stop despite not violating the algorithm (though that's not really a big deal on a shearwater but could still be annoying)

Buhlmann wrote in Decompression Sickness that the only reason for spacing deco stops at .3 atm that he could find is that "we've always done it that way" and that in practice smaller increments would likely be hard to maintain in open water. In a controlled environment like a chamber, Herr Doktor said one could "ride the ceiling" continuously and do away with discrete stops altogether.
 

dm9876

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Buhlmann wrote in Decompression Sickness that the only reason for spacing deco stops at .3 atm that he could find is that "we've always done it that way" and that in practice smaller increments would likely be hard to maintain in open water. In a controlled environment like a chamber, Herr Doktor said one could "ride the ceiling" continuously and do away with discrete stops altogether.
very true and it will also slightly accelerate your deco due to the average supersaturation being higher since with the staged stops you only briefly touch the supersaturation limit when you first arrive at a stop. However riding the ceiling would also mean that you're using the model in a more aggressive way than which most of the testing of the model was likely done. Having said that, most implementations add a lot of extra conservatism than the original model anyway. I don't hear of anyone using GF100/100 :)

It is a fascinating thought that we are still doing it that way because we always did it that way. I wonder if there have been any studies to compare the two approaches. I'm not even sure how you would fairly compare them.

Dean
 

boulderjohn

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It is a fascinating thought that we are still doing it that way because we always did it that way. I wonder if there have been any studies to compare the two approaches. I'm not even sure how you would fairly compare them.
The ascent method was not random. I am a little fuzzy on the details, but I know a century ago the debate was going on. Was it better to do a series of stops during ascent (as Haldane advocated) or do a slow continuous ascent (as Hill wanted)? The series of stops won the debate. Haldane's experiments with goats indicated that a series of stops was superior to a slow continuous ascent.
 
https://www.shearwater.com/products/swift/

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