Increased nitrogen off-gassing 10ft/3m VS 20ft/6m on 100% oxygen

[broncobowsher]

I remember my rebreather training from a couple years ago. Doing deco at 20' is nearly as good as doing it at 10' Less surface surge to deal with. And being on a rebreather, you don't use any more gas like you would on OC. Shearwater even has a setting for the last deco stop at either 10 or 20'

 

[inquis]

for each tissue compartment, there are 2 relevant gradients

In a simplified sense, yes.

DCS occurs when the volume of inert gas reaches a critical point.

If you mean bubbles form and grow too large, then yes, that's the gist of it.

 

[boulderjohn]

I think you're asking about the oxygen window? This article might answer your questions, at least from a theoretical perspective.

The effect of hyperbaric oxygen on the oxygen window | GUE

www.gue.com

But I don't think anyone has done a large-scale study to empirically determine whether it is better to do part of your oxygen deco at 10 ft, so we don't really know for certain. Obviously on an open-circuit deep dive you have to stop at 20 ft to do your gas switch and then stay for some time to prevent too many bubbles from forming. But after that point is it better (from the perspective of minimizing DCS risk or minimizing total deco time) to stay at 20 ft for the remainder of your shallow deco time, or move up to 10 ft and spend a significant amount of time there? Who knows.

From a practical standpoint for ocean diving in rough conditions, buoyancy control at 10 ft is kind of a hassle. I mean I can do it but I'm constantly watching my depth gauge and making minor adjustments. Whereas sitting at 20 ft is more relaxing.

Your article is from 2004, or about 6-7 years before Jarrod Jablonski told me they no longer believe that.

 

[Nick_Radov]

Your article is from 2004, or about 6-7 years before Jarrod Jablonski told me they no longer believe that.

Believe what? The physiology described in the article still seems basically accurate 18 years later.

But the question

 

[boulderjohn]

DCS occurs when the volume of inert gas reaches a critical point.

To be honest, no one is really sure what causes DCS.

There are two forms of gas in the tissues--dissolved and free (bubbles). In the early days of dive theory, dissolved gas is all people worried about. The idea is that if the dissolved gas could be in a safe range relative to ambient pressure, the gas would stay dissolved or any bubbles formed would be minor. Think of a bottle of pop. Before you open it, you don't see any bubbles, because the CO2 is all dissolved, open it, and it becomes exposed to atmospheric pressure, and bubbles will form. If you open slowly, the gradual release of the pressure in the soda will keep the bubbles under control. If you are at high altitude, the lower ambient pressure will make that harder to control. Open a bottle recklessly at high altitude and you will be in a shower of exploding bubbles.

We now know that there are bubbles already formed during the dive, and we do want to control them. What does it mean to control them? Well, it's all unproven theory, with really no way to prove it. Is the size important? The number? About 20 years ago, it was all the rage to transfer focus from the dissolved gas issue to the bubble growth issue as the key factor. That wass when the popular idea was to stay deeper longer to keep bubbles from expanding under Boyle's Law. In the past decade things have switched away from that a bit.

 

[Jack Hammer]

So the solid question is: For pure oxygen, is 10ft 1.4 PPo2 or 20 ft 1.6 PPo2 more efficient for nitrogen off-gassing?

I think the question has been reasonably answered. There are solid reasons to do either. The dive circumstances should be a factor in choosing which is appropriate when. Also, lots of people take an additional 3-5 minutes to surface from the "last" stop.

(Also, check your math, 100٪ at 10ft/3m is 1.3. You would need to be at 13ft/4m on 100% to be at 1.4. (doesn't change the answers))

 

[boulderjohn]

Believe what? The physiology described in the article still seems basically accurate 18 years later.

But the question

No, as I said earlier, the physiology in the article is wrong. Read mark Powell's description of it in Deco for Divers. In the the first edition, you can find the relevant portion in Chapter 5, with the key conclusion on page 134.

 

[lowwall]

The DCS issue aside, it's the basic physics of diffusion that the greater the pressure difference between two volumes, the faster that molecules will move from the area of high pressure to the area of low pressure.

Other factors that affect diffusion speed are temperature, the relative concentration of different molecules, surface area and membrane porosity. But in this case we are holding all of those constant. It's only the pressure differential that varies.

Specifically, the two volumes are the dissolved gases in the tissue and the surrounding blood. The gases in the blood are very near ambient while the gases in the tissue are at some higher pressure. As we ascend, the pressure differential between the gases in the tissue and the blood increases and thus the speed in which the N2 moves from the former to the latter increases.

The very fastest offgassing plan is always to ascend straight to the surface and then breathe 100% O2. Of course that ignores the whole reason you are trying to offgass N2 which is to reduce the likelihood of DCS. Still if you concerned about minimizing your N2 load for the next dive(s), doing your final O2 stop at 3m instead of 6m would have a small, but real, advantage.

If this doesn't show up in the results from a particular decompression algorithm, then that shows the programmer or person creating the algorithm has made simplifying assumptions, not that the physics is invalid.

 

[Stoo]

This thread will likely go on for 400 pages and there will no resolution.

Do what I do, hang at 15'.

 

[inquis]

Other factors that affect diffusion speed are [...] the relative concentration of different molecules

Exactly, which is included in the partial pressure.