Using gas mixtures to eliminate surface intervals?

[Charlie99]

Not true, the gases are independent.

In some ways the gases are independent, but not in others. For example, bubble growth is driven by the SUM of all inert gas partial pressures compared to the ambient pressure.

Diver0001, you can probably find some good info if you go searching on things like isobaric counterdiffusion.

Another related sort of problem is that of changing to a nitrox deco mix an the ascent from a dive on trimix.

Both VPM and RGBM track both the individual gases and their sum. You should go ahead and run some profiles through them and see what they say.

And the deeper you go on dive 2, the faster you offgas the residuals from dive1. Its like being at negative ATAs from the perspective of that residual gas.

Not true for dissolved gas. The gradient stays the same ---- 0 on one side, independent of depth; and whatever the tissue loading is on the other side. The increased pressure will help with the bubbles, though.

 

[rjack321]

Depends on whether you believe isobaric counter diffusion could possibly apply here. You've already been to the surface. Your gas load can't be that high or you'd be dead.

The residuals cannot on-gas nope, nada, zilch. So if you didn't get bent on the first surfacing you won't be on the second.

Unless you don't do a proper ascent/deco. Hell yes you can be bent from bubbles of whatever's in mix 2. But from a practical perspective dive 1 doesn't exist.

 

[rjack321]

Not true for dissolved gas. The gradient stays the same ---- 0 on one side, independent of depth; and whatever the tissue loading is on the other side. The increased pressure will help with the bubbles, though.

This was in response to the microbubble concerns

 

[JessH]

I think there is a snake under the grass. Microbubbles formed by one gas will have a an added gradient to enlarge them from the second gas

What if the second dive is deeper than the first? Won't that crush the Microbubbles formed by the first dive?

Another thought along the same lines would be to switch gasses in order to extend bottom time. Like switching from nitrox to heliox at some point so that you will start offgassing nitrogen while still on the bottom. Of course I assume that this is more likely to cause problems with counterdiffusion.

Am I correct in assuming that it is really the sum of the disolved gasses that matters in terms of limiting bubble formation and that each gas won't act independently in this respect? This would mean that you can not ignore the first dive but during the second you will be offgassing from the first.

~Jess

 

[Charlie99]

Am I correct in assuming that it is really the sum of the disolved gasses that matters in terms of limiting bubble formation and that each gas won't act independently in this respect? This would mean that you can not ignore the first dive but during the second you will be offgassing from the first.

That's a pretty good summary of my understanding of the situation.

What if the second dive is deeper than the first? Won't that crush the Microbubbles formed by the first dive?

Crushing of the bubbles helps to offgas them, but it doesn't eliminate them. If you do a short spike dive, it will crush the bubbles at depth, but then they come right back to the same size as you immediately ascend back up.

 

[Dr Deco]

Hello readers:

Gas Mixtures

Several decades ago, the idea immerged that a combination of breathing gases would not form bubbles because each dissolved gas would be below its “critical limit.” The concept did not work because the dissolved gasses acted together to form bubbles (actually enlarge preexisting micronuclei). Dissolved gases do not segregate when they become free gas in bubbles. If this were true, it would be a wonderful way to separate nitrogen, helium, neon, oxygen, and so forth.

Nitrogen Residuals

When you dive with a gas mix such as nitrogen and helium, you must account for each of the dissolved species (e.g., trimix). It does not make any difference how the dissolved gases entered the tissues - simply that they were there. Thus, you could dive two inert gases together, or alternate them on different dives. One must still account for all inert gases dissolved in the tissues.

Dr Deco :doctor:

 

[Diver0001]

Thanks Dr. Deco. Learned something new again.

R..

 

[TeddyDiver]

And the deeper you go on dive 2, the faster you offgas the residuals from dive1. Its like being at negative ATAs from the perspective of that residual gas.

You meaning "opening the oxygen window wide open" like in a chamber with O2:huh:

 

[rjack321]

Dr. Deco,
Respectfully, I think your missing something here. If all the residual gases stay dissolved on the ascent form dive 2, who cares? Dissolved gas doesn't cause problems. If you do get a bubble, the inert fraction in that bubble is going to offgas like crazy since that inert doesn't exist in the inspired gas. The bubble will quickly become just the inert in the inspired gas. So from a practical persepective,
1) Dive 1 microbubbles will offgas as soon as you start breathing gas 2. And
2) any residual dissolved gas from dive 1 will either stay dissolved and do no harm, or form a mixed gas bubble and quickly turn into a bubble of inert 2.

So the only chance of causing a problem that needs management is ala an isobaric counter diffusion-like mechanism. Whereby the dissolved inert 1 doesn't have enough of a dissolved fraction to form a bubble on its own, but combines with the new inert in mix 2 and those bubbles - despite off gassing inert 1 - grow on ascent.

Since isobaric counter diffusion only seems to appear with fairly high gas loadings, and these are NDL dives, I propose that this phenomenon will not occur to any sustantive degree. I'd be willing to test it (with good ascent rates and stops) if this was any more than a thought experiment.

 

[rjack321]

You meaning "opening the oxygen window wide open" like in a chamber with O2:huh:

Yes, for bubbles