Diving Hypoxic Diluent on a Normoxic Dive

silent running

That's what I don't get, I think He would expand faster than N2 during ascent. I would think that whatever He that's left on ascent from a dive with 10/90 would expand more/faster from 20m to surface than whatever N2 is left in the loop from a dive with air dil ascending the from the same 20m...

Steve

What you are thinking is correct but at the pressures we are talking about here, 1-10 bar, the difference is very very small, I doubt you would pick it. Someone who has had their coffee today can do the math

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dteubner

At the pressures we are talking about all of these gasses behave as ideal gasses as described by the universal gas law.

1 litre of helium at 90m will be 10 l at the surface
1 l of oxygen at 90m will be 10 l at the surface
1 l of nitrogen at 90m will be 10l at the surface
1 l of 10/60 trimix at 90m will be 10l of 10/60 at the surface.
etc

The differences in rates of expansion of different gas components is a theoretical issue and of absolutely no relevance at all to the point in question.

Dave T

silent running

Dave, thanks for the explanation. But, wouldn't there be a difference in the amount of pressure inside cylinders holding the volumes listed above? 2 small cyls, one holding 10 ltrs of He and the other with 10 ltrs of air, wouldn't have the same psi, right? And if these pressures are different, then wouldn't there be different rate of expansion also? And wouldn't even a little bit of variation of these densities and expansion rates affect the PO2? I guess that even if this is true, it might not be detectable. Sorry to be thick headed, and I'm perfectly willing to accept that I might have imagined the more frequent injects on the shallow end of the 8/65 ascent, just making sure I understand... -Andy

Janos

Wrong.

Basic physics gives us: Pressure x Volume / Temperature is constant, irrespective of the gas. The psi would be identical, until you start getting up to 3000 or 4000 psi, where you'll notice a tiny, tiny difference.

Janos

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vulcan

I used the standards posted on their website here:

http://www.iantd.com/standards/IANTD_Standards_2003[1].pdf

I can't get the link to work, if you cut and paste this in to your browser you should get there.

on page 55 as my reference. They're dated 2003 so may not be current.

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dteubner

Exactly what Janos said. The densities of the gasses (which are quite different) do not have anything to do with the pressure/volume relationship. In your example the cylinder of air would be a bit heavier but have exactly the same pressure as the cylinder of helium.

The universal gas equation states that for an 'ideal gas'

PV=nRT

Where:
P = pressure of the gas
V= volume of the gas
n = quantity of gas present (expressed in moles)
R = gas constant whcih varies with the units used
T = temperature of the gas.

Only when the gas is not 'ideal' does this relationship break down. The gasses we are talking about depart from the 'ideal' a bit in our tanks but not our loops.

Dave T

dave t

yes those are the current ones, they state that dives must be done on normoxic trimix

silent running

Dave, thanks for going further with the explanation. I had trouble in tmix class with the ideal gas law and understanding anything much past gasses only behaving differently under very high pressures.

But I still wonder if there really were more injects on ascent from the dives with hypox dil than with air. Do we have a good way of knowing how much He we off-gass back into the loop? Assuming a high He dil, it would be more than any N2 we have taken up during descent-more going in and coming out faster than the smaller, slower amount of N2. So wouldn't off-gassing from a high He descent affect the PO2? I don't imagine it would take much to cause a few more injects during ascent, considering that injects occur as soon as PO2 falls even a little below SP. Wouldn't this be consistant with the general idea behind using He to cut down on deco obligations-comes out faster-and the move towords slowing ascent rates? -Andy

silent running

Oh, and sorry for the sideways hijack folks.