Isobaric Countertransport

[Phineas Gage]

That's the name of a section in the B. Wienke book I've been reading, where I came across this interesting tidbit...

Evidently, the warmth factor is only part of the reason for not using a light gas (i.e. He) in an enclosed exposure suit. For the most part the text is concerned with the saturation/desaturation effect of switching from heavy to a light gas (or vice versa in the desaturation case), but it also makes a point of noting that having a lighter gas in your drysuit than you are breathing can also lead to "skin lesions and vestibular dysfunctionality". Crazy.

Why is that? Has anyone here ever experienced that phenomena first hand? The only thing I can guess is that the gradient between small blood vessels close to the skin surface and the gas you are enclosed in is sufficient to induce microbubbles forming just under the skin?

Of course, if I'm reading this correctly, then that means there is a slight advantage to using a really dense gas like Argon in your exposure suit that goes beyond just the thermal benefit.

Oh, and pardon my naivity if this is old news. I'm still trying to grok it all.

[Joshua Smith]

I think you're probably right on the money with the pressure gradient stuff. But, to be honest- this super detailed molecular decompression stuff bores me to tears, and I have a feeling that the 6 guys in the world who understand the most about it are just taking educated guesses. And come to think of it, I did a dive once where I ran out of Argon, and switched to my bailout bottle, which had trimix in it, while I was using air for dilluent- and I surfaced with zero skin lesions. (The trimix was noticably colder than the argon, though!) But, to be fair, I only had a litle bit of deco on that dive, and I wouldn't want to be a guinea pig, and do something like a long, deep air dive with a drysuit full of pure Helium, for example.

[Phineas Gage]

Yeah, that's what I was thinking. If you were to do a dive with Helium in your drysuit and become saturated on Nitrogen, then maybe it would happen?

And yes, it is very dry stuff for the most part. Just digging the key points out of all the "extra" stuff can be a chore.

[CaptnJack]

For all practical purposes its not relevant since we are only talking 4 possible gases here, N2, O2, He and Ar. You would never inflate a drysuit with He since you might as well not be using a drysuit at that point. N2 and O2 being denser than He are obviously not even a theroretical problem.

Isobaric counter diffusion is kinda like the oxygen window. It probably exists but its not important in day to day measure with a micrometer cut with an axe decompression diving.

[Joshua Smith]

Isobaric counter diffusion is kinda like the oxygen window. It probably exists but its not important in day to day measure with a micrometer cut with an axe decompression diving.

I think we agree- but isn't "Isobaric Counter Diffusion"(ICD) similar, but seperate, from "Isobaric Counter Transport"(ICT)?

ICD is about switching from a high He breathing mix to a low/no He mix, and forcing He out of the body, causing a nasty inner ear hit, no?

And ICT is regarding skin/ very small veins and arterioles to offgas suddenly due to density of gas surrounding divers body, resulting in a skin bend, right?

[CaptnJack]

ICD is about switching from a high He breathing mix to a low/no He mix, and forcing He out of the body, causing a nasty inner ear hit, no?

Yeah same concepts but for all practical purposes they don't exist. I know I have done gas breaks from O2 onto 18/50 with no ill effects whatso ever. If you have a PFO switching to O2 with a heavy gas load still aboard can be a big deal. I don't have a PFO.

And ICT is regarding skin/ very small veins and arterioles to offgas suddenly due to density of gas surrounding divers body, resulting in a skin bend, right?

I think so. Basically having a large inert gas gradient across the skin. If you are breathing 18/50 and inflating with Ar that gradient is maxed out just like it would be if you inflated with N2 or good ol' air tho. The suit gas is at ambient pressure regardless and density is not really important for us (except from a thermal perspective). I <think> ICT is only an issue for saturation divers decompressing in a warm/heated chamber but still breathing trimix.

I agree that most of this is mumbo jumbo for our puny lil dives. 1,000ft saturation dive, albit in a bell/chamber is a different bailiwick with more unusual gas dynamics going on.

[Joshua Smith]

I agree that most of this is mumbo jumbo for our puny lil dives. 1,000ft saturation dive, albit in a bell/chamber is a different bailiwick with more unusual gas dynamics going on.

Lol- it kind of reminds me of reading about string theory, or something. Fascinating, at least the parts I can understand are. But it doesn't change a thing for me.