Counterdiffusion (high-nitrogen-content gas after high-helium-content gas)

[Karel_de_Goeiste]

What are your toughts on (Isobaric) counterdiffusion? Or more specific: diving a high-nitrogen-content gas after a high-helium-content gas? So for example:
- During the same dive: switching from trimix (or heliox) to a high-nitrogen-content gas for decompression AND/ OR
- A repetitive dive: a (shallow) air or nitrox dive soon (lets say a couple of hours) after a (deep) trimix dive.

The theory behind this: "Helium has a diffusivity that’s approximately 2.65 times that of nitrogen (Lambertson, 1989), and because of that disparity it can diffuse into the skin quickly while nitrogen diffuses more slowly. The slow diffusion of nitrogen from the fluids and tissues of the body while the helium saturates the skin can cause supersaturation in some superficial tissues that can result in gas bubble formation" ... "These bubbles can cause itching followed by joint pain and have been more recently associated with inner-ear decompression sickness, although the bubble formation could contribute to other types of decompression sickness as well." - Isobaric Counterdiffusion in the Real World

Do you have experience with these kind of dives? And if so: did you experience any (DCS) problems?

 

[inquis]

because of that disparity it can diffuse into the skin quickly while nitrogen diffuses more slowly

You're missing the fact the He diffuses OUT of the skin more quickly than nitrogen enters, so there are no IBD issues for such tissues.

The only area for concern is the inner ear, which has He diffusing into it from the fluid behind it as well as the N2. The numerous threads where this is discussed all conclude it's not an issue while keeping a reasonable END. (E.g., don't switch to air at 200+ ft.) The switch from bottom (or deep deco) trimix to EAN50 at 70 ft is done thousands of times every year.

EDIT: after reading the article again, you may have been asking about the case where your suit inflation gas contains He. The solution is to simply not do that and use a dedicated suit inflation bottle with air (or possibly argon). My above/original comments assume this is the case.

 

[Karel_de_Goeiste]

You're missing the fact the He diffuses OUT of the skin more quickly than nitrogen enters, so there are no IBD issues for such tissues.

The only area for concern is the inner ear, which has He diffusing into it from the fluid behind it as well as the N2. The numerous threads where this is discussed all conclude it's not an issue while keeping a reasonable END. (E.g., don't switch to air at 200+ ft.) The switch from bottom (or deep deco) trimix to EAN50 at 70 ft is done thousands of times every year.

EDIT: after reading the article again, you may have been asking about the case where your suit inflation gas contains He. The solution is to simply not do that and use a dedicated suit inflation bottle with air (or possibly argon). My above/original comments assume this is the case.

Thanks for your reply! Yes, the article does mention possible risks specifically related to suit inflation, but my question is not specifically related to that.

​

 

[Wibble]

How deep are you thinking about?

Received wisdom appears to be that for moderate depths, say ~75m/250ft, ICBD is not an issue. It becomes a concern for much deeper dives .

 

[Capt Jim Wyatt]

When we do deeper trimix dives like 180 or deeper we never use Helium as an inflation gas for our dry suits due to this isobaric counter diffusion issue.

 

[Raphus]

When we do deeper trimix dives like 180 or deeper we never use Helium as an inflation gas for our dry suits due to this isobaric counter diffusion issue.

Really? Never heard of that reason.
I don't use it because of the price and the bad isolation characteristics.

Icbd is a problem for deeper dives, with high helium contents. A reason, why for deep dives the 50 nitrox stage has 20 % helium added. Then the change from helium to nitrogen partial pressure is reduced

 

[Matan]

What are your thoughts on (Isobaric) counter diffusion?

a lovely theory which I never cared and never will care about in any dive I'll probably do in my lifetime.
and I have shifted like so many other divers from 70% of helium to nothing - I'm still here.
is this theory real - yes. practical for 99.999999% of all dives - IMHO no.

When we do deeper trimix dives like 180 or deeper we never use Helium as an inflation gas for our dry suits due to this isobaric counter diffusion issue.

OR perhaps it's just a total waste of money to inflate a drysuit with a gas that costs more and will freeze you like a popsicle

 

[Capt Jim Wyatt]

OR perhaps it's just a total waste of money to inflate a drysuit with a gas that costs more and will freeze you like a popsicle

Hah--maybe that is it!!

I *think* there is some validity to the IBCD issue but I cannot state what it is. Its one of those things I heard about and believed...maybe.

 

[Nick_Radov]

When we do deeper trimix dives like 180 or deeper we never use Helium as an inflation gas for our dry suits due to this isobaric counter diffusion issue.

Why would anyone ever inflate with helium in the first place??? Never heard of that.

 

[OTF]

Why would anyone ever inflate with helium in the first place??? Never heard of that.

I'm not recommending it, but the reason this can be tempting is that using a separate suit inflation bottle adds another (small) tank+reg to carry on dives when you're already feeling overburdened with crap. Trimix is a terrible suit gas though - helium is a noticeably poorer insulator than air.