Deco For Divers, Ed 2, And Speed Of Helium Off-gas

[Jax]

Mark Powell's Deco for Divers was so good, I bought the second edition. I've noticed quite a few updates, but one is not addressed.

In the Trimix section, Helium is stilled called a "fast gas" and that it's smaller modules will go in and out of tissues faster.

Recently, there was a report of tests with sheep that show the diffusion of Nitrogen and Helium from the cells is actually a constant, so the Helium really isn't "fast" and the off-gassing of Helium is about the same as Nitrogen.

Does anyone have further information on this?

 

[rongoodman]

My understanding has always been that He is "faster", but less soluble than N2, so there is less of it to off and on-gas.

 

[rjack321]

You mean this?
ARTICLES | Journal of Applied Physiology

Was only published last year, and only applies to brains and hind limbs of sheep. No human trials, and hence nobody has altered their VPM, Buhlmann or RGBM parameters to accommodate. Although lots of people decide they know better than their computer or tables (for reasons exactly like this one) and game their algorithms in various ways.

 

[Jax]

You mean this?
ARTICLES | Journal of Applied Physiology

Was only published last year, and only applies to brains and hind limbs of sheep. No human trials, and hence nobody has altered their VPM, Buhlmann or RGBM parameters to accommodate. Although lots of people decide they know better than their computer or tables (for reasons exactly like this one) and game their algorithms in various ways.

Yes, that is it, thank you. March 01, 2015.

I was wondering if anything further came from it.

 

[Kevrumbo]

Just another challenge to the Deep Stops Theory, specically deco coming up from operational depth on a high He mix. Because of Helium's higher diffusivity and lesser solubility than Nitrogen, the idea was to keep it in dissolved phase in venous blood longer with deepstops, preventing the formation of numerous "showers" of pathological microbubbles causing type II or chokes symptoms upon surfacing.

The study above seems to imply that the off-gassing rates of Nitrogen and Helium from tissue are the same.

 

[Jax]

Just another challenge to the Deep Stops Theory, specically deco coming up from operational depth on a high He mix. Because of Helium's higher diffusivity and lesser solubility than Nitrogen, the idea was to keep it in dissolved phase in venous blood longer with deepstops, preventing the formation numerous "showers" of pathological microbubbles causing type II or chokes symptoms upon surfacing.

The study above seems to imply that the off-gassing rates of Nitrogen and Helium from tissue are the same.

This is something I have always wondered about. Gases diffuse without regards to the density or molecular weight of the other gases. So, when you have blood cells in blood moving around the body at the same speed, how would the albeit smaller particles move out quicker? The only thing that should move gas "quicker" would be a higher partial pressure. The partial pressures don't change.

 

[dmaziuk]

This is something I have always wondered about. Gases diffuse without regards to the density or molecular weight of the other gases. So, when you have blood cells in blood moving around the body at the same speed, how would the albeit smaller particles move out quicker?

E.g. the smaller physical size should make it easier/faster to get through the cellular membranes. Of course for that to matter, the membranes must be the limiting factor.

 

[Kevrumbo]

This is something I have always wondered about. Gases diffuse without regards to the density or molecular weight of the other gases. So, when you have blood cells in blood moving around the body at the same speed, how would the albeit smaller particles move out quicker? The only thing that should move gas "quicker" would be a higher partial pressure. The partial pressures don't change.

E.g. the smaller physical size should make it easier/faster to get through the cellular membranes. Of course for that to matter, the membranes must be the limiting factor.

But Deep Stop/Dual Phase Bubble Model Theory also postulates about Helium's greater diffusivity, lesser blood/tissue solubility, and potential affinity to occupy free phase micronuclei/pre-cursor "bubble seeds", caused by tribonucleation. Hence holding a deep stop to keep the surface tension & internal pressure of these bubble seeds high to prevent He from diffusing into them, as well as keeping He in dissolved state longer in tissue --> dissolved state venous blood for return to the lungs for more efficient inert washout. In other words, you don't want bubble formation in fast tissue (type II DCS), or bubbles in blood returning to the lungs (the "chokes"). . .

 

[StefinSB]

But Deep Stop/Dual Phase Bubble Model Theory also postulates about Helium's greater diffusivity, lesser blood/tissue solubility, and potential affinity to occupy free phase micronuclei/pre-cursor "bubble seeds", caused by tribonucleation. Hence holding a deep stop to keep the surface tension & internal pressure of these bubble seeds high to prevent He from diffusing into them, as well as keeping He in dissolved state longer in tissue --> dissolved state venous blood for return to the lungs for more efficient inert washout. In other words, you don't want bubble formation in fast tissue (type II DCS), or bubbles in blood returning to the lungs (the "chokes"). . .

Hi Kev. I am not sure if I understand the first part correctly. Are you saying that thermodynamics favor tribonucleation of He bubbles when compared to N2 for example? If so do you have any reference paper for that? Thanks.

 

[Kevrumbo]

Hi Kev. I am not sure if I understand the first part correctly. Are you saying that thermodynamics favor tribonucleation of He bubbles when compared to N2 for example? If so do you have any reference paper for that? Thanks.

Diving and Subaquatic Medicine, Fifth Edition

Science of Diving

Dual Phase Bubble Theory in general says you just have random free phase bubble seeds formed by tribonucleation, at critical radius and surface tension, with the potential to be occupied by Helium coming out of solution earlier than that Nitrogen because of Helium's greater diffusivity and lesser solubility.

Conversely in terms of shallow recreational NDL's , because of Helium's greater diffusivity and lesser solubility, you have less NDL times than you would with Air or Nitrox. Again it's these fast He gas particles looking for these bubble seeds to diffuse into, rather than being dissolved in blood/tissue readily like Nitrogen because of N2's greater solubility. . .

"Helium and nitrogen trade off for NDLs depending
on depth and time. For most NDLs in the shallow
zone, nitrogen is better. For deeper exposures
helium is better. For deep and deco diving,
helium wins, especially when coupled to
strategic mix switching on the way up. Overall
nitrogen is not the divers friend, even though
it wins the recreational NDL contest.

It's a matter of mass transport coefficients,
that is, solubility multiplied by diffusivity.
And the amount of gas stored in tissue/blood
safely, per unit volume. Both can be estimated
from sat experiments and reported data.

For shallow and short exposures, slow diffusing
nitrogen allows longer NDLs (than helium).
While for longer and deeper exposures, rather
insoluble helium wins the NDL race (longer
NDLs than nitrogen). . ."
Helium offgassing rate