Helium offgassing rate

[ianr33]

Dr. Deco
What is the exact efficient ratio of oxygen over air while decompressing? I have been told the efficientcy ratio is 1.41:1. I have heard of other figures bouncing around and figured you would give us the straight scoop.

100% O2 will speed up deco by a factor of about 3 compared to air. 50% speeds it up by a factor of 2.

No theoretical basis behind those figures just an observation from spending hours playing around with V-Planner. Its pretty amazing how well those factors of 3 and 2 work,certainly good enough to "wing" deco if you ever needed to.

 

[G~Diver]

G~Diver, the misunderstanding is don't interpret solubility versus diffusivity/on-gas/off-gass half times as diametrically opposed constructs in this instance; there is a dynamic of both going on with regards to the physical properties of Helium and Nitrogen in the Mass Transport/Biomass NDL's Paradigm. . .

BRW post#9
BRW post#4

I still don't get it. according to Dr. Deco solubility is the most dominant factor for most of dives with Helium. if some gas has zero solubility the on-gassing time would be infinite > infinite half time.
Even if it's a dynamic process, as long as solubility is the dominant factor I would expect the half times for He to be longer then N2.

 

[G~Diver]

I still don't get it. according to Dr. Deco solubility is the most dominant factor for most of the dives with Helium. if some gas has zero solubility the on-gassing time would be infinite > infinite half time.
Even if it's a dynamic process, as long as solubility is the dominant factor I would expect the half times for He to be longer then N2.

If Helium is less soluble then N2 how come the half times of He are 2.65 faster then N2 - as I think of it, if it less soluble it will take Helium longer time to dissolved , longer time to get to saturation > > longer half times.
Where is my mistake?

Dr. Deco - Can you please answer it ? this issue is bothering my mind..

 

[bleeb]

if some gas has zero solubility the on-gassing time would be infinite > infinite half time.

Or zero. If a gas has zero (or very low) solubility in a medium, it takes zero time for that medium to reach saturation, i.e. as much dissolved gas as it can hold.

You might also want to have a look at this thread: http://www.scubaboard.com/forums/technical-diving-specialties/224715-calculating-deco-mix.html
It's a bit technical (no pun intended), so it may take a bit of effort on your part to understand it if you don't have the background, but I think most of the information you're asking for is there.

 

[Kevrumbo]

Again to recite Bruce Wienke:

Helium NDLs are actually shorter than nitrogen for shallow exposures . . . Reasons for this stem from kinetic versus solubility properties of helium and nitrogen, and go away as exposures extend beyond 150 fsw, and times extend beyond 40 min or so.

Helium ingasses and outgasses 2.7 times faster than nitrogen, but nitrogen is 1.5 to 3.3 times more soluble in body aqueous and lipid tissue than helium. For short exposures (bounce and shallow), the faster diffusion rate of helium is more important in gas buildup than solubility, and shorter NDLs than nitrogen result. For long bottom times (deco and extended range), the lesser solubility of helium is a dominant factor in gas buildup, and helium outperforms nitrogen for staging. Thus, deep implies helium bottom and stage gas. Said another way, transient diving favors nitrogen while steady state diving favors helium as a breathing gas.

My conceptual take on this is that the Helium of a Trimix breathing gas is still saturating the blood plasma and being transported, along with a concentration of N2 being dissolved in solution because of its greater solubility. Through blood perfusion and gas diffusion, the He and N2 make it past the capillaries into the interstitial fluid, where N2 is more readily taken up by the cells/tissues, again by its greater solubility. However, due to Helium's greater diffusivity, it saturates the interstitial spaces between cells/tissues faster than the N2 being absorbed by the cells/tissues. It is this critical saturation level that results in the shorter NDL time for shallow recreational dives --not from absorbed Helium or N2 in Tissue Compartments for a prescribed M-Value per se-- but from all this free Helium in the interstitial fluid and blood plasma, looking for some totally random, free-phase and idiopathic "Bubble-seeds or micronuclei" to diffuse quickly into, and potentially causing a Boyle's Law expansion of the bubbles on ascent resulting in a DCS hit.

 

[TSandM]

Really, it's fairly simple. Solubility determines the total number of gas molecules that can occupy a given volume, for example, a cc of blood. Diffusivity determines how QUICKLY that number of molecules can get in there. Helium diffuses very quickly, so the total number of permitted molecules will accumulate fast. But there won't be as many of them in a given volume as there are nitrogen molecules.

Imagine an elevator. There's a sign above it that says, "Men - 12; women - 3." The door opens. 12 men can get in, but they walk slowly, so it takes a full minute to fill the car. Only three women can get in, but they're moving fast, so it takes 10 seconds to get all of them in.

 

[limeyx]

Imagine an elevator. There's a sign above it that says, "Men - 12; women - 3." The door opens. 12 men can get in, but they walk slowly, so it takes a full minute to fill the car. Only three women can get in, but they're moving fast, so it takes 10 seconds to get all of them in.

This can't be right, I've never met the woman that can do anything in 10 seconds, let alone 3

 

[Kevrumbo]

Geez Nick, now you've really opened yourself up for attack now! To butcher a William Congreve quote, "Hell hath no fury than [Women being made fun of by Men]. . ."

Anyway, the above explanation is also what I think was going on when I got the chokes after repetitive 30/30 Triox Dives, NDL/Min Deco Algorithm & Rules on bottom gas only (i.e. no Oxygen or Nitrox 50% deco gas) . . .I still had too much residual Helium in numerous microbubbles that overwhelmed my lungs' alveolar bed. . .

 

[Charlie99]

Anyway, the above explanation is also what I think was going on when I got the chokes after repetitive 30/30 Triox Dives, NDL/Min Deco Algorithm & Rules on bottom gas only (i.e. no Oxygen or Nitrox 50% deco gas) . . .I still had too much residual Helium in numerous microbubbles that overwhelmed my lungs' alveolar bed. . .

The situation you describe is one where the "oxygen window" as I understand it would have made a difference. There is a lot of different stuff that gets thrown around about the oxygen window, but one of the effects of having lower total venous pressure is that bubble formation in the venous system is less. My take on the oxygen window is that it doesn't enhance offgassing, but it will reduce venous bubbles. Ordinarily that doesn't really make that much difference except in a few cases like yours where massive venous bubbling overwhelms the lungs.

This just goes along with the general rule that bubble formation is driven by total pressures, offgassing/diffusion of dissolved gas is driven by partial pressures/concentration gradients.

 

[Kevrumbo]

Thanks Charlie . . .I now always take at least some leftover Oxygen in an AL40 when doing repetitive NDL Triox Dives, and breath it at the safety stop on the last (or next-to-last) dive of the day. . .