Helium the Perfect Diver's Gas

[Kevrumbo]

Bruce Wienke (RGBM) wrote:

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.

http://www.scubaboard.com/forums/3439114-post14.html

 

[DCBC]

Thanks Kevrumbo. Wienke and O’Leary went on to say:

The size of bubbles formed with various inert gases depends upon the amount of gas dissolved, and hence the solubilities. Higher gas solubilities promote bigger bubbles. He went on to list the Inert Gas Solubilities as follows:

Helium Nitrogen Oxygen
Solubility Blood .0087 .0122 .0241
Solubility Oil .0150 .0570 .1220

 

[hobodiver]

Its not due to chance, the smaller bubble size of helium is fairly well established. "Proving" why is not so simple since many there are too many variables with N2 and no way to eliminate those variable.

Most plausible reasoning: Lower solubility = fewer moles of gas = smaller bubbles. Or possibly because helium is a true inert gas and can't actually dissolve like (e.g.) salts dissolve into a polar liquid like water.

I wasn't saying it was due to chance. I was merely pointing out that if you hypothesize that helium will produce smaller bubbles because Helium is a smaller atom, and then you observe it producing smaller bubbles, that doesn't mean it's because the atoms are smaller.

 

[ianr33]

Helium Nitrogen Oxygen
Solubility Blood .0087 .0122 .0241
Solubility Oil .0150 .0570 .1220

So helium has only around 70% the solubility of nitrogen in blood and 26% the solubility in oil. Narcotic potential is supposed to be related to lipid solubility . Do you have a figure for that?

Assuming Helium has significant/measurable solubility in lipids why is it considered to be non narcotic?

Put another way,if you dive 4/96 to a depth of 1000 feet will you get narked? just something I've always wondered.

 

[Blackwood]

So helium has only around 70% the solubility of nitrogen in blood and 26% the solubility in oil. Narcotic potential is supposed to be related to lipid solubility . Do you have a figure for that?

Assuming Helium has significant/measurable solubility in lipids why is it considered to be non narcotic?

Solubility isn't the only contributing factor. If a non-narcotic substance (like, I don't know... sugar) dissolves into your tissues, it won't have a narcotic effect.

 

[rjack321]

So helium has only around 70% the solubility of nitrogen in blood and 26% the solubility in oil. Narcotic potential is supposed to be related to lipid solubility . Do you have a figure for that?

Assuming Helium has significant/measurable solubility in lipids why is it considered to be non narcotic?

Narcosis is not due to the solubility directly, there is just a rough correlation between solubility and narcotic potential. Inert gases like N2 are considered to interfer with the lipids in cell membranes. Just because helium "dissolves" (using that term a bit loosely) in lipids doesn't mean that it is actually capable of interfering with the lipids in cell membranes.

Put another way,if you dive 4/96 to a depth of 1000 feet will you get narked? just something I've always wondered.

Commercial divers actually add tiny amounts of N2 back in to avoid HPNS at these kinds of depths (the only ones doing this dive). But otherwise narcosis is minimal.

 

[M_Bipartitus]

Most plausible reasoning: Lower solubility = fewer moles of gas = smaller bubbles. Or possibly because helium is a true inert gas and can't actually dissolve like (e.g.) salts dissolve into a polar liquid like water.

I agree with this as the ideal gas law indicates that, for a given pressure and temperature, the volume will be related to moles, not atomic or molecular size. The ideal gas law should apply with only very small errors at any temperature and pressure the human body can take and helium is the most ideal of gasses. With fewer moles of dissolved gas the bubble will be smaller.

 

[DCBC]

Commercial divers actually add tiny amounts of N2 back in to avoid HPNS at these kinds of depths (the only ones doing this dive). But otherwise narcosis is minimal.

Yes this is true with Trimix dives, but not with Heliox. HPNS is controlled by slow compression and by including "compression stops." Depending upon the saturation depth, this can take as long as one or two days.

 

[boomx5]

Yes this is true with Trimix dives, but not with Heliox. HPNS is controlled by slow compression and by including "compression stops." Depending upon the saturation depth, this can take as long as one or two days.

I'm curious...have you ever experienced HPNS yourself or witnessed someone else deal with it?

 

[Kevrumbo]

"The Meyer-Overton hypothesis states that narcosis happens when the gas penetrates the lipids of the brain's nerve cells. Here it apparently interferes with the transmission of signals from one nerve cell to another. Exposure to nitrogen-oxygen mixture at high pressure induces narcosis, which can be considered as a first step toward general anesthesia. . . and narcotic potencies of inert gases are attributed to their lipid solubility." (see PADI Encyclopedia of Recreational Diving Ch.5/p22)

Of interesting empirical note, from Wienke BUBBLE MODELS AND DECOMPRESSION COMPUTATIONS:
A REVIEW p.34:

To track gas transfer across bubble boundaries, we need mass transport coefficients . . . Table 4 lists [mass transport coefficients] for the same lipid-aqueous surfaces, using Eisenberg [28], Frenkel [33], and Bennett and Elliot [10]

Table 4. RGBM Mass Transfer Coefficients. . .

Gas (μm2/sec fsw)
Ne 10.1 × 10−6
He 18.4 × 10−6
Ar 40.7 × 10−6
O2 41.3 × 10−6
N2 56.9 × 10−6
H2 72.5 × 10−6

Notice that helium has a low mass transport coefficient, some 3 times smaller than nitrogen.