79%Helium - 21% O2 blending? Why not?

[lamont]

Helium has a smaller effective molecular diameter than O2, or N2, but that's not really why it has different thermal properties. But, you need the O2 whether the 'inert' portion of the mix is N2, He, or some mixture of the two, so you can't get away from heat issues associated with O2 in the mix.

The thermal properties go back to whether the molecular structure of a gas is monatomic, diatomic, etc. -which is why thermally, O2 and N2 act similarly in this respect - and different from He or say Ar (physical chemistry and/or thermodynamics classes). The effective molecular diameter affects permeability and diffusivity if I remember correctly, but not directly related to thermal properties. Molecular weight, however, does have some relation to thermal properties and helium atoms are lighter than oxygen or nitrogen atoms, much less monatomic He being lighter than diatomic O2 or N2. But, He will permeate faster than N2 or O2.

Yeah, the heat capacity of He is lower than N2 or O2 because N2 and O2 have rotational and vibrational degrees of freedom in addition to translational (kinetic) degrees of freedom. Its been about 6 years for me now, but IIRC, you get 1/2kT for each degree of freedom, so for He you've got E = 3/2 kT and for N2 and O2 you've got E = 3/2 kT (translational) + 1/2 kT (vibrational) + 2/2 kT (rotational) = 3 kT. So the heat capacity of N2 and O2 should be roughly double He.

 

[WarmWaterDiver]

The way I remember it was as a multiplier to some power of R, the universal gas constant, with some other stuff in the equation, but monatomic was 1/2, diatomic was 3/2, etc. but to get the exact equations I'd have to go back to the textbooks and they're on the shelf at work, and I'm trying to spend less time there (hence dive vacation soon!). Been more like 15 years for me now - but you're right, the absolute root is degrees of freedom and molecular weight.

You had it in post#27, but lost me on discussing diameter vs. thermal properties in post #39. The diffusivity and permeability are definitely related to effective molecular diameter, and hence it is centrally related to on-gassing and off-gassing of inert gases in living tissues, among other things.

Atomic (ionic) hydrogen is the worst actor for high permeability especially where that's not desireable (hydrogen blistering) - the deep mixes using molecular H2 as an inert gas must have a more rigorous inspection and testing program than regular scuba.

 

[ScubaDadMiami]

When you take your trimix course, you will learn that there is a "sweet spot" for different mixes as far as decompression obligations go. The variables, besides the mix, include the depth and the time.

Forgetting the oxygen toxicity issue for the moment, if you are going on a short dive to significant depth (other than the narcotic issue with regard to nitrogen), using helium for such a dive would actually increase your decompression obligation. This is because Helium diffuses much faster (about 2.65 times faster) than nitrogen into your tissues while, at the same time, your tissues can hold about three to five times more nitrogen than helium in them before becoming saturated. So, the helium rushes in and reaches saturation quickly compared to nitrogen on these kinds of dives. One gas is not the same as the others when it comes to the maximum amount the tissues can take before reaching saturation nor how fast the gas goes into the tissues.

When you start to increase time, eventually, either gas will reach saturation. Since your tissues hold less helium before reaching saturation, and because the helium will diffuse out more quickly than nitrogen, your decompression time will start to be less with helium mixes. It is a balancing act between depth and time.

A good depth to work this out with is about 150 feet. This is a depth where the old deep air divers have dived (I would never do this anymore; I am just talking academics here) without being totally blitzed with narcosis. For short durations, you will actually get out of the water sooner on air or with low amounts of helium in the mix (just enough to ward off narcosis). Start to stay longer, and you will get out of the water faster by increasing the helium in the mix. Deco time for air versus trimix at this depth converges at about one to two hours. Longer than this and helium becomes more efficient. On shorter and shallower dives, decompression is shortest on air, followed by trimix, followed by heliox.

One last however. Add back in the narcotic effect of nitrogen, and it becomes clear that it is better to do a longer decompression schedule without narcosis. Add to this the issue of oxygen toxicity on the deeper stuff, and you start to see that helium takes care of both of these issues at the small cost of a little time and thicker thermal underwear. It's worth it!

Helium is about six times more thermally conductive than air. It used to be thought that breathing it would actually hasten chilling. However, respiratory heat loss is actually LESS with Helium than with air. Because Helium is much less dense than air, it does not have the ability to draw out the body heat during respiration that was once believed. On the other hand, it makes a dandy air conditioner if you put it into your dry suit. So, argon in the suit (or even just air) takes care of the chilling problem while breathing helium mixes really doesn't do much in the way of chilling the diver.

 

[chrisch]

.... while, at the same time, your tissues can hold about three to five times more nitrogen than helium in them before becoming saturated. ....

Ehh?? ***???

Explain. :11:

Chris

 

[lamont]

Yeah, I was goofing around with v-planner awhile ago and I ran a profile with 21/35 and then did one with 21/79 and got something like 2-3x the deco. That was kind of interesting. I also managed to trip an IBCD alarm for high ppN2 by cranking up the He in the bottom mix. I'm definitely drawing up a list of questions for a trimix course...

 

[ScubaDadMiami]

Ehh?? ***???

Explain. :11:

Chris

Helium diffuses quickly into your tissues but your tissues can hold much less of it, compared to nitrogen, before reaching saturation. So, helium goes in quickly and reaches saturation quickly. This results in longer deco for shorter dives compared to nitrogen mixes.

Nitrogen goes in slower but your tissues can hold more of it before they are fully saturated. As a result, on "bounce dives," you will incur less deco obligation with a nitrogen mix than you will with a helium mix.

Helium also offgases faster than nitrogen. The deeper you go and the longer you stay, Helium becomes more efficient, and it actually gets you out of the water sooner.

 

[Charlie99]

Ehh?? ***???

Explain. :11:

Chris

Bruce Wienke, "Mr. RGBM", has also discussed this topic. http://www.scubaboard.com/showthread.php?p=427565&highlight=mass+transport#post427565

To reduce ScubaDadMiami's post to just a sentence ..... High helium mixes have the shortest deco for long deep dives, but for shorter, shallower dives high helium mixes will increase deco as compared to nitrox.

The depth-time crossover point is sometimes called the "mass transport limit".

 

[ScubaDadMiami]

The depth-time crossover point is sometimes called the "mass transport limit".

Please supply that reference. I've not heard it used before. Thanks.

 

[Charlie99]

Please supply that reference. I've not heard it used before. Thanks.

See the link in my previous post. It's BRW's reply to my request for further explanation of his use of the term, "mass transport limit"

"For short,
shallow exposures, the slower diffusivity
of N2 results in less total gas buildup in bulk
tissue, and hence shorter NDLs than He. For
longer and deeper exposures, He wins because
of its lower solubility. The point where
He and N2 NDLs meet, or cross over, is called
the "mass transport boundary"." and

"In the region 100 -140 fsw, and beyond air
NDLs, the use of HELITROX (enriched heliair)
is really advantageous from the point of view
of staging, repets, and hang time. In that
regime, you are past the mass transport limit
point for N2, and optimizing deco with He. "

The thread has some other interesting comments and observations on "Differences in "quality" of inert gas saturation - He vs. N2"

 

[ScubaDadMiami]

Thanks. I have read his stuff extensively though I don't recall seeing the term before to describe it. Then, again, who can understand anything he writes anyway? :06: