Recreational Trimix

[DiveNav]

...... I got rid of the thing and got a computer that recognizes the benefit of slow ascents and deep stops ...

and which one did you get ?

Alberto (aka eDiver)

 

[Chris Brown Tech]

I did a recreational double dip on the Spiegel Grove today. Decided to use my doubles which were already loaded up with 30/30. Figured displacing all that evil nitrogen with heavenly helium would give me nice, long no decompression limits. Did that ever turn out to be a stupid move!

As it turns out, 30% NITROX would have yielded a NDL of 26 minutes at 100' vs. 14 minutes on the relatively pricy trimix. My "stoke" buddies got a 28 minute NDL on their 32% Nitrox and I ended up doing a lot of deco time on my fancy back gas.

I'm fully aware that the primary benefit of increased helium percentages is reduced narcosis as opposed to increased NDL bottom time. However, I was shocked to discover that more helium actually dramatically reduced NDL limits all other things being held constant.

Can someone please explain this to me?

Im assuming that you used a computer and discovered this NDL whilst on the dive? If so, what computer were you using? My shearwater using GF for example, seems to give a lot more deco for shallow helium dives than other models. Also depends on what GF I have it set to.

This is why IANTD has a 28/15 mix table for Advanced Recreational Trimix. If you subscribe to the "time spent offgassing the helium on a shallow dive will exceed the benefit of inspiring less N2" approach, then its a good mix to use on those recreational / mini-deco dives on the Spiegel. I use it all the time in my classes. Run the numbers through Vplanner and you will see. More deco with higher helium mixes on those shallow dives.

Deeper & longer is where the benefit of inspiring less N2 due to helium content (and therefore incurring less deco) outweighs any "penalty' from using He and having the algorithm slow your ascent and add more time in to offgass it.

 

[MXGratefulDiver]

and which one did you get ?

Alberto (aka eDiver)

Liquivision X1 ...

... Bob (Grateful Diver)

 

[waterpirate]

GUE doesn't condone smoking, that's why we just drink it now .

Peace,
Greg

Finally someone who thought that was funny,
Eric

 

[Slamfire]

OTOH, I can point to mixed teams with 21/35 and 21% where the 21% diver has been bent. I'm very unconvinced that helium necessarily penalizes decompression.

Not picking on you, but what are possible reasons for helium not penalizing deco? I once heard somebody say [paraphrasing from memory] that "helium is not soluble therefore it is not as relevant for determining deco obligations (ie. not penalizing deco?). And we know helium is not soluble because we don't get narced on helium. This is similar to sugar, water and sand. Sugar is soluble in water, just like nitrogen is soluble in the body. We know nitrogen is soluble because we get narced with nitrogen. We should pay more attention to nitrogen when it comes to deco. Sand is not soluble in water. It does not matter how long you stir sand in water, it will not dissolve. Helium is like that in your body."

Frankly, that whole explanation really turned me off. First of all, you cannot measure solubility using narcosis parameters. Correlation is not causation. And even if you still wanted to, how do you measure narcosis? What are the units? Secondly, it is not accurate to say helium is not soluble. It should not be treated as an absolute because it is not. Helium is simply less soluble than nitrogen.

From Bruce Wienke, Technical Diving in Depth, Reduced Gradient Bubble Model (RGBM) In Depth: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.

Then I read an explanation like Bruce's. It does give you measured parameters. It may not be any more accurate at reflecting what really happens inside the human body, but, at least for my taste, it does provide an explanation that is more concrete. It does not lend itself as easily to misinterpret the role of helium in decompression as one that is totally irrelevant. Helium is relevant. You may dive 30/30 just like you dive 32%, simply because you have a diving protocol that does apply to both mixes. However good that diving protocol is, not everybody dives it. If there where no explanation about Min deco becoming mandatory deco it could easily be extrapolated that the PADI RDP table for EAN32 is the same as a table for 30/30.

Deeper & longer is where the benefit of inspiring less N2 due to helium content (and therefore incurring less deco) outweighs any "penalty' from using He and having the algorithm slow your ascent and add more time in to offgass it.

I've often thought that the off-putting explanation may have gone that way as an effort to oversimplify things and make an explanation easy to be understood. Well, Chris said it simply enough without having to resort to inaccuracies.

 

[Blackwood]

Solubility of Gases in Water

At body temperature, N2 is about 14% more soluble in water than He. Not sure whether that has anything to do with human fizziology.

Further, Lawrence, et. all considered Helium to be negligably soluble in fat lipids.

Other tissues? Shrug.

 

[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.

 

[lamont]

Not picking on you, but what are possible reasons for helium not penalizing deco?

I don't really care for theoreticians, I'm an experimentalist. There's an entire agency of people diving helium mixes with the same overall decompression time as nitrox mixes, and it works. If your theory can't explain the phenomena, then the theory needs to be thrown out. Deep stops worked for Pyle long before anyone could explain why he felt better after dives he used deep stops on -- reality is whatever it is, and it doesn't care if humans have a validated theory about it or not.

You answered your own question, however, by quoting BRW. That sounds like the right direction.

If there where no explanation about Min deco becoming mandatory deco it could easily be extrapolated that the PADI RDP table for EAN32 is the same as a table for 30/30.

Except I specifically called out that you needed to do minimum deco with 30/30, called out what the minimum time was, and it was in the context of talking about a diver executing a decompression dive. If you simply read what I wrote it would not be "easy to extrapolate" to anything.

I've often thought that the off-putting explanation may have gone that way as an effort to oversimplify things and make an explanation easy to be understood. Well, Chris said it simply enough without having to resort to inaccuracies.

Not a single thing I wrote was inaccurate.

 

[Slamfire]

.. ..

Helium is relevant for deco planning.

You do refer to the relevancy of helium when you mention min deco becomes mandatory. I want to be more direct for the sake of those that may not be familiar with min deco protocols. After all, this is not the DIR forum.

Treating helium as irrelevant can be dangerous. Just saying, "dive it like 32%" implies it is irrelevant (but, again, you referred to min deco becoming mandatory).

You express your doubts about "helium penalizing" and you mention that you know of mixed teams were the air diver got bent but the 21/35 divers didn't. Again, to the DIR uneducated this may seem as implying helium is irrelevant, though that may not be your intention. I could easily counter saying that I know of divers that got "helium bent", indirectly implying that if they hadn't use helium, they would have been fine -- but that would be misleading. And in any case our arguments would not be statistically significant.

The reality is more related to the fact that by properly applying DIR protocols (ie. different ascent speeds for different segments of the dive, usage of specific standard gases for specific missions, etc.) the repercussions of helium interactions are de facto taken care off, leaving nitrogen as your main concern. Unfortunately, this is not the DIR forum and the OP and many readers may not be aware of DIR protocols.

 

[Slamfire]

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.

Notice that neon has a low mass transport coefficient, almost 2 times smaller than helium. We should all be diving neox. Or at least quadmix (Ne, He, O₂, N₂). We are all such nickel rockets... I kid, I kid.

On a more serious note, thanks for posting this, Kev. Very interesting, concretely measured information.