Helium Fraction and Standardized Gases

[lamont]

Again, the simple logical means to an end --if you're trying to off-gas Nitrogen loading from your bottom mix, why are you switching to a intermediate "standardized deco gas" with significantly more Nitrogen than your bottom mix???Intuitively, if you can eliminate possible factors that can preclude a DCS hit (even rare but always seriously acute Inner Ear DCS) wouldn't you sensibly do so?

Seriously, go talk to JJ. He was basically the crash test dummy for all those theories. Since he's obviously an idiot, you should go point out to him that GUE standardized gasses are violating those rules in that article from 1999 and they're going to get him bent. Make sure to mention to him that the WKPP has disproven the entire idea of standardized gases and that he needs to start teaching "best mix".

See how far you get with that...

 

[Kevrumbo]

Because I always enjoy a good debate, the gases listed in the very article you site are only a few percentage points off from the GUE standard gases. Below is from your beloved article, sections in italics and parentheses are my words:

"trimix 25 / 35 at 190'/58m (21/35 is 190 deco gas. A whopping 4% less oxygen, but you also are on a 1.6 deep with 25%. Completely unsurvivable if you have a toxic episode); trimix 35 / 25 at 120'/36m (35/25 is 120 gas); trimix 50 / 15 at 70'/21m (yup); 100 percent oxygen at 28'/8.6m [in a dry habitat], with periodic breaks using trimix 15 / 45."

I don't think anyone is using 240 deco gas (18/45) for a 280-300 profile as it doesn't provide much benefit vs the cost/ risk of bringing the bottle.

So what, exactly, are you arguing for? What gases would you use? In one breath you're chastising the gases, and in another you're celebrating an article that advocates what you are so against in the previous breath. Are you arguing for introducing 240 gas on all 280-300' dives? That's old news, and there are a ton of dives that show that its not really needed. If you don't need it, don't take it...

______________________

The original post #1 again with additional context:

Originally Posted by boulderjohn
What's the deepest deco gas you would use on a 300 foot dive?
Originally Posted by PfcAJ
Depends on the exposure. 120 or 190 deco gas (35/25 or 21/35).
Originally Posted by johnkendall
Generally 21/35, I tend not to use 35/25 until the exposure is long enough that the 36-24m stops get longer than about 3-4 mins.
Or if I'm in a cave where the profile requires more gas in that region.

Why do you use an intermediate deco gas (21/35) that has a higher fN2 than your bottom mix (12/60 or 10/70 trimix in this case, dive to 90m/300')?

---->(i.g. 12/60 or 10/70 bottom mix have a fN2 of 28% and 20% respectively, while 21/35 intermediate trimix deco gas has an fN2 of 44% --why are you switching to a deco gas that has more Nitrogen percentage wise, than that of your bottom mix???

Again, the simple logical means to an end --if you're trying to off-gas Nitrogen loading from your bottom mix, why are you switching to a intermediate "standardized deco gas" with significantly more Nitrogen than your bottom mix??? Intuitively, if you can eliminate possible factors that can preclude a DCS hit (even rare but always seriously acute Inner Ear DCS) wouldn't you sensibly do so?

Think about it (and this is exactly what I'm arguing for)! The much better & consistent strategy is to utilize deco gases that titrate down, or at least hold the fraction of Nitrogen nearly constant (i.e. no significant fN2 increases as you ascend through the deco stops); that means using a "best mix" deco blend over standard mix.

Here's an excerpt of a deco gas planning write-up for some of the deeper longer cave dives of the WKPP (utilizing a dry habitat for Oxygen deco):

A trimix of 10.5 percent oxygen/ 80 percent helium was selected owing to the average bottom depth of 280'/85m. Considerations in this selection were:

Since many tissue compartments will reach saturation and decompression will take longer than a few hours, the high helium content has advantages for off-gassing effficiently later in the dive. The amount of time helium takes to reduce its partial pressures in tissues by one-half are about 2.7 times faster than the half-times for nitrogen. . .

As decompressions times lengthen to two and a half hours or more, counterdiffusion of excessive amounts of nitrogen can become a real problem. It can have the effect of doing a deep air dive in the middle of decompression. As shallower stops are made near the end of deco, the diver's body can be loaded with enough nitrogen that it offsets any advantages gained in eliminating helium. Because of nitrogen's greater molecular weight, greater solubility in body tissues and slower half-times, it can take longer and be more difficult to eliminate than helium. This is a special concern at the final deco stop where oxygen is used to remove inert gas from the slowest tissue compartments. . .

[Non-standard, intermediate] decompression mixes that achieve an acceptable balance of these factors are a trimix of 19 percent oxygen / 50 percent helium at 240'/73m; trimix 25 / 35 at 190'/58m; trimix 35 / 25 at 120'/36m; trimix 50 / 15 at 70'/21m; 100 percent oxygen at 28'/8.6m [in a dry habitat], with periodic breaks using trimix 15 / 45.

This selection allows the fraction of helium to gradually taper off while the fraction of oxygen gradually increases and the fraction of nitrogen remains nearly constant. Helium off-gases efficiently with the reduction in pressure and the increasing oxygen fractions. Nitrogen loading during deco is kept below target limits upon arrival at the [oxygen] dry habitat stop. . .

From Erik C. Baker, Decompression Strategies Enable Deep, Long Explorations of Wakulla Springs, Immersed Magazine p.30, Fall 1999.
See also Erik Baker and the Varying Permeability Model: Technical VPM Publications

Decompression from an N2-based dive is longer with N2 containing deco mixes because some N2 is continuously diffusing into tissue during deco. Decompression from a He-based dive can be longer with N2 containing deco mixes because N2 is diffusing into tissue as He is diffusing out of tissue. The decompression obligation of a tissue compartment is based on the sum of gas partial pressures in the compartment. This means that if a tissue is loaded with N2 as He is being removed, its tissue has a greater decompression obligation than when no N2 is added to tissue during He off-gassing. . . The gas partial pressure gradient for movement from tissue into blood is not controlled by ambient pressure; it is controlled by the gas partial pressure in the tissue and in arterial blood. As long as the arterial [inert, non-metabolic] gas partial pressure is zero, the gradient for [inert, non-metabolic] gas removal from tissue is maximal . . .It should be intrinsically obvious that removal of a gas from tissue can be speeded by elimination of the gas from the inspired mixture. If the arterial partial pressure of a gas is zero, then no gas will diffuse into tissue while the gas is diffusing out of the tissue. . .Gas Exchange, Partial Pressure Gradients and the Oxygen Window, p.12, J.E. Brian M.D.

 

[Bombay High]

I have a question that i guess is not DIR, but I would like to know why.
On deep dives why not eliminate N2 from the mixes altogether? Bottom and deco ? It eliminates the inner ear DCS threat, since you have only one inert gas. Also the threat from high heliumPP is eliminated by a slow descent.
I have used exclusive 02/He mixes on dives to 1000 ft and N2 is never introduced into the mixes ever.
While that maybe because it is commercial diving, why is that not considered by tech divers.

 

[Crush]

I think that what kevrumbo is saying to all those who disagree with him is that your standard mix might work in practice, but how does it work in theory?!

 

[Kevrumbo]

I have a question that i guess is not DIR, but I would like to know why.
On deep dives why not eliminate N2 from the mixes altogether? Bottom and deco ? It eliminates the inner ear DCS threat, since you have only one inert gas. Also the threat from high heliumPP is eliminated by a slow descent.
I have used exclusive 02/He mixes on dives to 1000 ft and N2 is never introduced into the mixes ever.
While that maybe because it is commercial diving, why is that not considered by tech divers.

The high cost of heliox for deep non-professional, non-commercial/non-military sport diving is prohibitive; and secondly, Nitrogen provides a buffer against High Pressure Nervous Syndrome (HPNS), typically at depths below 120m/400'. (You've never had issues with HPNS?)

I think that what kevrumbo is saying to all those who disagree with him is that your standard mix might work in practice, but how does it work in theory?!

Thank you Crush for understanding and acceptably paraphrasing the valid questions restated again below (with no equally valid "DIR Practitioner" answers or replies given yet):

Why do you use an intermediate deco gas (21/35) that has a higher fN2 than your bottom mix (12/60 or 10/70 trimix in this case, dive to 90m/300')?

---->(i.g. 12/60 or 10/70 bottom mix have a fN2 of 28% and 20% respectively, while 21/35 intermediate trimix deco gas has an fN2 of 44% --why are you switching to a deco gas that has more Nitrogen percentage wise, than that of your bottom mix???

Again, the simple logical means to an end --if you're trying to off-gas Nitrogen loading from your bottom mix, why are you switching to a intermediate "standardized deco gas" with significantly more Nitrogen than your bottom mix???

 

[Crush]

Most excellent... LMAO.

 

[TSandM]

I think you might actually get a more intelligent discussion on this if you posted it on the GUE forum, to be honest with you. It IS, I think, a good question, and one to which I don't know the answer. Is it cost? Is it a concern about helium's relative insolubility and bubble formation? Is it simply the desire to use gases already available (although not all the deco mixes are also bottom mixes)? The essence of standard gases is that they had to be CHOSEN -- there have to be reasons why the gases which are used were the ones selected, as opposed to higher helium deco mixes. The thing to do is ask the folks who did the choosing.

 

[Bombay High]

The high cost of heliox for deep non-professional, non-commercial/non-military sport diving is prohibitive; and secondly, Nitrogen provides a buffer against High Pressure Nervous Syndrome (HPNS), typically at depths below 120m/400'. (You've never had issues with HPNS?)

I understand the cost factor, but none of the DIR type divers sound that hard up for cash. HPNS is a non issue if you are compressed conservatively. All deep commercial dives are now usually on a deep mix comprising 2/98 or 4/96. No N2. Also not introduced into the deco, so the vestibular DCS (Isobaric) is a non issue.
I have never had the twitches and shakes.

Just wanted to understand the DIR stand on it.

 

[limeyx]

I think you might actually get a more intelligent discussion on this if you posted it on the GUE forum, to be honest with you. It IS, I think, a good question, and one to which I don't know the answer. Is it cost? Is it a concern about helium's relative insolubility and bubble formation? Is it simply the desire to use gases already available (although not all the deco mixes are also bottom mixes)? The essence of standard gases is that they had to be CHOSEN -- there have to be reasons why the gases which are used were the ones selected, as opposed to higher helium deco mixes. The thing to do is ask the folks who did the choosing.

To be serious here for a second I guess (if I have to)

Kevins original assumption is not necessarily correct (as others have pointed out)
He comes from the "If you are trying to offgass Nitrogen" .. why would you.
And indeed, if you were ONLY offgassing Nitrogen then he would have a point.
(i.e. If it was a "deep air" dive then obviously one wouldn't make that kind of switch becauase its entirely counter-productive)

BUT you are off gassing Helium as well as the Nitrogen, so it's not necessarily as straightforward as all that, and I guess is something of a judgement call as to how much to reduce the PP of Helium, coupled with PPO2 limits etc.

Also, as others have pointed out, in general the GUE gases are "Max O2/Min He" so saying 21/35 is a 190 deco gas is really saying "at MOST 21 O2 and at LEAST 35 He"

The reason (as has also been pointed out) for the lower O2 contents than 1.6 are due to the risk/reward of a tox event (who is going to rescue you at those depths?) and I think due to an exposure issue -- if you have a 190 bottle or 240 bottle, chances are you'd like your lungs working well as you are likely to have long deco at 70 and 20

A GUE intsructor (who I am not going to name so as not to drag his name into it did say that for dives with very long bottom times(at least an hour I think) at depths of 200+, the WKPP has seen some small benefits to making the 50% bottle 50/25, and increasing the He on the deeper bottles, but he said that for the dives that most of us are doing (30 min BT say), then the cost of the Helium usually outweighs any deco benefit that they have observed.

I've read some of those papers referenced and honestly, how much real-world data is there to back up the "bring 4 deco gases or varying composition for a 200 foot dive" are there compared to the thousands of dives done on 50% (or 60%) and O2/80% ?

 

[boulderjohn]

Because I have yet to receive a reasonable & valid "DIR Practitioners" answer to the question (perhaps I should address it to GUE and Jarrod Jablonski, as well as UTD & Andrew Georgitsis):

I It IS, I think, a good question, and one to which I don't know the answer. Is it cost? Is it a concern about helium's relative insolubility and bubble formation? Is it simply the desire to use gases already available (although not all the deco mixes are also bottom mixes)? The essence of standard gases is that they had to be CHOSEN -- there have to be reasons why the gases which are used were the ones selected, as opposed to higher helium deco mixes. The thing to do is ask the folks who did the choosing.

I noticed that James (amascuba) gave Lynne a "like" on her post, so I assume he is still reading the thread, and I hope he will chime in with what he remembers from when Andrew was asked this exact question in our Ratio Deco class. Actually, the memory I am drawing from is also from his answer to a different question. My memory is imperfect, and perhaps he can help.

As I said earlier in his thread, he said first of all that the main purpose at that point was to off gas the helium. He said, in fact, that if it were not for the need to drive out the helium, he would agree with the point about the increased amount of N2. (I am dead certain on this part.)

He also talked about the belief in the advantage of using standard gases, even if they are not always the ideal gases. He talked about the origins of the standard mixes, and as I recall it had a lot to do with convenience--they were the easiest mixes to get based on what was banked locally. (My memory is fuzziest here.) He also talked about how the origins were affected by history--the switch to 21/35, for example, was based in part on the fact that in older deco models, 190 was when they switched to air.

Finally, remember that the increase in N2 is not as great as the differences in the FN2 suggest, because at the deepest switches you have come up from greater depths relatively quickly. Because of the decreased ATA, the PPN2 is not that much greater with the higher percentage of N2.