Helium Fraction and Standardized Gases

[lamont]

Nick basically answered the question perfectly, which is what I've been trying to say.

It'd be interesting if this discussion took a more useful turn down the road to where 50/25 and 21/45 would be used in WKPP-level dives, but I doubt it'll go there...

 

[limeyx]

Nick basically answered the question perfectly, which is what I've been trying to say.

It'd be interesting if this discussion took a more useful turn down the road to where 50/25 and 21/45 would be used in WKPP-level dives, but I doubt it'll go there...

I have used 50/25 on a couple of dives that were between 220-270, where I had previously used just 50%, and noticed no difference at all (the bottle didnt even ride THAT much better)

I believe the WKPP does routinely use much more Helium than the GUE defaults, and you can see that on their pages that for the push teams and for deep support, they really pump up the He fractions on the deco gases due to the extended BTs

And I may be going out on a thin branch here with a thin knowledge of deco , but I do agree with John Adsit (Boulder John)

When people start to go on about not "increasing the % of N2 in a deco gas" I start to think about people who use nominal tax rate instead of "actual rate paid" to make a point.

The point here is that even if you increase the fraction of N2 a bit, you ongassed much deeper so the dissolved N2 will be at a higher PPN2.

Yes, by the time you get up to say 190, and switch to a higher N2 gas, you may be ongassing in some compartments, but for sure not all, or maybe even most since you breathed the bottom gas at say 250 or 300 feet so the PPn2 is higher (the dissolved gas doesn't "magically" change to the ambient pressure on ascent (otherwise back-gas deco would be impossible)

 

[limeyx]

Nick basically answered the question perfectly, which is what I've been trying to say.

It'd be interesting if this discussion took a more useful turn down the road to where 50/25 and 21/45 would be used in WKPP-level dives, but I doubt it'll go there...

I remember reading a post by Clare Gledhill on some support-work she did with the WKPP where they were getting 190 bottle fills, which were 21/45 (I think, if i remember correctly)

 

[boulderjohn]

It'd be interesting if this discussion took a more useful turn down the road to where 50/25 and 21/45 would be used in WKPP-level dives, but I doubt it'll go there...

I don't know about others, but I am not sure what kinds of discussion topics are allowed in this forum. My sense is we can talk about what DIR is for clarification purposes, and we can ask why DIR philosophy is what it is--again for clarification purposes. I have been wondering about this throughout this thread, since I did not think challenging a philosophy or suggesting changes was within bounds.

Once while desperately trying to fill a whole pile of tanks before our scheduled departure time, I made the efficiency of my process more of a factor than I should have and had multiple operations going on at the same time. I made the error of opening a whip to top off some tanks before closing off some tanks that were already on another whip. Consequently, I got some helium in a tank intended for EANx 50 before I caught the error. (BTW, people like me are reasons you should always analyze.) Well, my immediate thought was "No problem--it's better that way."

The main reason I would not be so inclined is just economics. In an ideal world, I would use way more helium than I do. Not being independently wealthy, though, I am happy to use a less costly blend when it doesn't make a big difference.

 

[amascuba]

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.

That's basically what I remember.

 

[lamont]

I don't know about others, but I am not sure what kinds of discussion topics are allowed in this forum. My sense is we can talk about what DIR is for clarification purposes, and we can ask why DIR philosophy is what it is--again for clarification purposes. I have been wondering about this throughout this thread, since I did not think challenging a philosophy or suggesting changes was within bounds.

This forum is for discussion of DIR.

It doesn't always have to maintain within the rigid boundaries of what is taught.

I only lose patience with threads like the go-for-the-right-post-or-manifold-first because its been beat to death on a million threads on the internet and is very well covered in all of the courses.

The SM threads are also generally not that useful either. When started by non-DIR posters its usually trolling of one form or another. I let Kevin's thread run, but it didn't go anywhere, which is kinda the point why I closed the first thread -- its either trolling or its not going to be useful.

The posting-non-DIR-information rule is difficult to enforce and doesn't allow any discussion and is really what the (largely unused) DPF is supposed to be for. for the canonical answers, without any debate -- so in that forum its always the right post first, period. I'd moderate incorrect information out of the forum, but it usually gets shouted down fairly quickly. I do think it'd be better to have the DIR forum be at least opt-in, no moderation required. Then we would avoid the accidental trolls who just didn't recognize the thread was in the DIR forum which I think is the bulk of the incorrect information issue... We'd also have a bigger weapon to use against people who opted-in with zero experience and then spouted off nonsense like they're an authority...

 

[Kevrumbo]

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% ?

Nick don't confuse the original post by waxing rhetorically about O2/Helium concentration gradients, or N2 only dives, a chide about deep air, and a "nameless GUE Instructor". Look at and study the implied & given base assumption of my question:

Coming off a bottom mix of 12/60 or 10/70 to standard intermediate deco trimix 21/35 --look at the fraction of Helium: you have a concentration of either 60% (if using 12/60) or 70% (10/70), and upon switching to 21/35 on deco at 57m you have a Helium fraction now of 35%. You have now a decreasing concentration gradient, going from 60 or 70% Helium in the bottom mix to a lesser inspired gradient of 35% in the intermediate deco mix of 21/35, which is the proper tactic for off-gassing the Helium from tissues. That's a given --noted and understood Nick: not an ignored or "incorrect assumption" as you contend.

By the tactic above for decreasing the gradient of the inert Helium then, explain Nick --why you can't or choose not to do the same with the inert Nitrogen? Your hearsay speculations and others above still don't answer or even address the thesis below satisfactorily, with either cited theory, models or even basic intuitive & valid physical/physiological principles.
_____________________________
The original post #1 again recap'd 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.

 

[boulderjohn]

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???

Let's say you were diving 12/60 at 10 ATA--your PPN2 is 2.8. If you ascend relatively quickly to 190 feet and then switch to 21/35, your PPN2 is now 2.97. Yes, it is more, but not as much more as the FN2 difference would suggest.

No, it is not ideal, but I think the answer has already been stated that the belief is that staying with standard gases provides more benefit overall than using a best mix philosophy. That is certainly a debatable point, but I do not think which approach is favored by either GUE or UTD is in doubt.

 

[Kevrumbo]

Let's say you were diving 12/60 at 10 ATA--your PPN2 is 2.8. If you ascend relatively quickly to 190 feet and then switch to 21/35, your PPN2 is now 2.97. Yes, it is more, but not as much more as the FN2 difference would suggest.

No, it is not ideal, but I think the answer has already been stated that the belief is that staying with standard gases provides more benefit overall than using a best mix philosophy. That is certainly a debatable point, but I do not think which approach is favored by either GUE or UTD is in doubt.

No boulderjohn . . .per the article below, for gas removal on decompression, ambient pressure differences (i.e. ascending depth changes) do not control the gradient for inert gas movement from tissue into blood; it is controlled by the gas partial pressure IN the tissue and arterial blood.

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.

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.

 

[lamont]

No boulderjohn . . .per the article below, for gas removal on decompression, ambient pressure differences (i.e. ascending depth changes) do not control the gradient for inert gas movement from tissue into blood; it is controlled by the gas partial pressure IN the tissue and arterial blood.

If you reduce the depth, you reduce ambient pressure, you also reduce fN2, which reduces the ppN2 in the lungs, which will reduce the dissolved gas tension/pressure of N2 in the blood.

If it didn't, decompression wouldn't work at all.