Helium Fraction and Standardized Gases

[PfcAJ]

Dan, I think the issue might be your choice of 50% helium in your backgas for a 300ft dive... unless I read that wrong, of course.

 

[LiteHedded]

bingo. it's not just about the END you can handle

for instance, I know I wouldn't be cool on that gas at that depth

 

[danvolker]

Dan, I think the issue might be your choice of 50% helium in your backgas for a 300ft dive... unless I read that wrong, of course.

I think 10/50 is a good mix for under 300, and precludes necessity of a travel gas. Remember, we did 100's of dives to this depth on air prior to George introducing us to helium around 96....I did higher helium mixes a few times, and could never feel any real difference between sharpness on a dive in any mix greater than 50%....the difference from air of course, was staggering

Most of us work for a living, and choice of gas is related to common sense issues--- if you don't care if diving each week costs you $1000 instead of $100 to $200, fine..if I could feel a difference, and my performance could measurably change, I might still think about using more helium....As it is, 10/50 is my "deep tool", 21/35 is my 130 to 180 tool.

Also, some of you guys are diving Cave, and even theoretical potential for increased sharpness can be enough to push you to higher helium percentage...Meaning -- even if you can't really feel the difference, it may be worthwhile to go with it for the safety measure it might represent. Relatively speaking, the Cave diving is way more brain intensive than deep ocean diving in South Florida

 

[limeyx]

Nick, thank you for now engaging in respectful dialog instead of immediate outright dismissal as a heretic in the Holy Name of Most High DIR Dogma (I apologize for the personal insults in the posts above ).

You didn't answer the questions I put forth though, just did some vague arm-waving about M-values and saturation and percentages.

The standard DIR gases are obviously not (by definition) perfect for every dive, but they seem to give a lot of people excellent results and are easily modified by simply adding more He on deco gases for extended decompressions.

Like I said, people who have actually gone out and *done* those dives have concluded that for reasonable exposures the current gases work fine, and for bigger exposures more Helium in the deco gases has anecdotally resulted in some people feeling better after the dive.

Just like I said from the beginning.

If you do the PPN2 calculations I asked above, you will see that the PPn2 increase, while there (if you go from 12/65 to 21/35) is not a huge spike.

Also, what now if the dive is at a different depth than you expected ? are you going to bring a Helium T-bottle on the boat and madly start re-blending deco bottles if you can't do the planned dive and have to change things up a bit ?

 

[PfcAJ]

In the DIR forum, we try to keep our answers within the constraints of DIR. A gas with a planned END greater that 100ft isn't DIR.

I think 10/50 is a good mix for under 300, and precludes necessity of a travel gas. Remember, we did 100's of dives to this depth on air prior to George introducing us to helium around 96....I did higher helium mixes a few times, and could never feel any real difference between sharpness on a dive in any mix greater than 50%....the difference from air of course, was staggering

Most of us work for a living, and choice of gas is related to common sense issues--- if you don't care if diving each week costs you $1000 instead of $100 to $200, fine..if I could feel a difference, and my performance could measurably change, I might still think about using more helium....As it is, 10/50 is my "deep tool", 21/35 is my 130 to 180 tool.

Also, some of you guys are diving Cave, and even theoretical potential for increased sharpness can be enough to push you to higher helium percentage...Meaning -- even if you can't really feel the difference, it may be worthwhile to go with it for the safety measure it might represent. Relatively speaking, the Cave diving is way more brain intensive than deep ocean diving in South Florida

 

[danvolker]

In the DIR forum, we try to keep our answers within the constraints of DIR. A gas with a planned END greater that 100ft isn't DIR.

South Fla tech dives to wrecks like the Rb Johnson/coryn chris, the Hopper Barges off singer Island, the Sylvina Express, The Skycliffe ( can't really get deeper here than 230) and the deep reef off of jupiter from 220 to 270, and over a dozen more well known in the area, are all multi level dives where it is theoretically possible to get deeper than 280, but the bulk of the dive--what you are really doing, is more like 225 to 260. Sorry if I don't play this discussion the way the DIR forum thinks I should.

If I do the Castor in Delray to video Goliath Groupers, I can get to 118, maybe ( like if I swim down to the junction between the bottom of the wreck and the sand, and spend a minute or two there..Most of the dive will be between 90 feet and 105 feet........but there is no way in the world I will make this a trimix dive, and I don't know any ocean diving DIR divers that would either. .. I do know quite a few DIR divers you know

 

[Kevrumbo]

You didn't answer the questions I put forth though, just did some vague arm-waving about M-values and saturation and percentages.

The standard DIR gases are obviously not (by definition) perfect for every dive, but they seem to give a lot of people excellent results and are easily modified by simply adding more He on deco gases for extended decompressions.

Like I said, people who have actually gone out and *done* those dives have concluded that for reasonable exposures the current gases work fine, and for bigger exposures more Helium in the deco gases has anecdotally resulted in some people feeling better after the dive.

Just like I said from the beginning.

If you do the PPN2 calculations I asked above, you will see that the PPn2 increase, while there (if you go from 12/65 to 21/35) is not a huge spike.

Also, what now if the dive is at a different depth than you expected ? are you going to bring a Helium T-bottle on the boat and madly start re-blending deco bottles if you can't do the planned dive and have to change things up a bit ?

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.

Nick (and John), you cannot do one representative static "snap-shot" ppN2 calculation for a dynamic deco event involving many different kinds of tissues with different M-values & half-times; as well as the physical properties of TWO inert gases in this instance --Helium and Nitrogen-- and both their effects on deco physiology especially with regard to IBCD (Iso-Baric Counter Diffusion). Not a "hand & arm-waving Jedi Mind Trick" excuse to dodge your question Nick, just your misunderstanding of attempting to apply a singular set of data to diverse yet indirectly interrelated phenomena.

Conceptualize and apply practically & simply Nick (and don't get hung-up or needlessly dependent on red-herring calculations, speculations & arguments to obfuscate my point). Read the following -abridged & applied from Mark Powell's Book "Deco For Divers", Ch.7 Mixed Gases (you know that book Nick --you got it for Jamie's B-day-- maybe you should actually read & comprehend it):

From a decompression view, trimix deco involves two inert gases of Helium & Nitrogen that have very different physical properties and dealing with these properties makes trimix deco theory more complicated than air or nitrox decompression. Helium molecules are much smaller than Nitrogen molecules and so will diffuse in & out of tissues much faster than Nitrogen; the half-times for Helium are usually taken to be 2.7 times faster than Nitrogen for the same tissue . . .another difference is that Nitrogen is more soluble in tissue & blood, some 1.4 times as much as Helium -up to 4.5 times more in lipid tissue (fat); and the larger size of potential bubbles formed by Nitrogen over that of Helium is due to the larger amount dissolved in the tissues & blood.

The result of a gas switch from 12/60/28 bottom mix to an intermediate deco mix of 21/35/44 at 57m is that there's now a lesser amount of inspired inert Helium and so the Helium will come out of the tissues even faster (35% He in deco mix vs 60% in the bottom mix). However, the inert gas gradient has actually increased for Nitrogen (28% fN2 from bottom mix increases to 44% inspired Nitrogen on deco mix), so it will not optimally come out any faster and some tissues may even briefly start on gassing Nitrogen again. The switch to 21/35 has accelerated the off-gassing of helium but has done nothing to accelerate the off gassing of nitrogen. . .

The primary and most controversial issue of Counter Diffusion in this example of a deep dive to 90m, is related to the physical properties of Helium & Nitrogen and the "spiking" of the Nitrogen inert component as seen in the use of 21/35/44 in the above example. It is when one inert gas enters a tissue faster than another can leave it. The inert gas entering the tissue can cause the tissue tensions to rise above the critical M-value or super saturation point causing bubbles to form. This bubble formation can occur with no change in depth, hence the name Iso-Baric meaning "same pressure" [and is also the reason why that your's & Boulderjohn's snap calculations justifying minor, seemingly ameliorating ppN2 delta changes over an ascending ambient absolute pressure range have no bearing on this point --can you understand that Nick???].

With deeper Trimix dives using higher percentages of Helium . . .[switching to a higher Nitrogen component in the deco mix over the bottom mix], Nitrogen with its high solubility can cause problems with ICD if introduced into a tissue that is already saturated with Helium which has low solubility. The Nitrogen dissolves quicker than the Helium can come out, creating a super saturation situation, hence bubbles of both Helium and Nitrogen are formed in the tissues. Deco For Divers, Mark Powell p.191

______
So the questions and the thesis still remains:

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, a 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???

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 . . .
---->By this tactic above for decreasing the inspired gradient of the inert Helium then, --why can't you do the same simultaneously with the inspired inert Nitrogen?

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.

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

I've been considering a similar strategy here locally in SoCal, of using "best mix" deco gas progressions of trimix blends for gradual off-gassing of the helium & nitrogen inerts, coming off a hypoxic trimix bottom mix. (This is assuming of course, you have a gas logistics support source that uses partial pressure blending and can afford using higher helim concentrations for the deco gases).

For instance, from bottom hypoxic mix of 12/60/28 used at 90m, instead of a standard deco mix of 21/35/44 and switch at 57m --use a best mix of 21/51/28 at the initial 57m deco stop. The next switch at 36m, instead of standard deco mix of 35/25/40, use 35/37/28. The next deco gas switch at 21m, use 50/25/25 and finally 100% O2 for the last 6m deco stop before surfacing.

In essence you're providing a decreasing inspired gradient for the helium, and although you still have a constant nitrogen fraction of 28% to 25% until the O2 stop -- it's mitigated by gradual ascent thru the deco stops with successive decreases in ambient absolute pressure (from 6.7ata to 4.6ata and 3.1ata in this example), and you are not significantly ongassing any more amount of nitrogen than you would if you decided to use standard gases as intermediate deco mixes instead (i.g. 21/35; 35/25 and 50% nitrox). And of course finally by the end of the prescribed 6m O2 stop, all inerts should be at a low level to permit ascent to the surface. . .

The most efficient way of getting rid of inert gasses is in dissolved form in the venous bloodstream returning back to the lungs to be exhaled via respiration; together with prescribed ascent rates, reduction in ambient pressure and deco stops according to whatever particular decompression algorithm in use. But these tactics along with controlled ascending changes in ambient pressure are for later in the deco profile strategy.

First things first --you also gotta get inert gas out of the tissues into the venous blood, which is the point of J.E. Brian's article:

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.

I'm talking about early in the deco profile, after the deep stops to keep the free phase bubbles in check and where you don't want sudden ambient pressure reductions to induce a Boyle Expansion of these free phase bubbles in blood & tissue. Additionally, you don't want to add any more inert gas loading by switching to a deco gas with a greater inert component than your bottom mix --in this case a Nitrogen slam or spike-- that can potentially diffuse back into these free phase bubbles and cause problems later in the deco profile (or worse on the surface post-dive).

(see also Ross Hemingway's V-planner website: Decompression myths and mistakes)

 

[lamont]

Nick (and John), you cannot do one representative static "snap-shot" ppN2 calculation for a dynamic deco event involving many different kinds of tissues with different M-values & half-times; as well as the physical properties of TWO inert gases in this instance --Helium and Nitrogen-- and both their effects on deco physiology especially with regard to IBCD (Iso-Baric Counter Diffusion). Not a "hand & arm-waving Jedi Mind Trick" excuse to dodge your question Nick, just your misunderstanding of attempting to apply a singular set of data to diverse yet indirectly interrelated phenomena.

Conceptualize and apply practically & simply Nick (and don't get hung-up or needlessly dependent on red-herring calculations, speculations & arguments to obfuscate my point). Read the following -abridged & applied from Mark Powell's Book "Deco For Divers", Ch.7 Mixed Gases (you know that book Nick --you got it for Jamie's B-day-- maybe you should actually read & comprehend it):

Kevin, as a reviewer for your "thesis", you should be aware of this work which is directly relevant and which your "thesis" must address:

Biophysical basis for inner ear decompression sickness

Your homework assignment would be to take the system of coupled ODEs simulating the inner ear compartments in that paper and show how the gas tension in the vascular tissue spikes in response to gas switches to 21%, 21/35, 21/45 and 21/55 after varying lengths of bottom time on 10/70. A series of graphs like Figure 3 would be nice.

If I had a copy of something like Mathematica that would be fairly easy to run, but I haven't played around with anything like that in 15 years so I'm a bit rusty and it would take me a few weekends to get up to speed. You seem to have tons of time to post on the internet, so why not create some actual data that would bring some utility to this entire conversation.

And this model addresses everything you have mentioned and accurately reproduces IBCD when run with paramaters of isobaric switches on saturation dives that have been observed to produce inner ear hits. It is 8 years old, so Doolette may have better models now, but this is a good start, and way better than your 1990s-era authorities.

If you could actually produce this then we'd actually learn something... If not, then there's little point to this thread, and you still haven't poked your nose into the GUE forums where you can find better authoritative answers than anyone here... Still waiting for your post over there that throws JJ under the bus for diving "best mix" against his own agencies philosophy...

 

[Kevrumbo]

Kevin, as a reviewer for your "thesis", you should be aware of this work which is directly relevant and which your "thesis" must address:

Biophysical basis for inner ear decompression sickness

Your homework assignment would be to take the system of coupled ODEs simulating the inner ear compartments in that paper and show how the gas tension in the vascular tissue spikes in response to gas switches to 21%, 21/35, 21/45 and 21/55 after varying lengths of bottom time on 10/70. A series of graphs like Figure 3 would be nice.

If I had a copy of something like Mathematica that would be fairly easy to run, but I haven't played around with anything like that in 15 years so I'm a bit rusty and it would take me a few weekends to get up to speed. You seem to have tons of time to post on the internet, so why not create some actual data that would bring some utility to this entire conversation.

And this model addresses everything you have mentioned and accurately reproduces IBCD when run with paramaters of isobaric switches on saturation dives that have been observed to produce inner ear hits. It is 8 years old, so Doolette may have better models now, but this is a good start, and way better than your 1990s-era authorities.

If you could actually produce this then we'd actually learn something... If not, then there's little point to this thread, and you still haven't poked your nose into the GUE forums where you can find better authoritative answers than anyone here... Still waiting for your post over there that throws JJ under the bus for diving "best mix" against his own agencies philosophy...

Sorry Lamont . . .I'm throwing the burden of proof back at you to show & explain definitively why this practice of switching to a deco gas with higher inert Nitrogen component is still an acceptable practice --despite all basic intuitive physical principles and common sense.

You still have not given a valid coherent answer as a group of "DIR Practitioners" to justify this fundamentally flawed practice. . .

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?

 

[MXGratefulDiver]

You still have not given a valid coherent answer as a group of "DIR Practitioners" to justify this fundamentally flawed practice. . .

Sure he has ... because it works.

This bickering makes my head hurt ... somebody just needs to go diving ...

... Bob (Grateful Diver)