Split from Deep diving advice ... The Exley side-bar

[boulderjohn]

Not to get too in depth into the tri-mix world since I'm not likely to ever need/want it, but could someone give me a high level description of the reasons behind the choices for gas? Eg "Helium reduces Ox-Tox past 150 feet, but also has some narcotic qualities that are offset by N %" or something like that...

As you indicate, oxygen toxicity is a serious danger as oxygen partial pressure increases, so the deeper you go, the less O2 you want in the mix. Nitrogen is a factor in both narcosis and in DCS, so the deeper you go, the less nitrogen you want. Helium is a good alternative to both gases for several reasons, so divers add more and more helium as they go deeper and deeper. Unfortunately, too much helium is implicated in High Pressure Nervous Syndrome (HPNS), which explains Sheck's rationale in using some nitrogen in the mix rather than going to a pure mixture of helium and oxygen.

---------- Post added March 18th, 2014 at 02:01 PM ----------

A little more information for those not familiar with trimix....

Sheck and Bowden were diving "heliair," a concept Sheck invented and which has been called "the poor man's trimix." That unfortunate nickname suggests that there is something wrong with it, but there really isn't. It is a cheaper way to make triimix than the full way of doing it, but it is an acceptable trimix blend. The thinking behind it is the same as the thinking behind the trimix blends commonly called standard gases.

To make trimix the full blown way, you first put in the amount of helium you want, and then you do a mathematical calculation to figure out how much oxygen to add. Then you top it off with air. The math involved is actually figuring out what blend of nitrox to add to the mix and then partial pressure blending that nitrox mix on top of the helium.

With heliair, you put in the amount of nitrogen you want and then top it off with air. You have essentially decided to use a nitrox percentage of 21 at all times. This saves a lot of math, and it eliminates the need to do an additional blending step requiring O2 tanks. Notice that "tanks" is plural. Because of the pressure differences in the tanks doing the filling and the tanks being filled, putting oxygen on top of helium is problematic if you are in the field and do not have an expensive oxygen booster with you. With high helium fills, you will need a whole pile of such tanks, with a whole lot of wasted oxygen.

The standard trimix blends (like 21/35) are actually done in much the same fashion. They became popular in south Florida, where lots of shops banked 32% nitrox. The standard trimix blends are helium topped off with EANx 32. Again, less math and no need for additional oxygen, as long as you have premixed nitrox 32 ready to go.

 

[Akimbo]

Hannes Keller and Dr. Albert Bühlmann probably pushed these compromises more than anyone in history. In addition to very high PPO2s for reasons already mentioned, they developed tables that very cleverly used the differences between Nitrogen and Helium saturation rates in different tissues to minimize decompression.

On 30 October 1962 Hannes Keller and Peter Small spent five minutes at 1,000 feet in a wet-pot in preparation for their disastrous dive off Catalina on 3 December. Decompression lasted 270 minutes using these mixes:

Depth	Oxygen	PPO2	Helium	Nitrogen
1000'	8%	2.50 ATA	92%	0%
500'	15%	2.42 ATA	60%	25%
165'	30%	1.80 ATA	0%	70%
133'	50%	2.51 ATA	0%	50%
50'	100%	2.51 ATA	0%	0%

Definitely don’t try this one at home. People died even though they pulled it off once.

 

[DCBC]

...I would assume Exley's plan would have been similar.

1. Free descent to 300 feet on air.
2. Switch to a second travel gas (50 He, 39.5 N2, 10.5 O2) until 600 feet.
3. Switch to the bottom gas to complete the deep part of the dive (69.5 He, 24.1 N2, 6.4 O2).

The only information I have that Exley's plan was any different was that his bottom mix was 6% oxygen, meaning he would have had a little less nitrogen and a little more helium.

In the spirit of the original thread, I'd like to repeat that I don't like to criticize another diver for his choice of equipment or the mixture(s) he selects. A diver should logically choose a mixture that will work for him at the planned depth. As we are different in our tolerances, the choice is a personal one.

That said, Sheck's choice would be too aggressive for me and my preference would be to opt for a mixture richer in Helium. I suspect Sheck's choice was due to the fact that he felt comfortable with higher Nitrogen and Oxygen partial pressures and the dive plan likely minimized exposure time (a faster descent time on Helium that I would be comfortable with). As has been mentioned, there's some indication that Sheck had experienced some problems with HPNS (likely exasperated by a faster descent rate) which likely influenced his choice of mixture.

Other than the minor incidents that I've described, I am not usually bothered by HPNS. Perhaps this is largely due to a planned slower descent rate. In OW my descent rate is 60 FPM to 300 FSW and 30 FPM after that (I haven't experienced any difficulty with HPNS).

For saturation diving (as a matter of interest) the descent rate is varied from 30 fpm to usually 0.5 fpm. As discussed, a fast descent rate can cause HPNS. Also a slow descent reduces Hyperbaric arthralgia (joint pain which occurs during compression).

The recommended maximums descent rates are:

0–60 fsw, 30 fpm max
60–250 fsw, 10 fpm max
250–750 fsw, 3 fpm max ( I prefer 2 fpm)
750–1000 fsw, 2 fpm (1 fpm)
1000-1500 fsw, 1 fpm (.5 fpm)

This "Compression Arthralgia" is something I have never experienced, even on dives when I had high workloads at 280 feet --on air or trimix.
Wondering why it appears in some divers and not in others. I do think this is why some deep divers use scooters, so the effects of this are less significant in performing the dive....and maybe less exacerbated--as I have heard some divers say they feel better after the dive if they do less work on it...again, I don't experience this, so it is alien to me.

Regarding Hyperbaric Arthralgia, I've experienced this in the form of a dry grainy feeling in the joints (when they are used). A popping of the joints also occurs; which increases with depth and changes from discomfort to mild/moderate pain on dives over 800 FSW. I'd estimate about 50% of divers experience this below 500 FSW (to a greater or lesser degree). On dives below 1000 FSW virtually every diver I'm aware has had this experience.

HA does dissipate. Generally speaking, the deeper the dive, the more severe the symptoms and the longer that discomfort/pain can be expected. Symptoms are also delayed in that they may start at 500 FSW and keep getting worse after you've stopped at a deeper depth. This will stabilize and disappear after a number of hours (or couple of days). Again, no pain or discomfort is experienced if there's no movement. The wrist, shoulder, knee and hip are the areas most commonly affected. Because of the nature of Hyperbaric Arthralgia, I doubt that this was a factor for Sheck.

---------- Post added March 19th, 2014 at 07:51 AM ----------

...Sheck and Bowden were diving "heliair," a concept Sheck invented and which has been called "the poor man's trimix."

Thanks for the excellent explanation John, but to clarify, Sheck didn't invent Heliair. An inventor named Elihu Thomson theorized that helium might be an appropriate substitute for the nitrogen in a diver’s breathing supply in 1919. He made the suggestion to the U.S. Bureau of Mines (which held a monopoly on helium marketing and distribution at the time) and recommended that further research be undertaken. Interestingly enough, Thompson became the President of MIT the following year.

In 1924, the Navy and the Bureau of Mines jointly sponsored a series of experiments using Helium (originally blending Helium with Air, then with Oxygen mixtures). The following photo was taken in 1929 of the Navy's first HeO2 manifold.

The main problem experienced with diving at that time wasn't Narcosis, but CO2 excess. For this reason, the Navy developed a semi-closed circuit unit, the MK V MOD 1 helmet. This was equipped with a carbon dioxide absorption canister and a venturi-powered recirculator assembly.

As you've mentioned, the principle components of Heliair are the same as Trimix. The difference is the method in-which they're mixed. There are advantages and disadvantages of each, but Trimix is a more accurate method that provides an exacting blend to suit the maximum safe PP02 and personal N2 tolerance of the individual.

 

[boulderjohn]

Thanks for the excellent explanation John, but to clarify, Sheck didn't invent Heliair.

Please note that I never said that Sheck invented the idea of using helium in a mix. I said he invented the specific blending method called "heliair." I ran into that information while checking out the details of the history of heliair to be sure I was not spreading misinformation.

I agree that as a blending method, the heliair process is limiting because it does not allow for the precise best mix for your dive. The ratio of oxygen to nitrogen will always be 21:79, and you have no way to change that. As someone who has blended a fair amount of trimix without benefit of an oxygen booster, though, I can guess that it was the only practical way to do it where he was diving. I would not argue with someone who believes that if you are going to dive at such an extreme, you should be using the ideal mix, and if the ideal mix is not available, you should not be doing the dive. On the other hand, he may well have thought that the mix he was using was ideal, for the reasons you mention. I can't say for sure what mix I would have used, because I am pretty sure I would not even consider doing the dive under any circumstances.

 

[thez_yo]

...
With heliair, you put in the amount of nitrogen you want and then top it off with air. ...

Do you mean Helium?

 

[boulderjohn]

Do you mean Helium?

Ooops. Yes. I meant you put in the amount of helium you want and then top it off with air.

 

[John C. Ratliff]

Please note that I never said that Sheck invented the idea of using helium in a mix. I said he invented the specific blending method called "heliair." I ran into that information while checking out the details of the history of heliair to be sure I was not spreading misinformation.

I agree that as a blending method, the heliair process is limiting because it does not allow for the precise best mix for your dive. The ratio of oxygen to nitrogen will always be 21:79, and you have no way to change that. As someone who has blended a fair amount of trimix without benefit of an oxygen booster, though, I can guess that it was the only practical way to do it where he was diving. I would not argue with someone who believes that if you are going to dive at such an extreme, you should be using the ideal mix, and if the ideal mix is not available, you should not be doing the dive. On the other hand, he may well have thought that the mix he was using was ideal, for the reasons you mention. I can't say for sure what mix I would have used, because I am pretty sure I would not even consider doing the dive under any circumstances.

John, you are correct, but I would like to amplify this for those who are not so math-inclined to understand the term, "ratio." The ratio of 21% oxygen and 79% nitrogen in air is actually a 1:3.76 ratio, or one part oxygen for every 3.76 parts nitrogen in the mixture of air. Now if we start with a mix that contains 50% helium, and we assume for ease of calculations that the mixture in air is 20% oxygen and 80% nitrogen (slightly off, but it makes the calculations easier to understand), we have a cylinder which is 50% helium, 40% nitrogen and 10% oxygen. For deep diving, the percentage of oxygen in the air can be decreased to preclude oxygen toxicity, but the ratio of oxygen to nitrogen remains the same. If we put 75% of the mixture as helium, the ration of oxygen and nitrogen remains the same (1:3.76 in reality, but shown below for math simplification as 1:4), but the percentage of oxygen in the mixture goes down to 5%, and nitrogen to 20%.

Mixture...............Air............50% He.........75% He
%Oxygen..............20.............10..................5
%Nitrogen.............80.............40.................20
%Helium.................0.............50..................75
%Total................100...........100................100

SeaRat

PS, You cannot use these mixtures at surface pressures. Any oxygen mixture below 19.5% is considered "Immediately Hazardous to Life and Health," or IDLH, by OSHA and NIOSH at the surface (one atmosphere absolute). These mixtures shown above can only be used at a specified depth during a dive where the oxygen partial pressure is high enough to sustain life.

 

[DCBC]

Please note that I never said that Sheck invented the idea of using helium in a mix. I said he invented the specific blending method called "heliair."

John, I'm not saying you're being misleading, rather that you are mistaken. It is my understanding that the USN mixed air with Helium before HeO2 was developed as their deep-diving standard. Sheck didn't invent Heliair; it was the first part of the Navy's 1924 research project into Helium mixtures. Sheck may be credited with being the first person to introduce this mixing process to recreational diving (I don't know), but there's a difference between this and inventing the concept.