100% O2 for DECO Stops

[BILLB]

The problem with helium and decompression has to do with its ability to difuse faster than N2. For this reason, a diver in decompression needs to take deeper stops and at more intervals to properly off gas the inert helium.

There is another phenomenon, know as isobaric counter diffusion. This can happen when a diver has not off gassed enough helium and switches to a heavy gas (i.e. Nitrox). This results in the slower diffusion of the N2 while the helium is off gassing at a rate 2.65 times as fast. This can result in the excess helium difusing through the skin causing blister and also doing the same kind of harm to the neural system as N2.

The problem in treating a helium DCS hit is keeping the helium diffusion under control and eliminating it without futher damage to the affected diver.

"Generally, for dives up to 80 to 120 minutes, Nitrox mixes require less decompression and swallow decompression stops than heliox through the transistion or "crossover point" depends on the depth of the dive and the decompression procedures used." (from "Extending the Envelope" by Dick Rutkowski).

Dick goes to to say that for deeper and longer dives (300 feet!) heliox gain the advantage. So, I guess the point is this: Recreational divers who like to dive beyound no DECO limits of heliox are creating another set of problems. In other words, Helium decompression is different and is govern by a different set of rules than our old friend nitrox.

 

[Munin]

Originally posted by BILLB
There is another phenomenon, know as isobaric counter diffusion. This can happen when a diver has not off gassed enough helium and switches to a heavy gas (i.e. Nitrox). This results in the slower diffusion of the N2 while the helium is off gassing at a rate 2.65 times as fast. This can result in the excess helium difusing through the skin causing blister and also doing the same kind of harm to the neural system as N2.

Isobaric counter diffusion seems to only be referred to by PADI (DSAT) these days. The problem with it happens when switching to a high inert gas (ie helium) from a high nitrogen mix. Even then it only happens at extremely high N2 levels, or near-saturation levels that are relevent only in commercial diving. Comex is the one that did the testing, and they did it by switching from air to heliox after long exposures.

When switching from N2 to He the N2 continues offgassing slowly, while the He ongasses quickly. This causes a rise in tissue tension and then possibly DCS. When switching from He to N2 during deco tissue tension drops, as the He diffuses out faster than the N2 diffuses in, thus no problems.

In any possible case nobody is going to be switching from a high N2 mix and tissue sat. level to a high He mix while ascending. ICD is not an issue.

I'm not a hyperbaric physician, so if I'm wrong about any of this I'd like to know. It should be right. BILLB is certainly right about the need for deep stops and the difference in deco between air and mix.

 

[The Iceni]

Hi all,

The above few posts seem to discussing deep Trimix and commercial dives and to be missing the point I was making - or rather the question I was asking.

Nitrogen off-gassing is accerated if the breathing medium contains no Nitrogen so why can't Heliox 50 be used as a single deco gas for Nitrox or air dives instead of oxygen with it's well proven hazzards?

Bill B said "When switching from N2 to He the N2 continues offgassing slowly, while the He ongasses quickly. This causes a rise in tissue tension and then possibly DCS. "

This seems to be counter-intuitive to my inexperienced brain.

I think I may need a tutorial on this one as I honestly don't understand it! Is this a sort of gaseous osmotic pressure?

 

[BILLB]

Hey Doc!

I believe I am doing an injustice here in taking sinpets from different articles to explain the pros and cons of helium usage in the recreational world of diving. I promise to stop this lunacy!

For you reference I have several studies I found at the University of Maryland. One is called: "The Physicological Basis of Decompression". Chaired and edited by Dr. Richard D. Vann. This paper was the result of a meeting of the "Undersea and Hyperbaric Medical Society, Inc. The UHMS Publication Number is 75 (PHYSC) 6/1/89. Another reference is contained in "The Physiology and Medicine of Diving" Third Edition. Edited by Peter Bennett and Davic Elliot. This was written in 1982 and chapter 15 has a fascinating article on Isobaric gas exchange and supersaturation by counter diffusion. Go to a web site called Abysmal.com and look into Argon useage in diving and the counter diffusion subject is examined there to in some detail.

You can get the medical articles from the University of Maryland. They charge a small fee to copy them and mail them to you. If you want to leave me a private email, I will be more than glad to copy these articles for you and mail them to you.

 

[voidware]

Guys, everyone is talking about using heliox for decompression. It is in my understanding that since helium is a "faster" gas it should not be used for decompression, rather for a bottom mix. Wouldn't it be much more beneficial to use, say, argon or neon in you deco mix? These slower gases would take longer to diffuse into the tissues while still diluting the O2. With long enough deco, you would eventually have to offgas the helium. The times might be nominal, so please correct me if I am wrong.

brandon

PS: if anyone has the times and tensions for heavier gases, I would like to try them and see what deco times come out.

 

[BILLB]

Voidware,

I agree that helium is NOT for DECO. A reference I use is contained in the book "Deep Diving" by Brett Gilliam. Table 11-1 called the physical properties of gasses.

Counter-diffusion is what it is all about. The only gas used in diving that is lighter than helium is hydrogen. So I guess if you were to deco out hydrogen, and helium was the only gas available, then it would indeed work up to a point. Slower gasses like nitrogen and neon (but very expensive) work even better. Argon would the best gas for counterfusion but it is twice as narcotic as nitrogen it's use is out of the question.

O2 is the last gas to be used and it is the best gas to use to "clean up" a decompression dive. Helium is not a gas to be used for deco. It is a usually bottom mix.

 

[The Iceni]

Thanks Bill,

To quote from the abysmal site "Digressing a bit, its should be noted the reverse process of switching from a heavier inert gas to a lighter inert gas (e.g. nitrogen to helium) can create a "super-saturated" condition, where the tissue inert gas pressure is greater than the ambient inert gas pressure, even with no change in depth from the switch! This is the basis of lengthy and often misguided discussions on isobaric inert gas counterdiffusion"

This does indeed seem to be like a dissolved gas osmotic pressure since the sum of the partial pressures of the various gasses in the tissues can (temporarily) exceed - or be less than -the ambient total pressure, but then we are dealing with gasses in solution not in the gaseous phase.

Can I take the liberty of running this by you?

Consider a liquid in equlibium under an atmosphere of 50% nitrogen and 50% helium at 1 bar which is then exposed to an atmosphere of pure oxygen at 1 bar. Assumimg that nitrogen and oxygen behave identically at the end of one helium half time the liquid will have reduced to 50% equilibrium with helium (0.25 bar) whereas the Nitrogen will still be close to 80% of its steady state value (0.4 bar).

In the other direction oxygen it will also have reached about 20% of its steady state value in solution of 1 bar, which is twice that of the vacated nitrogen.

The total tissue gaseous pressure will be;

0.25 bar helium, 0.4 bar nitrogen, 0.2 bar oxygen = 0.85 bar.

What if the first gas was helium alone at 1 bar and the second 100% oxygen? At the end of one helium half time this would give a liquid phase gaseous pressure of

0.5 bar helium plus 0.2 bar oxygen = 0.7 bar

If the first gas was 100% nitrogen and the second helium;

0.5 bar helium plus 0.8 bar nitrogen = 1.3 bar

So, if this is the way mixtures of gasses diffuse in the liquid pahse there is a very real potential for tissue overpressure if helium is used for deco.

Now you all know I am no physicist, which is why I would have thought that - as with the Van der Waals equation for partial pressures - when two or more gasses are mixed in solution their actual diffusion rates will not only depend on each gas's unique properties and their relative pressure differentials but will depend on the ratios of the concentrations of those gasses both in solution and in the gaseous phase in order to maintain the same total pressure.

As nature abhors a vacuum my naive hypothesis would have been as follows;

The helium will not off-gas 2.6 times as fast as the nitrogen but the three gasses will interact and diffuse in and out of the tissue at an equal geometric average determined by the various pressure gradients and concentrations of those gasses.

I GATHER THIS IS A LOAD OF TOSH!

I still find it a little odd, though, that helium can be used as a bottom gas at relatively high partial pressures for lengthy peroids of time and can also be used in relatively shallow dives with similar no stop limits to air but it cannot be used for very short periods as a part of the decompression gas because it can actually cause DCI - all because it diffuses quickly!

Very illuminating, indeed.

 

[BILLB]

Dr. Paul Thomas,

I like it!

The reason I am somewhat into this isobaric counter-diffusion is this was the last ITC lecture I had to give before I because an instructor. I am NOT a doctor or a scientist. I believe that the only reason the ITC director chose this was because he probably though there was a lack of information on this subject and I would probably have to "wing it" to make the grade! Well, I did not wing it. I came to the same conclusions you did but only in concept. The math that you did fantastic! I am saving your post for future use.

Hyperbaric medicine is a fascinating area. Much of it is simply beyond my comprehension. With good divers like yourself making posts like this, the picture becomes clearer each day.

Can we do a dive sometime, someplace?

 

[The Iceni]

Bill B said,

with divers like yourself making posts like this, the picture becomes clearer each day.

Can we do a dive sometime, someplace?

I would love to, perhaps when I get out of this slavery they call UK general practice I will be crossing the Atlantic more often. My last dive that side of the water was off Aruba. Beats the North Sea any day!

I am, perhaps, more of an absent-minded professor type nowadays and my practical diving skills are not as hot as they were in my youth but I am practising!

By the way, I heard today that a British research team has discovered that xenon is protective against neurological strokes. Perhaps xenon could be a diluent of choice. I have no idea of its properties but with a molecular weight of 131 it is much, much heavier than both oxygen and nitrogen so fits the bill pretty well but I bet it is highly narcotic!

Any millionaire divers out there like to give it a try?

Kind Regards

PS. At last, got the hang of quotes!

 

[Dr Deco]

Dear Readers:

One must remember that gases essentially do not interact. This is true whether they are in the gaseous or the dissolved state. They follow Dalton’s Law of Partial Pressures as found by him in about 1805.

The diffusion of a gas is related to its molecular weight. He heavier a gas, the slower it moves at any given temperature. For the same amount of kinetic energy (= heat energy) a light molecule will move faster (and diffuse faster) than a heavy one.

Solubility effects can also play a role when we are speaking of loss of a gas from solution. This does not generally enter into the equation since diving gases are generally chemically inert and do not bond well to the solvent. This is not true of car dioxide, which reacts with water and forms carbonic acid. The solubility of CO2 is very high in water because of this chemical reaction. [The same folds true for ammonia, but again, this is not a diving gas.]

When we are speaking of gas leaving or entering a solution, they will do so independently of one another and their pressures will be additive. If we were to have a very heavy molecule (xenon) attempting to diffuse out of a solution, it would move very slowly. :crawl:

If we had a very light molecule (helium) diffusing into the liquid, it would move quickly. :walking: One can see that if the exiting species was heavy enough, it would not leave at all, and we could end up with double the pressure of gases in solution as the light molecule enters easily and none of the heavy ones exit. We are now very supersaturated.

Now one problem concerns the formation of gas bubbles. While the sum of the partial pressures may be high, a very heavy and non-mobile gas molecule might not move through the liquid to reach the nascent gas bubble. We now see that TIME FACTORS are beginning to play a role. Solubilities are not the sole process as the system is a dynamic one and not in equilibrium (= independent of time).

Dr Deco
:doctor: