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Thoughts on Mixed Gas Diving November 2, 2008

Posted by Chris Sullivan in Technical Diving.
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The Periodic Table of the Elements is available from many online sources. I particularly like the colour one from Los Alamos National Laboratory. The reason I was looking at it was because I started thinking about gas densities based upon a lecture that discussed the physiological reasons for David Shaw’s cave diving death in Bushman’s Hole, South Africa, while attempting to recover the body of Deon Dreyer. To make a long story short the density of the gas he was breathing caused his bronchioles to collapse, making it impossible for him to get enough air to breathe. The harder he breathed the more air he needed, the more he needed the harder he breathed. He was on a rebreather at 270 metres – definitely not your typical dive.

One word of warning, I’m basing this on my high school chemistry class 35 years ago, so I might be talking off the top of my head a bit. As always, I won’t be insulted by polite corrections.

The mix he was breathing was 4/80, or 4% Oxygen (O2), 80% Helium (He), and 16% Nitrogen (N2). Under normal circumstances this gas would be pretty light. It certainly is compared to air. A mole of air at 21% O2, 78% N2 and 1% Argon (Ar) weighs 0.21x2x16 + 0.78x2x14.01 + .01×39.95 = 28.98 grams. The mix he was breathing is 0.04x2x16 + 0.80×4 + 0.16x2x14.01 = 8.96 grams, or only 31% of the density of air. Still, at 270 metres a gas is 28 times as dense as it is on the surface, so the density is the same as air at 77 metres or 250 feet. This doesn’t sound like that much as plenty of divers (but not all) have gone to greater depths on open circuit air without problems. Rebreathers, on the other hand, require a greater breathing effort due to the circulation through the CO2 absorbent material, so this gas density become a real problem.

To explain this calculation, a mole of gas weighs its molecular weight in grams. For this discussion, you don’t have to worry about how much a mole is (less than a cubic foot at atmospheric pressure), but this is true for all gases. My grade 11 chemistry teacher at Nottingham High School in Syracuse New York, Mr Newman, used to have a “mole box” made from cardboard to show how big they are.So to compare mass, weight or density, all you need is a relative figure on the molar weight. The atomic weight of Oxygen is 16, but breathable Oxygen comes in the form of molecules of 2 Oxygen atoms apiece (O2), so the molecular weight is 32. Helium, being the lightest of the components of the gas mix, as an atomic weight of 4.003, but it comes in single atoms, not molecules, so it only needs to be counted once.

16% nitrogen is about a fifth of the fraction in the atmosphere (78%), so at 270 metres the Equivalent Narcotic Depth is 28/5 + 1 atmospheres, or about 66 metres. If you accept Oxygen having equivalent narcotic properties as Nitrogen (it may actually have slightly more), then the calculation stays about the same, as 20% Nitrogen+Oxygen is still about a fifth of the atmospheric content (99%). Most people would be pretty whacked at 66 metres on air, especially if the CO2 started building up.

One of the questions about gas mixes (asked by John Chatterton) at the conference where this information was presented, was on the use of Hydrogen rather than Nitrogen as the third gas, using a mix call Hydreliox. Nitrogen was used to counteract High Pressure Nervous Syndrome (HPNS), which causes muscles tremors and other problems when Helium is breathed at great pressures. Very simply speaking it is kind of an anti-narcosis, so is counteracted by Nitrogen. I don’t know if Helium counteracts the Nitrogen Narcosis, on the other hand. Hydrogen, like Nitrogen, has narcotic properties, but it is much less dense. Hydrogen’s atomic weight is 1, so the Hydrogen Molecule (H2) has a molecular weight of 2. So a mole of 4/80 mix with 16% H2 instead of N2 would weigh 4.8 grams, or a little over half the density of the gas poor Mr. Shaw was breathing. As the Narcotic effect of H2 is less then N2, presumable a greater fraction of H2 would be possible that would lighten the mix even more.

The problem, as you may well know, is that when combined with Oxygen, Hydrogen makes a terrific rocket fuel. However, it has been used successfully as a diving gas because it is not flammable when the fraction of Oxygen is less than 5%,. Intuitively, you might think that the partial pressure of Oxygen would be the deciding factor, because things burn more rapidly in the presence of higher Oxygen partial pressures, but curiously this is not the case. My knowledge of the subject is too limited to explain why this is. At depth, 4% O2 is more than enough to sustain life, and even less would be fine, as unlike our Hydrogen example the minimum requirement for breathing depends on the partial pressure, not the fraction of Oxygen, which at 270 metres and 4% O2 is 1.12 atmospheres.

Nevertheless, the speaker who was asked this question had the opinion that mixes with Hydrogen were far too dangerous to consider, but I was left with the feeling that Chatterton was doing just that. Suffice to say that this really pushes the boundaries of diving and carries risks to match. Inside a rebreather, you’d have to make sure that the equipment, procedures and operator can all handle keeping the O2 fraction below 5% at any phase of the dive when Hydrogen was present. That sounds like an tremendous engineering challenge to me, but I’m sure it is one that someone will try to meet. All else being equal, it means that the density problem is pushed down to more than 500 metres depth.



1. Equivalent Narcotic Depth « Chronicle of an older diver - October 13, 2009

[…] instance, in the Deon Dreyer accident at Bushman’s Hole, he was diving at 270 metres with 4/80 trimix. If Helium was counted at 23% of the Narcotic effect […]


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