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In the process of putting together ZPlan, I've spent a lot of time tearing into the math behind Haldanean/Schreiner/Buhlmann decompression models, and have discovered a couple of points that raise questions. The first of these relates to what has been clssically termed "inert gas counterdiffusion". While this is a relatively obscure topic, IGCD bend hits do happen in recreational diving, and more importantly, properly thinking out this issue results in conclusions that strongly support the WKPP's experience that Helium based mixes are the best choice for the mid-range of decompression stops (I have nothing to do with the WKPP, except for having fun taking the occasional poke at George :) "Inert Gas Counterdiffusion", as the current theory holds, is thought to be a cause of DCS incidence that occurs when switching from mixes high in Helium to Nitrogen-based mixes. The theory runs that the sudden on-gassing of the Nitrogen somehow magically interferes with the off-gassing of the Helium, and bubbles form. This is typically in the fast and very-fast tissues - usually the skin or inner-ear. Inner-ear bends are *very* bad news, usually leading to permenant disability, including vertigo, nausea, tinnitus (ringing), and hearing loss, if not recognized and treated immediately. Skin bends are usually just uncomfortable, of course. Most everyone agrees that there are really two major factors that determine bubble formation: gradient and absolute pressure. "Gradient" is the difference between the pressure of a gas dissolved in the blood and the partial pressure of that gas in the inspired mix. When the gradient gets too high, microbubbles form. These bubbles expand as the absolute pressure decreases, in the same way your BC volume expands while ascending. Haldanean-based models use a mixture of gradient and pressure to determine a "safe" decompression curve, but don't specifically make the point that BUBBLES ARE FORMED BY HIGH GRADIENT, NOT ABSOLUTE PRESSURE CHANGES. On the surface, this seems like a minor point, and for non-mixed gas diving, it usually is. When we do, say, a deep air dive (for illustrative purposes only..poke,poke!) the gradient of Nitrogen is naturally controlled into a smooth, slow transition during decompression. This is because high partial pressures of Oxygen are toxic, so in order to avoid doing the O2 funky chicken, you must *slowly* decrease the N2 content in the mix - you can't just slam the dissolved/inspired gradient to the max at 200fsw by going on 100% O2 or you'd tox. Not so with Helium mixtures. When you do that deep switch from heliox or trimix to a nitrogen-based mixture, you are zapping the gradient to it's maximum in about five seconds, and you are doing it at the worst possible time, when the pressure of the dissolved helium is at its greatest. Plus you are usually ascending at the same time, which has got the helium gradient already reversed into "off-gassing" mode. Naturally, bubbles form. Makes alot more sense than some mumbo-jumbo about gas interference, eh? So obviously, the important point here is that deep switches from mix to Nitrogen-mixtures are a bad idea. You can get the helium gradient lowered to a level where it's safe to switch to nitrox by simply doing your first few stops on the mix, and switching to Nitrox or O2 for the "long haul" stops, where they're really needed. -Will -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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