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On Mon, 22 Dec 1997, Michel Therrien wrote: > William, are you sure of all you wrote? > Absolutely not, it's just a theory put forth because it appears that the models don't pay enough attention to gradient. > To my knowledge, bubble formation and growth does not depend on the > gradient, but simply on mechanical pressure, surface tension and tissue > tension (see Deeper Into Diving for more info.). If the gradient was a > factor, we could not decompress on pure oxygen. > That's not what I meant. If gradient plays a role, you'd never know with N2-based deco. You are forced to keep a smooth gradient curve, without massive trasitions, for the simple reason that you cannot switch to a really rich O2 mix until you've already diminished the gradient significantly at the deeper stops. There's never an opportunity to open the gradient wide enough to cause bubble problems. > However, the reverse does not seem to happen. For instance, we can stay in > a air or argon atmosphere while breathing helium without any problem. We > should also be able to switch from a 'fast' gas to a 'slow' gas without > problem (trimix to air or nitrox). > Right, that's the thing: we do see the reverse case, and that's what got me thinking. I have no problem with the N2-->He IGCD theory. The reason I've started to suspect that gradient is important is that it's treated very strangely in Buhlmann's model. Basically, he averages together the saturation levels and uses this to derive an absolute tolerable ambient pressure. That's funky, because it pays little attention to the relative gradient of each gas for the final ceiling calculation. While you are decompressing under Bulhmann, even a slightly lower gradient can *increase* your tissue tension and make for a longer deco, but it's OK to immediately raise the gradient to the maximum without consequence. In other words, despite the fact that he treats each gas totally independantly for on-gassing, only total tissue tension is considered when calculating off-gassing. We all know that Haldanean models are reasonably good at bubble *control*, but don't do much to address bubble formation. What I'd really like to see are some doppler tests done to compare bubble formation between dives with a deep and shallow N2-switch. -Will > > > >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'. > > > > > -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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