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reply NITROX DECOMPRESSION by the author of the original posting I+ve got these equations for the +oxygen window+ or +inherent unsaturation+: DELTA = P N2 - p N2 - B = P - P O2 - P CO2 - P H2O - p N2 Capital P denotes separated gas/ambient pressure Lower case p denotes tension/dissolved gas B = mechanical pressure; a tissue constant of compliance or resistance against deformation. If a bubble is tightly packed, this will to some extent prevent its growth. A guy named Harris has found the sum of P O2, P CO2 and P H2O to equal approximately 5.47 fsw p N2 is equal to: f ( P - P H2O ) fsw = f ( P - 2.04 ) fsw f = fraction/decimal percentage of nitrogen Thus, P - P O2 - P CO2 - P H2O - p N2 P - 5.47 - f ( P - 2.04 ) fsw P - 5.47 - fP + f2.04 fsw P - fP + f2.04 - 5.47 fsw ( 1 - f )P + f2.04 - 5.47 fsw ( 1 - nitrogen fraction ) equals oxygen fraction. This is the significant part of the equation. When I omitted the rest for simplicity I was left with the oxygen fraction times total pressure or P O2. I said that air at 130 feet had an oxygen window of about 1 atmosphere or 33 feet. That+s .21 times 5 atmospheres absolute ( ATA ) = 1.05 atmospheres or .21 times ( 130 + 33 ) feet absolute = 34.23 feet Let+s do the whole equation: ( 1 - f )P + f2.04 - 5.47 fsw ( 1 - .79 )(130 + 33 ) + .79( 2.04 ) - 5.47 fsw 34.23 + 1.6 - 5.47 fsw = 30.36 fsw The actual oxygen window is slightly less than P O2 - 3.87 feet less. Thus, P O2 is a practical and fairly accurate approximation, especially at depth. For smaller oxygen windows, 3.87 feet off makes a difference. P O2 (air) at 70 feet is approximately 21.6 feet According to the full equation I get : 21.6 feet - 3.87 feet = 17.76 feet At one atmosphere the oxygen window is small indeed. It+s: ( 1 - .79 )33 + .79( 2.04 ) - 5.47 fsw = 3.07 fsw That means that even the seemingly insignificant altitude exposure of flying in a pressurized air plane will cause some gas separation. When David Story commented on my oxygen windows as shooting way beyond, he+s probably referring to the normal, one atmosphere value. If a physicist could give us all a better understanding of this I would certainly appreciate it. But, to my best knowledge a practical rule is that: (sic) `the oxygen window is, slightly simplified, the partial pressure of oxygen at a given depth+, or in fact 3.87 feet less for air. Another practical way of viewing this: 130 feet ( about 5 ATA ) means one atmosphere of oxygen and four of nitrogen when breathing air. Four atmospheres of nitrogen can give the maximum nitrogen tension caused by four atmospheres. (Seemingly a beautifully redundant sentence, but tension is never expressed in atmospheres since it depends on solubility as well as pressure.) What does it take to hold a tension caused by four atmospheres in perfect, trouble-free solution? Four atmospheres! Thus, subtracting one atmosphere simply +takes in the slack+ before one actually starts to decompress - there is no supersaturation in any tissue before one ascends shallower than four atmospheres or 100 feet. (Note that all calculations are made with atmospheres absolute or feet absolute - remember to include/add the sea level pressure of 1 ATA / 33 fsw.) We never breathe 100 % inert gas, in that case there would be no +slack+ before supersaturation upon ascent. A non-diver is saturated at a P N2 of .79 atmospheres, not 1 atmosphere which is the maximum an ambient pressure of one atmosphere would balance perfectly. I think this is the essence of +inherent unsaturation+. The higher the O2 percentage, the lower is the inert gas percentage and more +slack+ is provided - the inert gas tension will never rise beyond the partial pressure of the inert gas. Nitrox 32 at 130 feet gives a P N2 of .68( 130 + 33 ) feet absolute = 111 feet absolute. What pressure does it take to keep a full load of N2 in solution? 111 feet absolute, or ( 111 - 33 ) feet = 78 feet Ascending from 130 feet to 78 feet takes the nitrogen load up to the pressure that balances the tissue tension. From there on we+re dealing with supersaturation and the inevitable nucleation of microbubbles. At 78 feet we+re dealing with 100 % saturation - Nitrox 32 ( 68 % N2 ) at 130 feet corresponds to 100 % N2 at 78 feet. One might ask, why am I talking saturation - according to m-values SCUBA diving will never saturate anything but the fastest tissues? The same physical laws apply for any tissue - more than 100 % saturation will initiate gas nucleation whether that tissue be blood or cartilage. The only difference is that fast tissues offgass a lot faster and will recover to accept further decompression faster. If one +takes in the slack+ of the oxygen window, the time he+ll have to spend at the ceiling in order to be able to proceed and still avoid nucleation will depend on tissue m - values. If we+re talking well perfused fast tissues, a couple of minutes will do. In practice we+ll probably break the oxygen window anyway, and accept some amount of microbubbles. The oxygen window gets increasingly critical as one approaches the surface - it+s practically impossible not to violate it from 30 feet and up. However, breaking it close to the surface is no big deal since the microbubbles formed won+t undergo too much further decompression and won+t grow to attain obstructive sizes. But, as I wrote in my previous posting, by switching to an oxygen-richer blend and wash out nitrogen at depth rather than undergo further reductions in pressure (Nitrox 60 at 50 feet) most, if not all, inert gas nucleation can be avoided. I+m actually combining the Haldanian and the +thermodynamic+ decompression approach. Readings: B. R. Wienke: Thermodynamic Decompression in AAUS +86 proceedings (Diving for Science +86) B.A. Hills 1966 A Thermodynamic and Kinetic Approach to Decompression Sickness. Library Board of South Australia, Adelaide A.R. Behnke 1967 The Isobaric (Oxygen Window) Principle of Decompression. Trans. Third Annual Conf. Marine Tech. Soc. 3, 213 Hans P. Roverud
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