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Theme: Re: Decompression Theory: Deep Stops
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Date: 27/03/00
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Author: Greg Roach
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Jason Shepherd
> Intuitively, we would calculate the ingress and egress of each gas > separately, and sum the tensions to give the current tissue tension; this is > then compared with inspired partial pressure to calculate the next iteration > of dissolved gas diffusion, and with M values or a+b values and ambient > pressure to calculate safety margin. I wouldn't say "intuitive", but that's pretty much the case. > However, Lettnin, in the handbook, quotes Buhlman 1983, as 'factors a and b > depend on the inert gas used, inert gas mixes, and on the half life of each > individual compartment.' So they do. You only have one M value (per compartment), which is calculated from one "a" value and one "b" value. > Anyone care enough to educate me on how decompression management systems > handle two inert gases? (Lettnin, BTW has a comprehensible explanation of > isobaric counterdiffusion - which I like - and a similar model of > narcosis/HPNS - which worries me). The "a" and "b" values are defined for individual inert gases. i.e. one set for helium and one set for nitrogen. Where your tissues have multiple inert gases, you need to generate a specific "a" and "b" for each compartment. This is done by using a weighted average of the N2 and He "a" and "b" values, weighted according to the pressure of each inert gas in each tissue. > Any chance of Gradient Factors making their way into Z-plan? No - Will Smithers (the author) has been asked, and has declined. He has also declined to fix the number of annoying bugs in Z-Plan. What you want is some decent gradient factor software, such as DivePlan (and a Psion5 to run it on), which is actively supported, free of charge, and written by a jolly decent chap ;-). See http://www.subaqua.co.uk/DivePlan/ BTW - Erik Baker gives a very good explanation of how to implement mulitple inert gases and gradient factors. I've put it on my web-site at http://www.subaqua.co.uk/decocalc.zip if anyone is interested. Greg -- Greg Roach, Stony Stratford, Bucks, England, greg@subaqua.co.uk |
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