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Half-times and parallel compartments May 12, 2009

Posted by Chris Sullivan in Technical Diving.
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I realized a couple of days after writing my last post about half times what the whole point of Bühlmann method of having the same compartment numbers for Helium and Nitrogen, but with different half times (these being proportional to the square root of the molecular weight of the gas, no less). When diving with trimix, for instance, you would add the partial pressures of the two gases in each compartment together to get the total inert gas pressure. For convenience, these pressures are measured in feet or metres of water.

The pressure in each compartment is then compared to the M-Value for the compartment to see whether the compartment pressure is more or less than the allowable overpressure  at the surface (or decompression stop, as required), and this becomes the basis of the decompression calculations. Some methods use the same M-Value for Helium as for Nitrogen, but Bühlmann has different values, and the accepted method is to linearly interpolate the He and N2 M-Values using the ratio of partial pressures in the compartment.

One of my new year’s resolutions was to not try to attempt writing any decompression code but I’m starting to feel it might be fun, now that my understanding of the topic is beginning to grow. Perish the thought. Well maybe just a little in MS-Excel.

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