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In a message dated 3/26/00 3:13:44 PM Central Standard Time, rthomps@co*.or* writes: << Now, my admittedly naive understanding of helium compressibility suggests that I really need to start with more than half-filling the cylinder with helium to obtain the desired mix. My question is: How much more? Specifically, if I'm using twin LP 112's with an ending pressure of 2640 psig, what fudge factor should I use to end up with tri-mix 18/50? Same question if, instead, ending pressure is 3500 psig? >> Virtually all mixing programs and mixing tables are based on a perfect gas model. (i.e., The compressibility of Helium is not taken into account.) Most experienced mixers use some "rule-of-thumb" or educated guess to adjust their mix. A typical technique is to add 50 to 100 psi Helium to mix and subtract an equal amount from the air or nitrox added. Hope this helps. Dive safely, Dale -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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