Posted by Kendall Raine on March 23, 2001 at 08:40:40:
In Reply to: Re: tri mix cert. posted by Steve on March 23, 2001 at 00:10:19:
You're intuition is right. You do need to know, but the threshold of materiality is different for He and O2. You use an O2 analyizer for F02 and arithmetic for FHe. You need to have confidence in the person filling your tanks, too. He is very sensitive to heat and real gas laws are different from ideal gas laws. That's why you need to incorporate compressibility factors for He and O2. They're effectively different for different proportions and different temperatures.
The more times you blow back a mix the more error factors compound. That's why you always test the FO2. Getting the PO2 a little wrong for the mission depth can kill you. Getting the PHe wrong a little probably won't do anything. Personally, I don't test for FHe because a slight error is immaterial for deco purposes. By the same token, I only blow back a trimix once because 1) the FHe gets diluted down beyond the point where I want it and 2) error factors compound beyond where I'm comfortable.
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