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RE: Differential Helium loss > Could you elaborate on this. I've seen it four times so far; once with trimix and 3 times with heliox. The trimix was the first, and only involved a pressure drop of about 150 psi and an O2 rise of about 2%, so I'm not totally sure it was helium loss or thermodynamic effects, or something else. In the cases of heliox, I had blended pony bottles of Heliox-21 (normoxic heliox), and later discoverd that the FO2 had increased to 25, 27, and 30% (respectively for the three times it happened). Because of the way I initially mixed & analyzed the bottles, I'm fairly confident the errors were due to helium loss, and not mixing problems. Howver, I did not make accure pressure readings at fill-time, so I can't corroborate the loss in terms of reduced pressure. > How long did it take? About two months passed between fill and noted change in the trimix case. Only a couple of weeks were involved with the pny bottles (but since they are small 13cf cylinders, I would expect them to be more sensative to this problem). > What do you do to minimize it. I blend the gas the night before I use it. But most importantly, ANALYZE the mix as soon before using it as possible. To make heliox, I decant helium onto oxygen; so if this problem happens, I just top-off with helium. If it happens in trimix, it's a little trickier. Unless you have a booster pump to add the helium at high pressures, you'd change your trimix ratio by topping-off with an air compressor. I'm tempted to say "tighten the valves", but I suspect this doesn't make too much of a difference. Incidently, I've been told there is no valve in existence that can hold back high pressure helium indefinitely. However, I know it's not always a problem because I had a 30 cuft pony bottle of trimix that maintained pressure anx FO2 for more than a year. Nevertheless, people who incorporate backup helium-mix cylinders into their protocol should periodically verify the mixture if they go for long periods without using the backup cylinders (which is how I ended up with an unused 30 cf cylinder of trimix for more than a year). > Whats a good time/rolling exercise procedure to minimize the > total fill time, assure mixing, and avoid helium loss? I don't know. I've mixed trimix in a variety of places, a variety of ways, and at a variety of different fill rates, but I have not noticed any trends (yet). Hope that helps. Aloha, Rich Richard Pyle deepreef@bi*.bi*.ha*.or* ******************************************************************* "WHATEVER happens to you when you willingly go underwater is COMPLETELY and ENTIRELY your own responsibility! If you cannot accept this responsibility, stay out of the water!" *******************************************************************
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