tplyons
Contributor
In all fairness, the DSAT Gas Blender manual and course teaches when you're blending helium mixes, if you didn't fill it, you start over with an empty cylinder.Two weeks ago I had a dive shop attendant tell me, rather condescendently, that it was recommended that I drain my tanks before starting to mix a new batch of trimix. I told him I NEVER do that. I wanted to reply that in school I was taught about PV=nRT and that he should try to gather a little more knowledge to alleviate the ignorance that is forcing him to drain tanks before mixing. I behaved and didn't.
I did tell him that I was not about to vent out 85 cu ft of 12/58 just because I wanted to mix in some 21/35 into the tanks. It is a lot of helium to throw out. And I was not asking him to blend anything for me. I just told him to top it up to 2600 psi with air. He then started asking questions about what was inside, and then I had to explain to him that it was pre-mix done on top of left overs from another dive. That's when he felt compelled to unsolicitedly share his mixing wisdom.
It reads: "As a blender, if you did not personally mix the gases for the previous dive, you have no way of knowing what combination of gases are in the remaining mix without an oxygen analyzer and a helium analyzer. In this case, you must always start with an empty cylinder." -- DSAT Gas Blender Manual, p. 45.
As I neither dive nor mix helium blends, I've never done this. But as far as Nitrox goes, I never drain unless I need to bleed down to hit a specific mix.