Nitrox - Trimix

[dofundive]

No one doesn't "have" to use argon. Again cost and availability may be a factor to some. Here in Milwaukee it costs around $6-$7 for an air fill. Argon costs $1 per cu ft. If I'm filling both of my 6 cu ft argon bottles I pay $12. In this case argon and air cost roughly the same and it's available. YMMV. There's also the whole physiological reason why argon is supposed to be better than air. There are many who can better explain that better than I can  .
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On a per cuft basis, Argon is the most expensive gas I buy, more than helium and astronomical compared to air. I'm a litte suprised a dive shop charges $12 to fill two 6 cuft bottles, mine doesn't even bother charging me. But, given that situation, argon is a no-brainer.

My shop doesn't carry He/Ar so I use air when I can't find Argon with a reasonable effort.

-Michael