[DougNR]

I don't believe a mix would change over a practical length of time you would let a tank sit idle unless you hade some serious corrosion going on, in which case the oxidation would reduce the o2 level. At that point you've got much more series issues at hand.

I've never felt I needed my own analyzer when filling at a shop, but I admit to concerns with regard to the accuracy of a few units I've used aboard charter boats when renting tanks through that service. The o2 sensors wear out pretty quickly and they need to be well cared for. Without any imperical data to back me up, I just 'feel' the reading seem to vary more when you're aboard.

Regards;