The only reason I wondered is the constant pressure in a Power Tank. It's at 3000 psi right up until there is nothing less, because the CO2 evaporates and creates air (is there a physics major in the house?).
Plain air in a scuba tank starts at 3000 psi for a good fill, then drops off to 500 when you head back to the boat (although I was always empty when I surfaced
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