Trivial The oxygen vendor will still charge you for adding no gas to a tank.

[elan_oots]

To be clear, what it's doing is *replacing* the air in your existing tank with fresh new air. I figured you wouldn't want to just top off an existing tank if it had impurities. It seems like it'd be pretty easy to either display the air contents in the air vendor so people don't do this, or check to make sure it's not full of pure O2 before replacing the gas. Either way, like Flourish said, this is intended behavior.