I understand the question. I believe you're asking this because when you buy a brand new propane tank and have it filled, they do some little purge test thing, mainly I think to purge air and moisture inside the tank.
I don't think they do that with CO2 maybe because its non-flammable?
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10