I have an aviation oxygen tank that I intend to convert to on-board air use. However, it is not as simple as just re-purposing the tank. In my case the cylinder is green and stenciled; Aviator Oxygen. No one will fill this tank with anything except oxygen as it is presently marked and I'm sure you can figure our why. I will have to take it to a gas supply company that will test it, bead blast it, repaint it, and refit the adapter. Only then will I be permitted to fill it with anything other than oxygen. I bought the tank at an estate sale for somewhere around $20 and the above work will be an additional $100. At some point you're just better off buying the proper CO2 tank to begin with.