Guy's...welders need their CO2 to be just as pure as we do so don't worry about exchanging tanks. These places are a rip off and will rape you for as much as they can. First thing...stay away from Praxair, they are by far the worst. Next, setup an account with Airgas and tell em you want contractors prices. My buddy took our Argon tank down to AG a few months back, they charged him 70.00 to fill it...I went a week later to fill our 02 tank, the guy told me he could have filled it for 29.00 if he would have used my account....