a displaceable purifier body improves the delivery of high purity gases from gas tanks or cylinders by providing a large flow area for filling the cylinder or tank with gas while preventing contamination of gases during the filling procedure and by automatically engaging the purifier for purification when withdrawing gas from the tank or cylinder . the purifier body or a separate seal body has an arrangement for reciprocal movement between a filling position wherein the gas entering the cylinder by - passes the purifier body and an engaged position wherein the purifier body moves towards the cylinder head assembly to prevent by - passing of exiting gases around the purifier . this arrangement offers little restriction to gas flow entering the cylinder and allows rapid filling of cylinders to make gas delivery more economical . the arrangement can also provide an indicator showing positive engagement of the purification system during gas withdrawal and a simple method for manually disengaging the purifier to allow exiting gas to bypass the purifier when desired .