This is the AS568A-012, 1/2" O-ring that seals your H.P. Regulator hose/port to the regulator first stage body valve used on aluminum and steel tanks. Fluorocarbon Elastomers (FKM): O-rings made of FKM have been the preferred choice for oxygen and nitrox compatibility in SCUBA diving applications. FKM is an elastomer with excellent oil and oxygen resistance at high and low temperatures, very good chemical resistance. Even for use with ordinary air, most experts agree that FKM O-rings outperform common Nitrile O-rings. Viton is DuPont Performance Elastomer's brand name for FKM.