... hardness for your option, including Buna-N (NBR), HNBR, VITON, Silicone, FluoroSilicone, EPDM, Teflon, Natural Rubber, BUTYL, SBR, Neoprene, HYPALON, etc ... DVGW certification and British WRAS (WRC) drinking water standard certification. Silicone Rubber O-Ring