Elastomer seals in an ultraviolet reactor work against warmth, pressure and direct UV exposure, which hardens and shrinks them over time. They are replaced whenever the joint is opened rather than reused: a seal that looks sound can still have taken a compression set that will not re-seat. Compound matters as much as size on a UV-exposed seal, so order against the position rather than measuring the old one. This is the OUTER seal. The inner sleeve O-ring for the BX20-30 is stocked separately under item 13141-1110 — this position takes both. Wedeco was founded in Herford, Germany in 1976 and is now a Xylem brand.