Elastomer seals in an ultraviolet reactor work against warmth, pressure and direct UV exposure, which hardens and shrinks them over time. They are replaced as a set whenever the reactor is opened rather than reused; a seal that looks sound can still have taken a compression set that will not re-seat. Size alone does not specify the part — the compound matters as much as the dimensions on a UV-exposed seal, so order against the position rather than measuring the old one. This seals the ultraviolet intensity sensor into the reactor wall and is renewed whenever the sensor is disturbed.