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. A leak at this position wets the sensor and ends it, so the seal is renewed whenever the sensor is disturbed.