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 is the end flange seal for the InLine 50. Original Berson number 26SE168V. Note that this catalogue also lists a 168 × 4 mm end plate seal under item 1077-1206 — the two are different positions at the same nominal size.