Berson reactors with an automatic wiper sweep the outside of the quartz sleeve on a timed cycle. Scale, iron and organic film attenuate UV-C before it reaches the water, so a wiper that has stalled or is wiping unevenly shows as falling measured intensity on lamps that are still good. The wiper mechanism passes through the reactor pressure boundary, so its seals are a scheduled wear item rather than a fit-and-forget part: a seal that has hardened lets water track into the drive. This is the 10 × 25 × 8 mm seal for the automatic wiper mechanism.