The UV intensity sensor reports the UV-C actually reaching the water. That is the measured variable a disinfection duty is controlled and alarmed on — not lamp hours and not lamp current — so a sensor that has drifted or fouled will under-report a healthy plant or over-report a failing one. This is the original-specification sensor for the Aquada Maxima, Wedeco part 37250. Sensors are calibrated to a particular lamp spectrum and reactor geometry, so an approximate substitute produces a reading that looks plausible and is wrong. Wedeco was founded in Herford, Germany in 1976 and is now a Xylem brand.