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, more seriously, over-report a failing one. Wedeco designates this sensor SO 20101; the designation is carried exactly as the manufacturer states it. Sensor types are calibrated to a particular lamp spectrum and reactor geometry, so substituting a different type produces a reading that looks plausible and is wrong. Wedeco was founded in Herford, Germany in 1976 and is now a Xylem brand.