Ultraviolet intensity sensor for the Trojan UVMax PRO 50. A UV intensity sensor reads the germicidal output actually reaching the water at 254 nm, which is the only direct evidence that the reactor is still delivering its validated dose. Lamp output falls with age and quartz sleeves foul, and both show up here before they show up anywhere else, so the controller alarms on this reading rather than on lamp hours alone. Specific to the PRO 50; the PRO 10 and PRO 20 take their own sensors and the three are not interchangeable. Contact us if you are unsure whether this part fits your UV system.