Ultraviolet intensity sensor, SI type, for Trojan UVLogic reactors, supplied with the short 3 ft cable. 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. Trojan part 793200-001. The same sensor is also supplied on a 21 ft cable as part 793200-004 and is stocked separately; cable length is what separates the two part numbers, so order against the cable run between the reactor and the controller. Contact us if you are unsure whether this part fits your UV system.