O-ring that seals the SI ultraviolet intensity sensor into its port on a Trojan reactor. It is the seal between the pressurised water in the chamber and the dry side of the sensor, so it is replaced as a matter of course whenever the sensor is withdrawn — a sensor refitted on a flattened or nicked O-ring is the usual cause of a weep at the sensor boss. Consumable. Contact us if you are unsure whether this part fits your UV system.