The temperature switch that protects the ultraviolet reactor against overheating. A UV reactor left energised at low or no flow will overheat, so the temperature channel is a protection function rather than an instrument: a failed switch either trips the plant needlessly or removes a safety interlock. This part is an Endress+Hauser Thermophant T TTR31 — an instrument sold under its own manufacturer's designation, which means it can be identified and specified generically as well as ordered here. Confirm the switch point and process connection against the existing unit before fitting.