The A1R conventional heat detector has a moulded self-extinguishing white polycarbonate case. Nickel-plated stainless steel wiper contacts connect the detector to the base. Inside the case, a printed circuit board holds the signal-processing electronics.
A pair of matched negative temperature co-efficient thermistors are mounted on the PCB in such a way that one thermistor is exposed to give good thermal contact with the surrounding air while the other thermistor is thermally insulated. Under stable conditions, both thermistors are in thermal equilibrium and have the same value of resistance.

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200-400 Datasheet