The Apollo 55000-127APO Series 65 Conventional BR Heat Detector 75°c uses a dual thermistor network that provides a voltage output proportional to the external air temperature. The detector features a wide voltage range (9-33 volt DC) and can be integrated on to security systems when used with the 12 volt relay base.
The detector works by using a matched pair of thermistors to sense heat. One thermistor is exposed to the ambient temperature whilst the other is sealed. In normal conditions, the two thermistors register similar temperatures, but when a fire develops, the temperature recorded by the exposed thermistor will increase rapidly which will result in an imbalance of the thermistors and cause the detector to change the alarm state.
As the temperature increases, the detector will detect this. The detector has an fixed upper limit which when detected will go into alarm if the rate of temperature increase has been too slow to trigger the detector earlier.