The heat detector operates by using a matched pair of
thermistors to sense heat. One thermistor is exposed to the ambient
temperature, the other is sealed. In normal conditions, the two thermistors
register similar temperatures, but, on the development of a fire, the temperature
recorded by the exposed thermistor will increase rapidly, resulting in an
imbalance of the thermistors and causing the detector to change to the alarm
state.
Rate of Rise detectors are designed to detect a fire as the temperature increases,
but they also have a fixed upper limit at which the detector will go into alarm
if the rate of temperature increase has been too slow to trigger the detector
earlier.
Fixed heat detectors only change to the alarm state at a preset temperature.
Fixed heat detectors only change to the alarm state at a preset temperature.
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