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                        Heat
                          DetectorThe 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. |