Bimetal strips form the basis of thermal fire detectors designed to operate at a fixed temperature or a ‘rate of rise’ in temperature. When temperature increases, the bimetal curves as the metal with higher coefficient of expansion lying on the outer side of an arc undergoes a greater increase in length. With one end fixed, the movement of the free end of the strip can be arranged to close an electric circuit that operates an alarm. This principle is utilized in thermal fire detectors designed to operate at a fixed temperature or a ‘rate of rise’ in temperature.
Figure above shows how a ‘rate of rise’ thermal fire detector operates using two bimetal strips. One bimetal strip has a higher thermal inertia either because it is lagged, as shown in the figure, or because it is thermally shielded from the space being protected. On a appreciable rate of rise in temperature, the contact ‘B’ on the faster response bimetal strip closes on contact ‘C’ of the slow acting bimetal strip. This causes an alarm signal to be produced by an alarm circuit connected between points ‘A’ and ‘D’. In the case of a very slow rate of rise in temperature, the difference in movements between contacts ‘C’ and ‘B’ will be such that a high temperature will be reached before alarm sounds. To ensure that the alarm signal is initiated before a temperature of 78 degree Celsius is reached, a second contact ‘F’ is provided on the slow acting bimetal strip. At the required space temperature, contact ‘E’ closes to contact ‘F’ and n alarm signal is initiated. At low rates of temperature rise, (less than 1 degree Celsius / minute) the alarm should not operate until the temperature exceeds 54 degree Celsius. Without the insulation, the upper bimetal strip would act as a basic fixed temperature thermal detector.
Thermal fire detectors are least sensitive type of detector. They have a high thermal inertia and the fire has to produce large amount of heat before the temperature at the detector is sufficient to cause it to operate. Consequently, they are normally used in spaces such as laundries, drying rooms, galleys, and pantries, where other detectors are susceptible to false alarm from water vapour or smoke.
In the most recent detector designs the bimetal strips have been replaced by thermistors (solid state devices of, for example, nickel, manganese and cobalt, whose electrical resistance changes significantly with temperature ). The principle of operation is, however, no different as one thermistor is exposed to the air and one is shielded.
“Fire Safety at Sea” by Dr. James Cowley