Heat detector
The importance of safety from fire and its accompanying fumes is highly recognized as a life saver for occupants of any structure. Fire alarm systems are in common use for early warning of a fire condition. Such fire alarm systems generally use smoke detectors or heat detectors capable of detecting either the presence of smoke particles in an area or an increase in temperature in the area as a result of a fire condition. A heat detector monitors the ambient thermal condition in the region where the heat detector is placed. Heat detectors generally trigger an alarm signal when either the actual temperature in the area being monitored exceeds a predetermined level, or when the rate of rise of the temperature in the area exceeds a predetermined level. Some detectors used in fire alarm systems incorporate both a smoke detector capability as well as a heat detector capability. A heat detector can be included as part of a smoke detector or operate as a stand alone device. Heat detectors may use a mechanical temperature sensing element connected to a suitable switch. Such a heat detector utilizes a spring-loaded plunger that is held fast at normal room temperatures by a conductor which melts at a preselected temperature and allows the spring to return to its relaxed state, which in turn causes a switch to close and operate a remote alarm panel or system. A second type of heat detector in use is constructed using a bimetallic switch. Varying environmental temperatures cause a bi-metallic element to snap between two bi-stable positions, which in turn causes a switch to be opened or closed as a function of the bimetallic element. A third type of heat detector includes both a fixed temperature sensor and a rate-of-rise heat sensor. In addition to the spring-loaded switch actuator, the actual physical housing of the heat detector acts pneumatically upon the switch. Thus, when a relatively rapid change in temperature occurs, the expansion of the air within the housing causes an integral bellows to expand and actuate the same switch that is used for fixed temperature applications. Another type of heat detector includes a thermistor to sense the ambient temperature. The resistance of the thermistor changes as the ambient temperature increases and decreases.
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