Fire, Flame, and Smoke Detectors

A fire occurs in four distinct phases. In the incipient phase, warming causes the emission of invisible but detectable gases. In the second phase, smoldering, smoke is formed so smoke detectors can be used. In the third phase, when the ignition temperature has been reached, flames and their emitted radiation (IR and UV) can be detected. In the fourth and last stage of the fire, heat is released the temperature of the space starts to rise, and the use of thermal sensors becomes feasible. 

To detect the incipient stage of fire, ionization-chamber-type sensors are used that analyze the composition of the atmosphere by measurement of its conductance. The ionization rises as the invisible combustion gas concentration rises. 

Once the fire starts to smolder and smoke is present, photoelectric sensors can be used to activate alarms. As the smoke density rises, less light passes from the source to the receiver, and an alarm is activated. 

There are two types of thermal sensors the rate-of-rise sensor and the absolute temperature sensor. Fixed temperature sensors are usually either bimetallic or low-melting-point fusible link devices that activate sprinklers or other extinguishing devices. 

Once there is a flame, it emits a flickering radiation, which is mostly in the IR wavelength. Therefore, IR sensors can be used to detect the presence of 

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