Fire detector

(engineering) A temperature-sensing device designed to sound an alarm, to turn on a sprinkler system, or to activate some other fire preventive measure at the first signs of fire.

A device designed to initiate a desired action in response to a change in its surroundings caused by an unwanted fire. Heat detectors respond when the surrounding temperature exceeds a specific minimum value or when the rate of rise exceeds a specified rate. Heat-detector sensing elements are normally mechanical, employing components which melt or expand when heated, but can also be electronic, using elements which produce thermoelectric voltages or change in conductivity.

Smoke detectors are also commonly used fire detectors. They respond when the concentration of solid or liquid particles in the air exceeds a selected value. In ionization-type smoke detectors, a small radioactive source is employed to make the air within the sensing chamber slightly electrically conductive. The presence of particles reduces this conductivity in proportion to the number of particles present. In photoelectric-type smoke detectors, a light source and light-sensitive receiver are arranged so that light from the source does not normally strike the receiver. Smoke particles reflect light from the source onto the receiver in proportion to the number of particles present.

Flame detectors contain receiving elements sensitive to light in certain specific infrared or ultraviolet wavelengths characteristic of flames. Flame detectors can detect a fire within a few milliseconds and are therefore most often used to activate explosion-suppression systems.

The actions initiated by fire detectors include sounding of audible and visual alarms, signal transmission to remote monitoring stations, activation of extinguishing systems, and emergency shutdown of equipment or processes which might increase the severity of the fire hazard. See also Automatic sprinkler system; Fire extinguisher.