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Fire Detectors

The sooner a fire is detected, the easier it is to put it out and the less threat to the occupants it presents. Fires develop in four stages which include the incipient, smoldering, flame, and heat stages. Various types of detectors will respond to the different stages of fire.

A fire detector is a device designed to start a desired action in response to a change in its surroundings caused by a fire. The general types of detectors which are in common use are thermal detectors, photoelectric or smoke detectors, flame detectors, products of combustion (ionization) detectors.

Heat detectors respond when the surrounding temperature exceeds a specific rate. Heat detector sensing elements are normally mechanical employing components which melt or expand when heated. They can also be electronic using elements which produce thermoelectric voltages or change in conductivity.

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.

In ionized-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 the conductivity in proportion to the number of particles present.

In photoelectric type smoke detectors, smoke particles reflect light from the source onto the receiver in proportion to the number of particles present.

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.

The actions started by fire detectors include sounding or 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.

Fire departments advise people to install smoke detectors in their homes. Smoke detectors sound an alarm if a small amount of smoke enters their sensors. Smoke detectors are attached to the ceiling or wall in several areas of the home. Fire protection experts recommend at least one detector for each floor of a residence. Most home fires that result in deaths occur at night when the family is asleep. Smoke detectors will awaken the family before the flame and the smoke build up to the point where escape is impossible.

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