How Do Smoke Detectors Work?

In the United States, a person dies in a fire or from a fire related injury every two hours. According to National Fire Protection Association data, most people who survived house fires were able to do so because the building was fitted with working smoke detectors. According to a related NFPA study, close to 1,000 lives could be saved each year if all home owners would use and properly maintain smoke detectors.

A smoke detector can be used either in an industrial setting, usually hooked up to a larger fire alarm system, or as an independent device in your residence. There are two types: the photoelectric detector, and the ionization detector.

The Photoelectric Smoke Detector

Inside this circular device is an enclosed space where, on one end, a beam of infrared light (or light from a LED) travels unblocked toward the other end of the enclosure where a photodiode (a tiny, tubular component that turns light into electric current) is mounted. The light beam does not hit the photodiode; it is directed slightly away from it. However, when smoke is present in the area, it enters the smoke detector and gets into the space where the light beam and light detector (the photodiode) are mounted. Smoke particles scatter the previously straight light beam and cause some of it to hit the photodiode. The photodiode will then convert the light into an electrical pulse that sounds the alarm. This type of smoke detector is effective in detecting smoke coming from smouldering fires, but some reports show that it may need substantial amounts of smoke particles to disturb the light beam before the alarm mechanism is activated.

The Ionization Detector

Ionization refers to the process where molecules become either positively or negatively charged. Inside an ionization detector is an air-filled compartment where two electrodes — in this case, small, thin wires that conduct electricity — are installed. A constant electric current passes between these two electrodes in the absence of smoke. When smoke enters the compartment, however, its particles become ionized, disrupting the constant electrical current between the two electrodes. This sudden change triggers the alarm mechanism in the device. This type of detector does a good job in detecting flames, but one drawback is that it's prone to triggering false alarms because of its sensitive smoke detection process.

New smoke detector designs that combine both technologies are in development, ensuring more accuracy and reliability in these very important devices. Fire safety experts around the nation encourage users to follow strictly the recommended usage and maintenance guidelines from smoke detector manufacturers. The usual causes of faulty (and therefore useless) smoke detectors are dead or disconnected batteries, dirt accumulation, and malfunction due to the device's old age. Most manufacturers advise replacing a smoke detector unit after ten years.

Live Science Staff
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