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Smoke Detectors and Alarms

Smoke AlarmSmoke detectors are one of the most important safety devices you can have in your building. Smoke detectors provide early warning and enable emergency action in the event of a fire.

They are inexpensive, easy to install, unobtrusive, and require very little maintenance, and no building should be without them.

A smoke detector (sometimes called a "smoke alarm") is a safety device that detects airborne smoke and issues an audible alarm, thereby alerting nearby people to the danger of fire. Most smoke detectors work either by optical detection or by ionization, but some of them use both detection methods to increase sensitivity to smoke.

Optical Detector
This type of detector includes a light source, a lens to collimate the light into a beam, and a photodiode or other photoelectric sensor at right-angles to the beam. In the absence of smoke, the light passes in front of the detector but does not fall on it. When visible smoke enters the beam, some light is scattered by the smoke particles, and some of the scattered light is detected by the sensor. An increased output from the sensor sets off the alarm.

Ionization Detector
This type of detector is cheaper than the optical detector, and can detect particles of smoke that are too small to be visible. It includes a tiny mass of radioactive americium-241, which is a source of alpha radiation. The radiation passes through an ionization chamber, which is an air-filled space between two electrodes, and permits a small, constant current to flow between the electrodes. Any smoke that enters the chamber absorbs the alpha particles, which reduces the ionization and interrupts this flow of current, setting off the alarm. Hot air entering the chamber changes the rate of ionization and therefore, the electric current level, which triggers an alarm.

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