Smoke Detectors

What are smoke detectors?

Smoke detectors (sometimes called a "smoke alarms") are safety devices that detect airborne smoke and issue 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 smoke detectors

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 smoke detectors

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.

Standards for Smoke Detectors

Underwriters Laboratories (UL) has three standards for smoke detectors: one for duct detectors, UL 268A; one for single and multiple station smoke alarms, UL 217; and one for systems type detectors, UL 268. Detectors should only be used for the applications for which they are specifically listed.

Testing, maintenance and service of smoke detectors

Smoke detectors are designed to be as maintenance free as possible. However, dust, dirt, and other foreign matter can accumulate inside a detector’s sensing elements and change its sensitivity. They can become either more sensitive, which may cause unwanted alarms, or less sensitive, which could reduce the amount of warning time given in case of a fire. Both are undesirable. Therefore, detectors should be tested periodically and maintained at regular intervals. Follow closely the manufacturer’s specific recommended practices for maintenance and testing.

Where to place smoke detectors?