Photoelectric Smoke Detectors

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Furthermore, ionizing detectors tend to give false alarms when exposed to the fumes produced by cooking or other household activities. Dual chamber models help alleviate the problem, but some studies have shown that, even at their best, ionization smoke detectors "cry wolf" almost twice as often as does the alternative type of device.

Photoelectric detectors employ a light emitting diode (LED) and a receiving photocell, which are set into a chamber that must be light-, but not smoke-, tight. Smoke particles enter the detection chamber through a convoluted path and scatter the beam from the LED toward the receiver. When the photocell catches enough light to reach its "threshold of detection", the alarm sounds.

Because large particles do a very effective job of scattering the light from the LED, photoelectric smoke detectors are particularly sensitive to smoldering fires ... such as might occur if a lighted cigarette fell onto a mattress. In fact, a photoelectric detector might respond to a smoky fire as many as 20 minutes sooner than would an ionization device. (On the other hand, the latter unit might pick up the fumes from an actual blaze as many as 30 seconds sooner than the photoelectric unit would.)

Obviously, then, each type of detector has its own particular capabilities, and many experts recommend that you have at least one of each variety. In the case of most fires, however, enough of both the large and small particles will be present to trigger either type of detector, but-and this is a very important "but"-in the rare case where a smoldering fire doesn't produce sufficient particles to set off an ionizing detector, the lag in alarm could be long enough to jeopardize human life. Furthermore, smoldering fires are the primary cause of fire-related fatalities in the U.S., because they often produce large volumes of toxic gases without giving warning signs. We (and many experts), then, believe that if only one detector is to be used, the photoelectric alarm might provide a greater margin of safety than would an ionizing device.

Though both ionization and photoelectric smoke detectors can operate on either batteries or household current, and their power demands are similar, the photoelectric device may require photocell replacement after being in use for five to seven years (a warning light indicates a weakening of the light-sensitive receiver). Ionizing detectors, on the other hand, should last for quite some time ... since their operating medium—americium-241—has a radioactive half-life of 458 years!

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