A home smoke alarm detector provides an early warning against fires by detecting the presence of smoke within a room. Most home smoke detectors are housed within disc-shaped plastic enclosure about 6 inches in diameter, and emit a loud audible alarm when the presence of smoke is detected.
A smoke alarm detector is normally powered by batteries, although some are powered by mains electricity with a battery backup. Smoke detectors either use photoelectric sensing technology, or ionization sensing technology to detect the presence of smoke, and some smoke alarms use both types of sensors to offer increased smoke detection sensitivity.
2. cover
3. case moulding
4. photoelectric sensor
5. infrared LED
These smoke alarms use an optical detector to detect the presence of smoke. In the optical chamber, a beam of light is projected by a bulb or an infrared LED. This beam of light passes in front of a photoelectric sensor, without hitting the sensor. If smoke enters this optical chamber, some of the light from this beam is scattered, and this scattered light will then hit the photoelectric sensor, thus triggering the smoke alarm.
The advantage of a photoelectric smoke alarm detector is that it is less likely to issue a false alarm from bathroom steam or from kitchen cooking fumes than an ionization smoke alarm.
The black structure on the right
is the ionization chamber
An ionization smoke alarm detector includes a tiny amount (about 150 milligrams) of radioactive americium – the alpha radiation produced by this americium is extremely minute and is quite safe. These smoke alarms contain an ionization chamber containing 2 metal discs connected to a battery – one plate has a positive charge, and the other a negative charge.
The alpha particles produced by americium cause the oxygen and nitrogen atoms within this chamber to ionize (ionization is the process of removing an electron from an atom), with the negative electrons being attracted to the positive plate, and the resulting positive atoms being attracted to the negative plate. If smoke enters the ionization chamber, the smoke neutralizes the charged particles within the chamber, which causes a drop in the current between the plates and thus setting off the alarm.
The advantages of an ionization smoke alarm detector is that its cheaper than a photoelectric smoke alarm, and its also able to detect tiny invisible smoke particles that a photoelectric sensor cannot detect. The disadvantage of an ionization smoke alarm detector is that it’s more prone to issuing false alarms.
A photoelectric smoke alarm detector detects smoke generated by smoldering (cool, smoky) fires quicker than ionization smoke alarms. An ionization smoke alarm detector is quicker to detect smoke particles generated by hot, flaming fires. Some smoke alarms include both photoelectric and ionization sensors, and if you can afford it, these are the best smoke alarms to buy.
Any smoke alarm detector should be powered by batteries, or by mains with a battery backup – a smoke alarm powered only by mains electricity is not much good if the power goes out. Most smoke alarms take 9V, AA or D-type batteries, but the 9V battery type is the best option, as these batteries last for years. Some smoke alarm units come prepackaged with the batteries, and with some units you have to buy the batteries separately. Most smoke alarms have a test button on the outer housing of the unit which tests if the batteries in the unit still have power, and whether the unit is still functioning – an essential feature, and this button should be pressed periodically.
The last consideration for buying a smoke alarm detector is whether the smoke alarm has a loud enough siren alarm. Some smoke alarms include a visual alarm in the form of flashing lights, but unless the audible alarm is loud enough to wake you from the slumbers of sleep, its not much good to you.
The Kidde PI9000 Battery-Operated Dual Ionization and Photoelectric Sensor Smoke Alarm
Images courtesy of Wikipedia
