How Smoke Detectors Work
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Parts
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Smoke detectors have two main parts. The first is a sensor, which detects smoke. In a typical home unit, the sensor is contained in an ionization chamber. When smoke gets in, it triggers the second part, which is a loud alarm. The alarm is designed to alert people in the vicinity and also to wake them up if a fire occurs at night. Smoke detectors are powered by either a 9-volt battery or household electrical current. Some are wired into the 120-volt house current and also have a battery for backup in case the fire causes the electrical system to fail.
Types
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There are two main types of smoke detectors. The first, photoelectric smoke detectors, are not commonly used because they are large and are not very sensitive. They use a beam of light to detect smoke in the vicinity. When the smoke gets so thick that it breaks the photoelectric beam, the alarm is triggered. However, it may take some time for smoke to get sufficiently thick to set off this type of detector.
The second type, ionization detectors, are much more common because they are sensitive and compact. These are the type of smoke detectors typically used in homes. The sensor contains a very small amount of radioactive material. This material generates alpha particles, which ionizes the surrounding oxygen and nitrogen atoms and creates a minor electrical current. When smoke gets inside, the current is interrupted and the detector's alarm is triggered.
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Placement
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Because of the way they work, smoke detectors must be placed in an area that allows them to be in the path of the smoke generated by a fire. For this reason, the Foster City Fire Department recommends installed them away from cooling or heating ducts. They should also not be mounted within 6 inches of the spot where a wall and ceiling meet. Otherwise, smoke will have difficulty entering the detection chamber.
Maintenance
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Because smoke detectors work on a fairly simple principle, the Foster City Fire Department says that they don't need much maintenance. They should be tested twice a year. If you are using a battery-operated unit, the battery should be replaced annually.
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Resources
- Photo Credit Photo: Holmestown Volunteer Firefighters
