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How Smoke Detectors Work
Smoke Detectors
Not all fires are alike. Some are slow burning and smoky, some are fast burning, producing high heat but less smoke. Each type of fire requires an appropriate technology.
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Heat Detectors
A metal heat detector is a device that detects the heat from a fire and sounds an alarm or triggers a water sprinkler system. Heat detectors do not detect smoke and will not meet fire codes that require a smoke alarm.Smoke Detectors and Alarms
Smoke alarm technology detects smoke from a fire and sounds an alarm. Several types of smoke alarm technologies are available.Photoelectric Sensors
Photoelectric sensor technology relies on an electric current that produces a beam of light. When the beam of light is interrupted, an alarm sounds. Photoelectric sensors detect slow, smoldering, smoky fires more quickly than other technologies.Photoelectric smoke detectors contain a light source and a light-sensitive electric cell. Smoke particles in the detector deflect the light onto the photoelectric cell, thus generating a current and triggering the alarm. Optical smoke detectors more quickly detect slow burning, smoky fires. Some fire protection experts recommend using both types of detectors.
Ionization Sensors
Ionization sensor technology contains a small amount of radioactive material encapsulated in a metal chamber. Ionization technology is faster at reacting to fast flaming fires that give off little smoke.Ionizing sensors contain a small amount of americium-241, a radioactive material. It is housed in an ionization chamber, which is basically two metal plates a small distance apart. One of the plates carries a positive charge, the other a negative charge. Between the two plates, the alpha particles emitted by the americium-241. The alpha particles collide with the air in the ionization chamber to produce charged particles. The positively charged metal plate attracts the negatively charged ions (electrons) and the negatively charged plate attracts the positively charged ions (mostly oxygen and nitrogen). The steady flow of ions creates a small but steady current.
The current is disrupted when smoke enters the ionization chamber. Smoke particles attach to the charged ions and restore them to a neutral electrical state. This reduces the flow of electricity between the two plates in the ionization chamber, and when the electric current drops below a certain threshold, the alarm is triggered. Hot air entering the chamber changes the rate of ionization and therefore, the electric current level, which triggers an alarm.
