Introduction
There are two common types of smoke detectors available on the
market. Each type is distinguished by its detection method. The
most common smoke detector uses ionization
sensors to detect smoke. The other type of smoke detector uses a photoelectric
sensor to detect smoke.
Ionization Detectors
|
Ionization detectors detect the presence of invisible particles (less
than .01 micron in size) in the air. Inside the detector, there
is a small ionization chamber that contains an extremely small quantity of
radioactive isotope called Americium-241. The Americium-241 will decay
or emit alpha particles at a fairly constant rate. The alpha particles,
which travel at an extremely high rate of speed, ionize or knock off an
electron (negative charge) from the atoms in the air (oxygen and nitrogen
molecules) passing through the ionization chamber. |
 |
| The free electron
(negative charge) is then attracted to a positively charged plate, and the
positively charged oxygen or nitrogen is attracted to a negatively charged
plate. This creates a very small but constant current between the
plates. When particles, such as combustion particles, dust,
steam or fumes enter the chamber, the current is disrupted when the
electrons attach to the particles. If the current drops too low, the
alarm is triggered. [Note: The radioactive material generally presents a
health hazard only at extremely close distances and with direct exposure
(inhaling). Due to the design of the detector and limited amount of
radioactive material, exposure is unlikely with proper handling. |
Ionization detectors are very sensitive and are designed to detect hot and
fast-moving fires that produce little smoke. The disadvantage to these
sensitive detectors is possible false alarms from
cooking vapors or excessive
dust.
Photoelectric Detectors
Photoelectric detectors detect the presence of visible particles (larger
than 3 microns) in the air. Inside the detector, there is a light
emitting diode (LED) that directs a narrow beam of infrared light across the
detection chamber. When smoke or particles enter the chamber, the
infrared light beam is scattered. A photodiode or photodetector,
usually placed 90 degrees to the beam, will sense the scattered infrared
light and when a preset amount of light is detected, the alarm will sound. Photoelectric detectors are not as sensitive and are designed to
detect cool or slow-moving (smoldering) fires that produce a lot of
smoke.
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