Types of Smoke Detectors & How They Work
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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