Information about Ionization, Photoelectric Smoke Detectors
How they work
Ionization detectors contain a tiny mass of Americium-241, which is a source of alpha radiation. The principle of using a small amount of radioactive material to ionize the air between two differently charged electrodes is to sense the presence of smoke particles.
The radiation passes through an ionization chamber, which is an air-filled space between two electrodes, and permits a small constant current to flow between the electrodes. Any smoke that enters the chamber absorbs the alpha particles which reduces the ionization and interrupts this flow of current, setting off the alarm.
Photoelectric detectors use the principle of utilizing a light source and a photosensitive sensor. When smoke particles enter the light path, some of the light is scattered by reflection and refraction onto the sensor. In other words, when smoke blocks the light beam, the reduction in light reaching the photocell sets off the alarm.
Ionization vs. Photoelectric
Ionization
Least expensive
Can detect particles of smoke too small to be visible
Warns when batteries are low or failing
Better response to flaming fires
Photoelectric
- Uses a light sensor
- Quick to detect slow burning, smoky fires
- Less sensitive to false alarms from cooking and bathroom steam than ionization
- Faster response to smoldering fires
BOTH ionization and photoelectric are effective smoke sensors and must pass the same tests to be certified as a listed Underwriters Laboratory (UL). The number of alarms a home has is more important than the type.
Installing several smoke alarms of each type will provide better coverage in the extreme cases of long-term smoldering or fast flaming fires. Most importantly, enough alarms should be installed in the proper locations.
Contact the Fire Prevention Division of the Murfreesboro Fire Department for more detailed information.