What is the difference between ionization and photoelectric smoke alarms?

Ionization smoke alarms are the most common type of smoke alarm and are quicker at sensing flaming, fast moving fires. This type of alarm uses a small amount of radioactive material to ionize air in an internal sensing chamber. As a result, the air in the chamber becomes conductive permitting current to flow between two charged electrodes. When smoke particles enter the chamber, the conductivity of the chamber air will decrease. When this reduction in conductivity is reduced to a predetermined level, the alarm is set off.

Photoelectric smoke alarms are fairly uncommon today. They are quicker at sensing smoldering fires than Ionization smoke alarms.  A photoelectric type smoke alarm consists of a light emitting diode and a light sensitive sensor located in a sensing chamber. The presence of suspended smoke particles in the chamber scatters the light beam. This scattered light is detected by the light sensitive sensor which sets off the alarm.

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