An ionization smoke alarm uses a small amount of radioactive material to ionize air in the sensing chamber. As a result, the air chamber becomes conductive permitting current to flow between two charged electrodes. When products of combustion enter the chamber, the conductivity of the chamber air decreases. When this reduction in conductivity is reduced to a predetermined level, the alarm is set off. Most smoke alarms in use are of this type.
Ionization models are best suited for rooms that contain highly combustible materials that can create flaming fires. These types of materials include flammable liquids, newspapers, and paint cleaning solutions.
A photoelectric type smoke alarm consists of a light emitting diode and a light sensitive sensor in the sensing chamber. The presence of suspended products of combustion in the chamber scatters the light beam. This scattered light is detected and sets off the alarm.
Photoelectric models are best suited for living rooms, bedrooms and kitchens. This is because these rooms often contain large pieces of furniture, such as sofas, chairs, mattresses, counter tops, etc. which will burn slowly and create more smoldering smoke than flames.