Smoke Alarms – Ionization And Photoelectric

There are two types of household smoke alarms in common use. These are known as ionization or photoelectric type smoke alarms.

How Does An Ionization Type Smoke Alarm Work?

This type of 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 smoke particles 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.

How Does A Photoelectric Type Smoke Alarm Work?

A photoelectric type smoke alarm consists of a light emitting diode and a light sensitive sensor in the sensing chamber. The presence of suspended smoke particles in the chamber scatters the light beam. This scattered light is detected and sets off the alarm.

Which Type of Alarm is More Effective?

There is no simple answer to this question. The two types operate on different principles and therefore may respond differently to various conditions. Some advantages to each type are set out below:

Ionization

• Fastest type to respond to flaming fires
• Lowest cost and most commonly sold
• Some models have a hush or temporary silence feature that allows silencing without removing the battery
• Some models are available with a long life battery

Photoelectric

• Fastest type to respond to slow smouldering fires and white or grey smoke
• Less prone to nuisance alarms from cooking

Notwithstanding these differences, to achieve ULC listing, both alarms must be tested to the same standard and meet the same requirements. Photoelectric smoke alarms may respond slightly faster to smouldering fires, while ionization alarms respond slightly faster to flaming fires. Since you can’t predict the type of fire that will occur, it is difficult to recommend which is best. Both alarms will detect all types of fires that commonly occur in the home. Installing both types of smoke alarms in your home can enhance fire safety.

Which Type of Smoke Alarm Should A Homeowner Purchase?

It is the consumer’s responsibility to assess the circumstances of their household and to select the most appropriate alarm. However, an important consideration in the purchase of a smoke alarm is conformance to a recognized standard. In Ontario, CAN/ULC-S531 is the recognized standard for both the ionization and photoelectric types of alarms. Both ionization and photoelectric type products conforming to this standard are available on the market. A homeowner will know that a smoke alarm meets the requirements of this standard by the ULC or cUL label on the device.