Ionization Vs. Photoelectric Smoke Alarms

Ionization Smoke Alarms

• Ionization smoke alarms have a very small quantity of radioactive material between two electrically charged plates. This ionizes the air inside the smoke alarm chamber and causes a current to flow between the two plates. When small particles of smoke enter the chamber they lower the conductivity of the air, activating the alarm. Ionization alarms tend to be the least costly. They are very effective at detecting hot, fast and flaming fires but take longer to detect smoldering fires. They can be accidentally set off by things such as lit birthday candles on a cake.

Photoelectric Smoke Alarms

• Photoelectric smoke alarms have a light source aimed into a sensing chamber at an angle away from the sensor. When smoke enters the chamber, it reflects light onto the light sensor; triggering the alarm. Photoelectric smoke alarms tend to be more expensive than ionization types. They are most effective at detecting fires that produce visible smoke from smoldering combustion, such as those started by faulty electrical wiring, but react more slowly to fast fires. Tests by the National Fire Protection Agency show they provide adequate warning for all types of fires.

Which Smoke Alarm Works Best?

• Controlled experiments have shown that ionization alarms react earlier than photoelectric alarms in fast-flaming fires, such as those involving paper or flammable liquids, while photoelectric alarms react earlier than ionization alarms in smoldering fires. Smoldering fires are most likely if a lit cigarette is left on upholstered furniture or on a bed. Because they react differently, it is best to use both in the home. Do not place an ionization alarm near to cooking surfaces as it may give a false alarm.

Smoke Alarm Tips

• Never disable a smoke alarm. If it gives a false alarm because someone burned the toast you can fan it with a tea-towel or newspaper to stop it. Batteries should be replaced at least once a year, and many people change their batteries when they change their clocks for daylight saving time. Smoke alarms should be tested, cleaned and replaced according to the manufacturers' instructions. Have a fire escape plan and make sure everyone in the home knows it.