02.Types of smoke alarms
There are two main types of smoke alarm for home use:
These contain a very small amount of radioactive material (Americium-241), which produces charged particles, or ions, in a test chamber. The alarm monitors the current generated by these ions. When particles given off in a fire enter the test chamber, the electric current changes, setting off the alarm.
- Basic models can be very cheap ($10 or less).
- Ionisation models can be prone to nuisance alarms from cooking, so shouldn’t be located near a kitchen.
- They're best suited to detecting fast-flaming fires that give off little visible smoke. However, most domestic fires tend to be smoky, smouldering fires, and ionisation alarms aren’t as quick at detecting these.
- While the amount of radioactive material in each alarm is too small to be a health hazard, there is a waste disposal issue. One alarm on its own contains a tiny amount, but thousands of alarms together make a significant amount of radioactive waste. Ionisation alarms therefore have rules about how you must dispose of them — check with your local fire brigade.
- These contain a photo cell and a light beam shining away from the cell. When smoke enters the test chamber, some of the light is scattered by the smoke particles and hits the cell, triggering the alarm. Photoelectric alarms are best at detecting smoky and smouldering fires.
- Dust or insects entering the alarm can cause false alarms, so they have to be cleaned occasionally. Most modern smoke alarms have insect screens to help prevent this problem.
Other types of alarm available for domestic use include:
- Carbon monoxide
- Heat alarms
These are generally for special situations where a photoelectric or ionisation alarm is unsuitable — for example, carbon monoxide alarms are often used overseas to ensure safe operation of central heating systems. Heat alarms are ideal for kitchens. Even if you have one of these types for a special purpose, you should still install a standard smoke alarm as well.