How a Smoke Alarm Detects Fire: Ionization vs Photoelectric
Smoke alarms use one of two sensing methods, and each reacts a little faster to a different kind of fire. Here is how both work and what fire agencies say about testing and replacement.
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A smoke alarm senses smoke particles in the air and sounds a loud horn. Two sensing methods are common: ionization and photoelectric. Both are small, cheap and reliable, but they notice different fires slightly sooner, which is why fire agencies often suggest having both kinds. This article explains how each one works, with details on the radioactive part inside many alarms and the replacement advice from the U.S. Fire Administration.
Ionization alarms: a tiny electric current
An ionization alarm contains a very small amount of americium-241, a radioactive element. It gives off alpha particles, which knock electrons off air molecules and turn them into charged ions. Between two metal plates, those ions let a small, steady electric current flow.
The alarm has two chambers, one open to the room and one sealed as a reference. When smoke particles drift into the open chamber, ions attach to them. The current in that chamber drops, the circuit notices the difference between the two chambers, and the horn sounds. According to Wikipedia, these detectors typically respond faster to flaming fires, in the range of 30 to 60 seconds.
Photoelectric alarms: scattered light
A photoelectric alarm uses a light source, usually an LED, and a light sensor called a photodiode. In the common chamber design the beam is aimed so that it normally misses the sensor. When smoke enters the chamber, the particles scatter some of the light onto the sensor, and the alarm sounds.
Wikipedia says this design responds faster to slow, smouldering fires, where there is plenty of smoke but little flame.
How much radioactive material is inside?
Americium-241 has a half-life of 432.6 years, meaning half of it would remain after that time. Wikipedia gives a typical alarm as holding about 37 kilobecquerels, roughly 0.3 micrograms. Alpha particles are stopped by a few centimetres of air or by the plastic case, and the same article says the amount does not pose a significant radiological hazard while the chamber stays sealed.
A short history
Wikipedia traces the idea to Walter Jaeger in the late 1930s. He was trying to build a sensor for poison gas, and noticed the meter reading changed when he lit a cigarette, showing that smoke particles affected the circuit. The article credits Duane Pearsall and Stanley Peterson with the first low-cost battery-powered home smoke detector in 1965, and says a mass-produced model, the SmokeGard 700, followed from 1975. The article carries a note that these dates are disputed, so treat them as approximate.
Testing and replacing: what the U.S. Fire Administration says
The U.S. Fire Administration, part of the U.S. government, gives this guidance on its smoke alarm page.
- Have both ionization and photoelectric alarms, or dual-sensor alarms that contain both, because either type alone can miss certain fires.
- Test every alarm once a month, whatever its type.
- Replace every alarm 10 years after its manufacture date.
- In alarms with a 9-volt battery, change the battery at least once a year. Hardwired alarms with a backup battery also need that battery changed yearly.
- For sealed 10-year lithium battery alarms, replace the whole unit as the maker instructs.
- Place alarms inside and outside each sleeping area and on every level, including the basement, and choose interconnected alarms so that all of them sound when one does.
Why a chirp means attention
Most alarms give a short chirp every minute or so when the battery runs low. A battery-powered alarm with a dead battery cannot detect anything, so a chirp should not be ignored. Wikipedia also describes standards such as the European EN 14604 for home alarms and notes that a 2014 study found 33% of devices claiming that standard failed in one or more areas, a reminder to buy alarms that carry a recognised approval mark.
This is general information. For rules that apply to your home or rental, check your local fire service and building regulations.
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