Every smoke detector has two parts that matter: something that senses smoke, and a horn loud enough to wake you up. Some run on a battery, and some are wired into the building’s power with a battery for backup. What changes from one detector to the next is how it senses the smoke. An ionization detector does it with a very small radioactive source and an electric current.
What’s inside
Inside the detector is a small chamber with two metal plates, one with a positive charge and one with a negative charge. Next to them is a tiny bit of americium-241, a radioactive material that gives off alpha particles. Those particles knock electrons loose from the air in the chamber, and the loose electrons and charged air molecules drift toward the plates. That movement makes a very small, steady electric current, and the detector watches it all the time.
What happens when smoke gets in
When smoke drifts into the chamber, the smoke particles latch onto those charged molecules and slow them down. The current drops, and when it drops far enough, the detector sets off the horn.
What it’s best at
Ionization detectors are quick on fast, flaming fires, the kind that throw off lots of very small smoke particles, like paper or a trash can going up. They’re slower on smoldering fires that put out heavier smoke and burn a long time before there are flames, like a cigarette down in a couch cushion. A photoelectric detector does better on those. Because each type is better at one kind of fire, a lot of buildings use both, or use detectors with both sensors in one unit.
Ionization detectors also tend to go off from cooking smoke and steam more than other types. If one keeps going off near the kitchen, don’t pull the battery. Move it, or swap it for a different type.
Is the radiation a problem?
The amount of americium in a detector is very small. It gives off alpha radiation, which can’t get through a sheet of paper and doesn’t travel far in air. It’s sealed inside the chamber, and as long as it stays there, it isn’t a danger to anybody. Just don’t take the detector apart or mess with the chamber. When it’s time to get rid of one, follow the instructions on the detector or check with your local trash service.
Keeping them working
Push the test button once a month to make sure the horn works. If it takes a regular battery, change it once a year. Replace the whole detector every 10 years, because the sensor wears out even if the horn still sounds when you test it. The date is usually printed on the back.
In a business
In a business, smoke detectors are usually part of a fire alarm system wired to a panel instead of standing on their own. We install fire alarm systems, we test the whole system every year, and we offer 24/7 monitoring. We also put smoke detectors in with home security systems. If you’re not sure what’s on your ceiling or whether anybody’s testing it, call us at 1-800-348-9604.