Testing the Smoke Detector

This week's experiment is the result of a fire at my parents' house. No one was hurt, but it did cause quite a bit of damage. In case you ever have a fire, you should regularly check your smoke detectors. In testing them, we will also learn some about how they work.

You will need:

  • smoke detectors. These should already be in your house.
  • a candle
  • a candle holder
  • a lighter
Warning: This experiment uses fire. Be careful. Think about things before you do them. Get permission from an adult and keep them handy, just in case.

The obvious way to check your smoke detector is to press the test button. While this tests the battery, it does not test to be sure that the detector part is actually working. For that, we will need some smoke. That is where the candle comes in. Place the candle in a holder. That will keep the hot wax from dripping on your hand, the carpet, the furniture, your dog, etc. Before you light the candle, we need to know more about what we are doing.

There are two kinds of smoke detectors. The first kind we will test for is the ionization type. These smoke detectors use radioactive material to detect smoke. Wait a minute! Radioactive?!?! Don't worry. It won't explode, glow in the dark or change you into Spiderman. The detector uses a very tiny bit of the element Americium-241. This gives off alpha particles, which ionize (give an electrical charge to) nearby atoms of nitrogen and oxygen. Metal plates collect the charge from the atoms. The tiny electric current this produces will keep the alarm from sounding. Particles of smoke will trap these charged atoms. That decreases the electric charge, and the alarm sounds.

Ionization detectors work best with small smoke particles, so we will test with a lit candle. Hold a candle flame about 6 inches below the smoke detector. If it is an ionization type detector, after a few seconds, the alarm will sound. Move the candle away and fan the detector to stop the noise.

If the candle does not set off the alarm, your detector may be photoelectric. These detectors rely on a beam of light and larger particles of smoke. Imagine shining a flashlight into a smoke filled room. The beam would be easily visible as the smoke reflected the light. Inside the smoke detector, a light beam normally shines past a photoelectric cell, without hitting it. These cells change light energy into electrical energy, which will trigger the alarm. As long as there is no smoke, the light beam misses the photoelectric cell, and all is quiet. When smoke enters the detector, the light reflects off particles to hit the photoelectric cell and trigger the alarm.

To test this kind of detector, we will blow out the candle and then hold just under the device. The rising smoke should reflect enough light to trigger the alarm.

If neither of these tests works, and the smoke detector battery is good, then you need a new smoke detector. They do get old, and you will need new ones every ten years or so. Experts recommend that you test your smoke detectors once a month. Of course, I wind up testing mine every time a fry something.