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So let’s say you live in a village nearby a horrible haunted castle, full of ghosts, goblins, and ghouls. And worst of all, in the haunted dungeon under the castle there lives a great fire-breathing beast. A dragon! And every once in a while the dragon decides it’s time to come outside for a “visit.” Unless you’re a knight in shining armor astride his mighty steed, magic sword held aloft, you probably won’t be able to stop the dragon. But if you’re smart you can survive the attack. What’s the best thing to have in your thatched-roof cottage to warn you that a fearsome dragon is burning the countryside? A smoke detector, of course. But you have to make sure that it works. We’ll do that today, and in doing so, we’ll learn the science behind how they work.
Materials:
- A smoke detector
- An extra 9v battery (just in case)
- A pot
- A stove
- Some water
- A potholder or safety glove
- An adult
Process:
- Fill your pot with water and have your adult help you bring it to a boil on the stove. You need a good rolling boil, with lots of steam coming off the top.
- Take your smoke detector off the wall (I hope that’s where it was!) and, with your pot holder or safety glove on your hand, hold it face down above the pot where the steam is the thickest.
- What happens? Does it start sounding an alarm?
- If your smoke detector works, great! You’re safe!
- If your smoke detector doesn’t work, check the battery and put your extra one in, because the old one may be dead. Retest it. Any luck?
- Remember to test all of your smoke detectors at least once a year and change the batteries whenever they start emitting tiny chirps.
- Ask yourself the all-important question: why did steam set off the smoke detector when it isn’t even smoke? Can you think of other ways to test your detector to figure it out?
Summary
The really important thing to remember is that a smoke detector is a smoke detector, not a fire detector. If you’ve ever accidentally burnt some toast and set off the smoke detector you’ve experienced this: it can’t tell the difference between smoke from an overactive toaster and smoke from a dangerous fire. It can’t tell the difference between smoke and steam, either. So what’s going on here? Well, there are two good answers to that question: one answers “why” smoke detectors work that way and one that answers “how” they work that way.
The How
Smoke detectors can work in two different ways. Some work in both ways to be extra safe. The first way uses tiny amounts of radiation. A small sample of a radioactive element, usually the element called Americium, is sealed inside a chamber inside the smoke detector. Another small sample is placed inside a second chamber, but this chamber has an opening so outside air can flow in (including smoke or steam). The tiny bit of radiation changes the molecules of air, ionizing them, which gives them an electrical charge. Normally the charge is the same in the closed and open chamber. Smoke is made of tiny particles of burned material and water vapor in the air, and when it gets into the open chamber, it messes up the ionization. Steam, which is just water vapor, does the same thing. Messing up the ionization changes the electric charge, and when the smoke detector detects that the charges in the two chambers are different, it starts an alarm because that usually means there’s smoke in the air.
The other way is to do something similar with light. If you’ve ever seen a movie where a spy has to shimmy through a maze of laser beams and then blocks one, setting off an alarm, it works almost the same way. Light is shined onto a very sensitive light receiver inside the smoke detector. Anything that’s not normal air (like smoke or steam) gets in the way of that light and block it from hitting the receiver. When less light than normal hits the receiver, the alarm goes off.
The Why
So why don’t we have a smoke detector that just detects smoke? Why have one that can’t tell the difference between smoke and steam? Well, it’s hard. Maybe impossible. Different substances create different types of smoke when they burn, so there is no one kind of chemical that a detector can find and know for sure it’s smoke.
But all smoke does cloud the air, messing with electrical currents and light transmission through the air, so using those as a way to detect smoke is easy. There may be some false alarms, but it’s far safer to have a false alarm now than to have a detector that doesn’t detect well enough or fast enough. A thousand false alarms won’t hurt you, but all it takes is one real fire to cause real damage, so we have smoke detectors that warn you about anything that resembles smoke from a fire … just in case.
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