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 Smoke Detectors

Brian Marshall

Smoke detectors are one of those amazing inventions that, because of mass production, cost practically nothing. You can get a smoke detector for as little at $7 at Wal-mart. But despite their minuscule cost, smoke detectors save thousands of lives each year. It is recommended that every home have one smoke detector per floor. In this article we will examine the two most common types of smoke detectors used today.

All smoke detectors consist of two basic parts: a sensor to sense the smoke and a very loud electronic horn to wake people up. Smoke detectors can run off of a 9-volt battery or 120 volt current.

Photoelectric Detectors (Party Killers)

Occasionally you will walk into a store and a bell will go off as you cross the threshold. If you look you will often notice that a photo beam detector is being used. Near the door on one side of the store is a light (either a white light and a lens, or a low-power laser), and on the other side is a photodetector that can "see" the light. When you cross the beam of light you block it. The photodetector senses the lack of light and triggers a bell. You can imagine that this same sensor could act as a smoke detector. If it ever got smoky enough in the store to block the light beam sufficiently, the bell would go off! There are two problems: 1) it's a pretty big smoke detector, and 2) it is not very sensitive. There would have to be a LOT of smoke before the alarm would go off - the smoke would have to be thick enough to completely block out the light, and that's a lot of smoke.

Photoelectric smoke detectors therefore use light in a different way. Inside the smoke detector there is a light and a sensor, but they are at 90 degree angles to one another, like this:

In the normal case, the light from the light source on the left shoots straight across and misses the sensor. When smoke or fog from a DJ fogger enters the chamber, however, the particles scatter the light and some amount of light hits the sensor:

The sensor then sets off the horn in the smoke detector.

Photoelectric detectors that are used in most new constuction of comercial buildings are better at sensing smoky fires, such as a smoldering mattress and DJ smoke and fog machines.

Ionization Detectors (Safe for most DJ foggers)

Ionization smoke detectors use an ionization chamber and a source of ionizing radiation to detect smoke. This type of smoke detector is more common because it is inexpensive and better at detecting smaller amounts of smoke produced by flaming fires not the steam type fog and smoke produced from DJ foggers.

Inside an ionization detector is a small amount of Americium-241 (perhaps 1/5000th of a gram). The radioactive element Americium has a half-life of 432 years, and is a good source of alpha particles. Another way to talk about the amount of Americium in the detector is to say that a typical detector contains 0.9 microcurie of Americium-241. A curie is a unit of measure for nuclear material. If you are holding a curie of something in your hand, you are holding an amount of material that undergoes 37,000,000,000 nuclear transformations per second. Generally that means that 37,000,000,000 atoms in the sample are decaying and emitting a particle of nuclear radiation (such as an alpha particle) per second. One gram of of the element radium generates approximately one curie of activity (Marie Curie, the woman after whom the curie is named, did much of her research using radium).

An ioniziation chamber is very simple. It consists of 2 plates with a voltage across them, along with a radioactive source of ionizing radiation, like this:

The alpha particles generated by the Americium have the following property: they ionize the oxygen and nitrogen atoms of the air in the chamber. To ionize means "to knock an electron off of". When you knock an electron off of an atom, you end up with a free electron (with a negative charge) and an atom missing one electron (with a positive charge). The negative electron is attracted to the plate with a positive voltage, and the positive atom is attracted to the plate with a negative voltage (opposites attract, just like with magnets). The electronics in the smoke detector sense the small amount of electrical current that these electrons and ions moving toward the plates represent.

When smoke enters the ionization chamber it disrupts this current - the smoke particles attach to the ions and neutralize them. The smoke detector senses the drop in current between the plates and sets off the horn.

Caution no matter what type of smoke detector you encounter when setting up a gig keep in mind they are there for a purpose, to save lives. Never ever disable, cover, unhook or remove the battery from any smoke detector. In most states tampering with a smoke detector is against the law.



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