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 as $7. And while they cost
very little, smoke detectors save thousands of lives each
year.
Inside
ViewTo see inside a
smoke detector, click
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It is
recommended that every home have one smoke detector per floor.
In this edition of HowStuffWorks,
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 house current.
Photoelectric Detectors
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 how this same type of 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. But
there are two problems here: 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. It takes quite a bit of smoke to do that.
Photoelectric smoke detectors therefore use light in a
different way. Inside the smoke detector there is a light and
a sensor, but they are positioned 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
enters the chamber, however, the smoke 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 are better at sensing smoky fires, such
as a smoldering mattress.
Ionization Detectors
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 the smaller amounts of smoke produced by flaming
fires.
Inside an ionization detector is a small amount (perhaps
1/5000th of a gram) of americium-241. 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 billion 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 1 curie of
activity (Marie Curie, the woman after whom the curie is
named, did much of her research using radium).
(For an extensive explanation of nuclear materials and
nuclear radiation, see How Nuclear
Radiation Works.)
An ionization chamber is very simple. It consists of two
plates with a voltage across them, along with a radioactive
source of ionizing radiation, like this:
(Click
here to see an ionization chamber.)
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.
Speaking of alarms, whenever the words "nuclear radiation"
are used an alarm goes off in many people's minds. The amount
of radiation in a smoke detector is extremely small. It is
also predominantly alpha radiation. Alpha radiation
cannot penetrate a sheet of paper, and it is blocked by
several centimeters of air. The americium in the smoke
detector could only pose a danger if you were to inhale it.
Therefore, you do not want to be playing with the americium in
a smoke detector, poking at it, or disturbing it in any way,
because you don't want it to become airborne. See How Nuclear
Radiation Works for further details.
For more information on smoke detectors and related topics,
check out the links on the next page.
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