Smoke detectors
Last updated: May 22, 2009.
We've all heard of Native Americans who used smoke signals to send
simple messages over long distances. But sometimes when we see smoke
it's sending a message that's very alarming: there's a fire nearby and our
life is in danger. If fire breaks out in the daytime, we can usually
smell it and do something about it. But if we're asleep at night,
fire can steal the oxygen we need to breathe or produce
toxic carbon monoxide gas that can send us into a deep
and deadly slumber from which we may never recover. In the United
States, more people die from house fires than from all natural
disasters combined. Fortunately, thanks to modern technology, there's
an inexpensive and very reliable way of detecting fires: the
electronic smoke detector. How does this amazing gadget work?
Photo: An optical smoke detector. The chamber where smoke enters is the circular opening on the right. The dark circle in the middle is a test button with a built-in LED that flashes to show the detector is working okay.
Optical smoke detectors
There are two different kinds of smoke detectors. One is a kind of
electronic eye; the other's a sort of electronic nose. The eye type of
detector is more properly called an optical smoke
detector and it works a bit like Tom Cruise in Mission
Impossible. Remember the scene when Tom dangles from the ceiling trying
to avoid all those light-detecting burglar beams? An optical smoke
detector is just like that inside. Let's take a look.
The detector is screwed to your ceiling because that's where smoke
heads for when something starts to burn. Fire generates hot gases and
because these are less dense (thinner—or weigh less per unit of volume)
than ordinary air they rise upward, swirling tiny smoke particles up
too. An opening in the bottom of the detector (1), shown upper right in our top photo, leads to
a chamber up above. An invisible,
infrared
light beam, similar to the ones that Tom Cruise dodged, shoots across
the chamber from a light-emitting diode or LED (2) to a photocell
(3). The photocell is an electronic light detector that generates
electricity for as long as light falls on it. Normally, when there is
no smoke about, the light beam shoots constantly between the LED and
the detector. An electronic circuit (4) detects that all is well and
nothing happens.
But if a fire breaks out, smoke enters the chamber and interrupts
the beam. Because no light is falling on the photocell, it does not
generate an electric current anymore. The circuit spots this straight
away, realizes something's up, and triggers the shrill and nasty alarm (5)
that wakes you up and saves your life.
Ionization smoke detectors
Another type of smoke alarm is less expensive than the optical type,
more common, and works in a totally different way. You can think of it
as an electronic nose because, like the hooter on the front of your
face, it uses a kind of chemistry to spot unusual molecules (smoke)
heading inward. Detectors like this are called ionization smoke detectors.
What does that mean? They have a radioactive form of a chemical
element called americium inside. This constantly spews out tiny
radioactive particles (called alpha particles)
that leak into a small chamber. As they do so, they crash into air
molecules and turn them into ions (electrically charged atoms). The ions
whizz between two electrodes
(electrical contacts, rather like the terminals
of a battery). As long as they're moving around, a
current flows between the contacts and a circuit in the smoke detector
thinks all's well. But if a fire breaks out, smoke particles get into
the detector and start to clog up the chamber. They attach themselves
to the ions and effectively shut off the electric current. The circuit
in the detector spots that change straight away and sounds the alarm.
Photo: An ionization smoke detector with the cover removed. The large black cylinder on the right is the ionization chamber where smoke is detected. Note the relatively simple electric circuit, made from basic components such as
resistors and capacitors.
Get a smoke detector... check a smoke detector!
If you've not got a smoke detector in your home, why not? They cost
just a few pounds/dollars and could save your life. Get one at once!
If you have got one, make sure you check it works once a week and
vacuum the dust out of it regularly. Get into the habit of checking
detectors whenever you clean the room you're in.
Photo: Check smoke detectors regularly. Make sure the power light is working (this one has a green indicator that shows it's OK) and vacuum out the dust from time to time. Note the "replace by" date too:
smoke detectors don't last forever!
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