It is a small round usually white or off-white device that mounts on the ceiling of a room. It sits silently by and monitors your house for the presense of smoke. Of course if there is a fire in your house it will produce smoke. Therefore, this cheap little device might save your life someday if used properly. Here's a handy little guide.

How much does a smoke detector cost?
About $5 to $20 plus the cost of batteries. Estimated gross cost to power 4 smoke detectors in your house for 10 years (including the original cost of the detector) is about $240-$300. Based on the cost of death it's not that expensive.

How many smoke detectors do I need?
There should be at least one detector on every floor of the house. Additional detectors will increase the chance of early detection.

Where should I place a detector?
Smoke detectors should be placed near bedrooms either on the ceiling--at least 6 to 12 inches away from wall--or on the wall, 6--12 inches down from the ceiling. This allows the detector to sense the smoke as it approaches the sleeping area. DO NOT PUT A SMOKE DETECTOR IN YOUR KITCHEN! It will be going off every time you cook or run hot steamy water. It makes people take out the batteries and distrust smoke detectors. If you must have a smoke near the kitchen place it is a hallway that leads away from the kitchen.

What maintenance do smoke detectors require?
Test the detector at least monthly by pushing the test button. Once a year clean the dust from alarm air vents with a vacuum or some canned air. Battery operated detectors should have the battery replaced each year or when the low battery warning sounds. A good tip is to replace the batteries every time the time changes for those areas that follow daylight savings time. Set your clocks and your smoke detectors every spring and fall.

Is there anything else I should do with my smoke detector?
YES! Hold practice drills with the whole family so they will know what to do if your detector ever alerts you of an emergency. Plan an escape route from your home complete with back-up plans if fire blocks your exit.

A device made to detect the presence of smoke, and, by association, fire inside a building.

Two types of smoke detectors are currently in widespread use: ionization detectors and photoelectric detectors. Their principles of operation are entirely different, and each has its own particular benefits and drawbacks.

Ionization detector

The ionization smoke detector is an extremely ironic byproduct of the Manhattan Project. The most immediately destructive scientific research program in human history produced, as a mere afterthought, a mildly radioactive isotope. Americium-241, a product of the decay of plutonium-241, has made possible an inexpensive safety device which has likely saved thousands of lives since the Project's intended output seared, poisoned, and vaporized hundreds of thousands of Japanese. I'm not sure what I think of that.

Americium, discovered by Glenn Seaborg at the University of Chicago in 1944 or 1945, has several isotopes. Americium-241 is not the most stable; its half-life is 432 years. Decaying, it emits gamma rays of relatively low energy, 59.5 keV (kilo-electron-volts), as well as alpha particles.

An ionization-type smoke detector contains about 0.9 microcurie of Am-241, though it's bound in oxide form (Americium oxide, AmO2). The total mass of the radioactive material is about 1/5000 of a gram. The source is positioned near an ionization chamber, essentially two electrodes with a potential across them supplied by a battery. The emitted alpha particles ionize the air molecules in the chamber, causing a minute current to flow between the plates. If significant numbers of smoke molecules are present, they'll neutralize enough of the ions to reduce the net current. The detector's electronics are sensitive enough to respond to very tiny changes (as anyone who has tried to fry fish near one can tell you!), and will set off the alarm immediately upon a current drop.

The gamma radiation emitted by the americium is incidental to its function within the smoke detector; however, it is able to escape the detector's confines, unlike the alpha particles. In practice, what this means is that an ionization-chamber smoke detector gives a tangible, but insignificant, dose of radiation to the inhabitants of the home or office in which it is installed. The average yearly dose received by a smoke-detector user is several orders of magnitude smaller than that imparted by a single chest X-ray scan. The only real danger from the isotope comes if the radioactive material is somehow freed, then inhaled or ingested.

Ionization detectors tend to be more effective in detecting fires with open flames, rather than smoldering ones.

Photoelectric detector

Photoelectric detectors are considerably more expensive to purchase and maintain than those of the ionization variety, and are less commonly used. For those who are paranoid about radiation in any form or quantity, though, they present a viable alternative to installing a decaying isotope on one's ceiling.

Two types of photoelectric smoke detectors exist. The first uses an LED to send a beam of light down a chamber. A photocell lies at the end of a branch of the chamber perpendicular to the light's path. Under normal conditions, the sensor remains unilluminated. But when large smoke particles enter the chamber, light scatters off them and some makes it to the photocell at the end of the perpendicular chamber, setting off the alarm.

A second variety of photoelectric smoke detector, somewhat less sensitive than the first and less popular, directs the light beam toward the photocell, and registers an alarm when the light reaching the detector is reduced.

Photoelectric smoke detectors are better at detecting smoldering fires, which tend to release larger smoke particles into the air.

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