How does a fire alarm work

asks nodrog:
A. By fire alarm, you mean a smoke detector which is designed to wake people up at the first sign of a fire so that they can evacuate their home safely.

Household smoke detectors are made up of two things: something which can sense the smoke and a loud alarm to wake everyone up.

There are two main types of smoke detectors: ionisation detectors and photoelectric detectors, which can either run off a battery or be connected to the house's electricity supply.

Q. What's the difference
A. Ionisation detectors are common because they are cheap and good at detecting small amounts of smoke.

They consist of an ionisation chamber and a source of ionising radiation - a minute amount of americium-241, a source of alpha particles. The ionisation chamber is two plates about a centimetre apart: one has a positive charge, the other a negative charge.
Alpha particles, which are constantly being released by the americum-241, ionise the oxygen and nitrogen particles in the air (by knocking electrons off them), creating negatively charged electrons and positively charged atoms. When this happens, the positively charged atoms are attracted to the negative plate, and the negatively charged electrons are attracted to the positive plate. This generates a small, continuous current.

If, however, smoke makes its way into the ionising chamber, the smoke particle attach themselves to the ions and neutralise them so they don't reach the plates. This is what triggers the alarm.

Photoelectric detectors work in different ways. In one type, the alarm is set off when smoke blocks a light beam reaching a photocell. In most, the light is scattered by smoke particles on to a photo cell, and this sets the alarm off.

Q. Which works best
A. Both are effective and have to pass stringent tests before they can be sold. Ionisation detectors respond more quickly to fires with large flames, whereas photoelectric detectors are better at detecting smouldering fires.

Ionisation detectors also have a built-in safety factor - when the battery starts to fail, the ion current falls too, setting off the alarm.

Q. Are they prone to false alarms
A. If steam or humidity causes condensation on the circuit board, it could set the alarm off on either type of detector.

One problem with ionisation detectors is that, because they are sensitive to minute smoke particles, they can be set off by normal cooking activity, and are often put out of action.

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By Sheena Miller