Smoke Detection. More detailed information on smoke detection and smoke detectors including the various types of smoke detector, their uses and best positioning.

Ionization
The ionization smoke detection device uses a radioactive source (typically Americium-241, an alpha-emitting radionuclide) to ionize the air within the sensing chamber. The ionization of air by the radioactive particle causes a very small flow of electrical current. When smoke from a fire enters the chamber, its presence causes a reduction in the current's flow. The electronic circuitry senses the reduced flow and triggers the alarm horn.

Photoelectric
Photoelectric smoke detection devices use the principle of scattered or reflected light to indicate the presence of visual smoke. They work much like the automatic eyes used to open doors. When there's no smoke, the chamber is dark. The light shines across the chamber and is received in a light trap on the far side. When smoke is present in the chamber, a photocell located at right angles to the light source senses the light scattered off the smoke particles and, at a certain level of illumination, triggers the alarm horn.

Both smoke detection devices sense the presence of smoke. The photoelectric detector senses the large, visible smoke particles. The ion chamber detector senses the small, invisible particles.

If a fire starts and slowly smolders in upholstery without visible flame, a good photoelectric smoke detection unit would be superior to a good ion chamber detector in terms of detection time. But, if the fire has flames, a good ion chamber will detect it faster than a good photoelectric detector. For this reason, it's a good idea to use both types in your smoke detection system to allow it to detect smoke as fast as possible in any conditions.

Combination
Recently, manufacturers have begun to introduce and market smoke detection devices that combine ionization and photoelectric sensors in the same unit. These units should combine the advantages of both sensors into an advanced unit that will detect smoke from a broader spectrum of fires (smoldering or flaming). As the demand for these units increases, the cost should decrease to a competitive level.

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