Photoelectric and Ionization Smoke Detectors

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Photoelectric and Ionization Smoke Detectors Powered By Docstoc
					There are different types of smoke detectors that have been developed for various
purposes. The importance of detectors is being realized through the level of safety
they provide to industries and also in residences.
  The types of smoke detectors that are widely used are ionization detectors and
photoelectric detectors. Ionization detectors use a detection chamber and ionization
radiation as its source. The source of ionization radiation contains at least 1/5000th of
americium-241 which produces alpha particles from nuclei of helium elements. The
ionization chamber is composed of two plates having a centimeter distance between
them. A battery is used to apply voltage to charge one plate positive and the other
negative. The americium-241 releases alpha particles to knock off electrons of the
atoms in the chamber causing the positively charged oxygen and nitrogen atoms to
bond with the surface particles of negative plate and electrons are attracted to the
positive plate, producing a minute and continuous electric current. The drop in current
between parallel plates causes the alarm to set off. This happens when the ionization
chamber senses smoke whose particles automatically attach to the ions to neutralize
and prevent them to reach the plate.
  Another type of smoke detector is the photoelectric detector. The photoelectric
device allows the smoke to block a light beam. This means a reduction in light going
to a photocell. When this happens, alarm sets off. In the photoelectric unit there is a
T-shaped chamber containing a light emitting diode (LED) that focuses the beam of
light across the horizontal bar of the T. At the bottom of the vertical base of the T, a
photocell is placed to produce a current upon exposure to the light. If there is no
smoke, the light beam crosses the top of the T in straight line. When smoke enters the
T-chamber, its particles cause the light to bend towards the photocell. When the
photocell is hit by sufficient light, the current sets off the alarm.
  Photoelectric detectors are as effective as ionization detectors in sensing smoke.
Ionization unit responds to flaming fires with smaller particles; photoelectric detectors
respond more quickly to all fires. Photoelectric detectors are more expensive than
ionization units but the latter provides a level of built-in security not found in
photoelectric detectors. The difference between these detectors is also apparent during
the change of batteries. The alarm of ionization detectors sound to hint that the battery
should be changed. The sound is caused by the drop in the ion current. In contrast,
photoelectric detectors can be provided with back-up batteries, but don't provide this
added benefit
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