There are two main types of smoke detectors: ionization detectors and photoelectric detectors. A smoke alarm uses one or two methods, sometimes plus a heat detector to warn of a fire. The devices may be powered by a 9 volt battery, lithium, or 120 volt house wiring.
Ionization detectors have an ionization chamber and a source of ionizing radiation. The ionization chamber consists of two plates (positive and negative charge) separated by about one centimeter. The battery applies a voltage to the plates. Alpha particles constantly released by the source knock electrons off atoms in the air, ionizing atoms of oxygen and nitrogen in the chamber. The positively charged oxygen and nitrogen atoms are attracted by the negative plates and electrons are attracted by the positive one, generating a small electric current. When smoke enters the ionization chamber, the smoke particles attach to the ions and neutralize them, so they do not reach the plate. The decline in current between the plates triggers the alarm.
In photoelectric type device, smoke can block a light beam. In this case, the reduction of light reaching a photocell triggers the alarm. Light is scattered by smoke particles on a photoelectric cell, initiating an alarm. In this type of detector there is a hollow T-shaped with a light emitting diode (LED) that shoots a beam of light on the crossbar of the T. A photocell, positioned at the bottom of the vertical base of the T, generates a current when exposed to light. When smoke is present, light is scattered by smoke particles, and some light is directed down the vertical part of the T to reach the photocell. When sufficient light strikes the cell, the current triggers the alarm.
The ionization and photoelectric detectors are both effective smoke detectors. Ionization detectors respond more quickly to the fire flames with combustion particles smaller; photoelectric detectors respond more quickly to smoldering fires. In both types of detector, steam or high humidity can cause condensation on the circuit board and sensor, which triggers the alarm goes off. Ionization detectors are less expensive than photoelectric detectors, but some users purposely disable them because they are more likely to trigger an alarm from normal cooking smoke due to their sensitivity to minute smoke particles.
Ionization Smoke Detectors
Photoelectric Smoke Detectors
Which Type is better?
The ionization and photoelectric detectors are both effective smoke detectors. Ionization detectors respond more quickly to the fire flames with combustion particles smaller; photoelectric detectors respond more quickly to smoldering fires. In both types of detector, steam or high humidity can cause condensation on the circuit board and sensor, which triggers the alarm goes off. Ionization detectors are less expensive than photoelectric detectors, but some users purposely disable them because they are more likely to trigger an alarm from normal cooking smoke due to their sensitivity to minute smoke particles.
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