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Ionization vs. Photoelectric Smoke Detectors

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Smoke detection devices are one of the most common technologies used in fire protection. They are a common sight in apartment complexes, houses, and places of business, but exactly how do they work? Smoke detectors rely on one of two processes: ionization smoke detection or photoelectric smoke detection.  The two technologies have different operating features and provide different advantages. For maximum fire protection, it is best to have both types of systems installed.

1)      Ionization smoke detectors

Ionization smoke detectors work through the action of an ionization chamber. The ionization chamber consists of two metal plates with about a centimer’s space between them and a minute amount of radioactive material between the two plates. An electric current running through the plates causes the radioactive material to ionize the air in the chamber. When smoke enters the chamber, it interrupts the stream of ions flowing between the two plates, decreasing the electrical charge of the plates, which is what ultimately sets off the alarm.

Ionization smoke detectors respond better to flaming fires, i.e., fires that begin with flames and have a fast-growing smoke layer where the smoke builds up close to the ceiling.  For this reason, ionization smoke detectors are best suited for interiors containing highly flammable materials and substances (e.g. cooking oil, gasoline, paper, cotton fabrics etc.) that quickly burst into flames.

2)      Photoelectric smoke detectors

Photoelectric smoke detectors work through the action of a light-sensing mechanism.  The detector’s sensing chamber contains a LED light and a light-sensitive device called a photocell or photodiode. When smoke enters the path of the light beam, it scatters the light particles, so that some of the light is deflected toward the photocell. When light strikes the photocell it becomes electrically charged, which is what activates the alarm.

Photoelectric smoke detectors respond better to smoldering fires, i.e., fires that start with a long period of smoldering and where the some layer grows slowly and accumulates closer to the floor. These types of detectors are best suited to rooms with a lot of slow-burning materials (e.g. countertops, mattresses, wooden furniture, upholstery etc.) that produce more smoke than flames.

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