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There's two main types i think? The ionic type which works using a
isotope in the detector, and the optical type which works by the smoke
obscuring the photo detector. I suppose the later one could be used to
detect the concentration of smoke.
Smoke detectors are designed to detect smoke within a protected structure. In a properly designed fire alarm system, they are strategically placed in all occupied areas. System smoke detectors are designed to be connected to a fire alarm system, or as an option to an intrusion alarm system. Fire systems manufactures two types of smoke detectors: Long Range Beam and Photoelectric smoke detectors.
Long Range Beam Smoke Detectors are designed to protect a very large area. They can protect an area up to 350 feet by 60 feet. Long Range Beam smoke detectors use a separate Transmitter and Receiver. The transmitter projects an infrared beam which is measured at the receiver. If the beam is obscured by smoke, the receiver signals an alarm. If the beam is obscured by dust or dirt, or is blocked by a foreign object, the Receiver signals a trouble condition.
Photoelectric Smoke Detectors use an internal sensing chamber that includes a light source and a light sensitive receiver. The chamber is designed so that no ambient light can enter, but smoke can flow through it easily. Normally the receiver sees very little light from the light source. Smoke causes the light inside the chamber to scatter. This scattering action causes more light to be seen by the receiver. The detector measures this increase in signal and signals an alarm.
Photoelectric smoke detectors feature a patented chamber design which is virtually immune to dust. Chamber Check automatic trouble indication monitors the chamber to dust buildup. If the chamber becomes too sensitive, the detector will signal a trouble condition, before a false alarm can occur.
More about the same: ionic detectors can detect smaller particles than optoelectronic ones but they must be used under severe radioactivity normatives because of the ionizer isotope they have, so they are seldom used nowadays.