There are several types of flame detector. The optical flame detector is a detector that uses optical sensors to detect flames. There are also ionisation flame detectors, which use current flow in the flame to detect flame presence, and thermocouple flame detectors.
Ultra-Violet flame detectors detect fires and explosions within 3–4 milliseconds due to the UV radiation emitted at the instant of their ignition. The detectors are sensitive to a narrow wavelength so that false alarms can be avoided and have a fast acting response to flames up to 25m away.
Triple IR flame detectors compare three specific wavelength bands within the IR spectral region and their ratio to each other. In this case one sensor looks at the 4.4 micrometre range and the other sensors at reference bands above and below. This allows the detector to distinguish between non flame IR sources, and flames that emit hot CO2 in the combustion process. As a result, both detection range and immunity to false alarms can be significantly increased. IR3 detectors can detect a 0.1m2 (1ft2) gasoline pan fire at up to 65m (215ft) in less than 5 seconds.
Most IR detectors are designed to ignore constant background IR radiation, which is present in all environments. Instead they measure the modulated part of the radiation. When exposed to modulated non flame IR radiation, IR and UV/IR detectors become more prone to false alarms, while IR3 detectors become somewhat less sensitive, and more immune to false alarms. Triple IR, like other IR detector types, is susceptible to blinding by a layer of water on the detector’s window.
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