Heat Radiation
Infrared flame detectors suffer from Infrared heat radiation which is not emitted by the possible fire. One could say, that the fire can be masked by other heat sources. All objects which have a temperature higher than the absolute minimum temperature (0 kelvins or −273.15 °C) emit energy and at room temperature (300 K) is this heat already a problem for the infrared flame detectors with the highest sensitivity. Sometimes a moving hand is sufficient to get the IR flame detector in an alarm. At 700 K a hot object (black body) already starts to emit visible light (glowing). Dual- or multi-infrared detectors suppress the effects of heat radiation by means of sensors which detect just off the CO2 peak; for example on 4.1 µm. Here it is necessary that there is a large difference in output between the applied sensors (for example sensor S1 and S2 in the picture above). A disadvantage is, that the radiation energy of a possible fire must be much bigger than the present background heat radiation. In other words, the flame detector becomes less sensitive. Every multi infrared flame detector is negatively influenced by this effect, regardless how expensive it is.
Read more about this topic: Flame Detection
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