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Non-contact Temperature Sensors
These detectors measure the amount of radiation emitted by a surface. As the temperature of a body increases, the amount of infrared emitted and its average frequency also goes up.
The target material radiating efficiency will have a bearing on the reading and the emissivity ( between 0 and 100%) needs to be known. This rating can vary between 95% for an efficient surface to 20% for polished metal.
The emissivity of the surface must be known. Varying oxidation or roughness will affect the reading. A coating is often applied to ensure a certain emissivity figure.
Care needs to be taken to avoid reading contamination from sources other than the target. A detector will have a certain field of view which can be altered to concentrate on the target.
One significant advantage of infrared temperature measurement is that it is non-contact. Thus moving bodies, fragile or wet surfaces and very aggressive media can be measured with relative ease. Very large surfaces can also be measured and simply averaged, or hot spots detected without masses of wiring and detectors.
Only surface temperatures can be read. If the bulk temperature of a process is to be recorded a sheath must be inserted into the process and it must act as a sighting target.
Infrared thermometers can record high temperatures (up to 3000 °C), well above Thermocouples and Resistance Thermometers (RTDs) although they suffer from higher inaccuracies and other problems such as radiant heat.
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