Radiation Thermometers

About radiation thermometers

All bodies emit electromagnetic radiation as a function of their temperature above absolute zero. The total rate of radiation emitted per second is given by:


E = KT4


The power spectral density of this emission varies with temperature. The major part of the frequency spectrum lies within the band of wavelengths between 0.3 m and 40 m, which corresponds to the visible (0.3-0.72 m) and infrared (0.721000 m) ranges. As the magnitude of the radiation varies with temperature, measurement of the emission from a body allows the temperature of the body to be calculated. Choice of the best method of measuring the emitted radiation depends on the temperature of the body. At low temperatures, the peak of the power spectral density function lies in the infrared region, whereas at higher temperatures it moves towards the visible part of the spectrum. This phenomenon is observed as the red glow which a body begins to emit as its temperature is increased beyond 600C.


Radiation thermometers have one major advantage in that they do not require to be in contact with the hot body in order to measure its temperature. Thus, there is no disturbance at all of the measured system. Furthermore, there is no possibility of contamination, which is particularly important in the food and many other process industries. They are especially suitable for measuring high temperatures that are beyond the capabilities of contact instruments such as thermocouples, resistance thermometers and thermistors. They are also capable of measuring moving bodies, for instance the temperature of steel bars in a rolling mill. Their use is not as straightforward as the discussion so far might have suggested, however, because the radiation from a body varies with the composition and surface condition of the body as well as with temperature. This dependence on surface condition is quantified by the emissivity of the body. The use of radiation thermometers is further complicated by absorption and scattering of the energy between the emitting body and the radiation detector. This energy is scattered by atmospheric dust and water droplets and absorbed by carbon dioxide, ozone and water vapor molecules. Therefore, all radiation thermometers have to be carefully calibrated for each particular body whose temperature they are required to monitor.


Various types of radiation thermometer exist. The optical pyrometer can only be used to measure high temperatures, but various types of radiation pyrometers are available which between them cover the whole temperature spectrum.


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