Almost all of the radiation thermometers that we are likely to use in industrial furnaces, with the exception perhaps of the gold-cup pyrometer, have digital displays that give temperature directly to the nearest degree or tenth of a degree. These nice digital displays often deceive us into believing they are accurate to the last digit. It is not until we obtain different readings from two similar instruments that we realize that one, or more likely both, of the instruments contains an error in its reading.
As we have seen in Chapter 4, the reading on a radiation thermometer depends not just on the temperature of the target, but on many other factors, including the thermometer's operating wavelength, the emissivity of the target, and the temperature distribution of all objects surrounding the target. Generally, the only meaningful way of comparing the readings on two radiation thermometers is to use a blackbody source in a controlled environment. This eliminates all the environmental factors from the comparison, enabling us to characterize the properties of the thermometers themselves. For this and other practical reasons, blackbodies are frequently used as calibration sources, and the best we can do is to calibrate a thermometer in terms of its ability to measure radiance temperature.
Technically, a calibration is a comparison that establishes a link in an unbroken chain of comparisons that relate the radiation thermometer's readings to the international temperature scale as realized by a national measurement institute. Thus, a thermometer's readings are compared to the readings of another independent, preferably more accurate, calibrated thermometer's readings, which in turn have been compared to another calibrated thermometer's readings, and so on, all the way back to the international temperature scale. In each step of the process a set of corrections, or a means to determine the correct readings, is provided, along with an uncertainty statement about the corrected readings. As you might expect, the uncertainties increase as we add more and more links to this so-called traceability chain.
In this chapter we briefly look at the steps involved in calibrating a radiation thermometer. Since blackbodies form an integral part of the calibration process, we will also examine the nature and limitations of practical blackbodies. It is recommended that radiation thermometers be calibrated on a regular basis, preferably at least once a year. Calibrations should be carried out by an accredited calibration laboratory, but can be done in-house provided that suitable blackbodies and reference thermometers are available. In this case, any reference thermometers used should be independently calibrated by an accredited laboratory.
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