Text: Blackbody Cavities Because calibration of a non-contact temperature sensor requires a source of blackbody radiation with a precise means of controlling and measuring the temperature of the source, the interior surface of a heated cavity constitutes a convenient form, since the intensity of radiation from it is essentially independent of the material and its surface condition. Figure 7-2: Effective Emissivity of Spherical Cavities In order for a blackbody cavity to work appropriately, the cavity must be isothermal; its emissivity must be known or sufficiently close to unity; and the standard reference thermocouple must be the same temperature as the cavity. Essentially, the blackbody calibration reference consists of a heated enclosure with a small aperture through which the interior surface can be viewed (Figure 7-1). In general, the larger the enclosure relative to the aperture, the more nearly unity emissivity is approached (Figure 7-2). Although the spherical cavity is the most commonly referenced shape, carefully proportioned cone- or wedge-shaped cavities also can approach unity emissivity. In order to provide isothermal surroundings for the cavity, the following materials commonly are used: Stirred water bath for 30-100°C (86-212°F) temperature ranges; Aluminum core for 50-400°C (122-752°F) temperature ranges; and Stainless steel core for 350-1000°C (662-1832°F) temperature ranges. And while blackbody cavities have their appeal, they also have some disadvantages. Some portable, battery-operated units can be used at low temperatures (less than 100°C), but blackbody cavities are, for the most part, relatively cumbersome and expensive. They also can take a long time to reach thermal equilibrium (30 minutes or more), slowing the calibration procedure significantly if a series of measurements is to be made. Figure 7-3: Typical Tungsten Lamp filament Tungsten Filaments As a working alternative to blackbody cavities, tungsten ribbon lamps, or tungsten strip lamps, are commonly used as standard sources (Figure 7-3). Tungsten strip lamps are highly reproducible sources of radiant energy and can be accurately calibrated in the 800°C to 2300°C range. They yield instantaneous and accurate adjustment and can be used at higher temperatures than those readily obtainable with most cavities. Lamps, however, must be calibrated in turn against a blackbody standard; the user typically is provided with the relationship between electric current to the filament and its temperature. Emissivity varies with temperature and with wavelength, but material is well understood enough to convert apparent temperatures to actual. Just as a blackbody cavity includes a NIST-traceable reference thermocouple, instrument calibration against a ribbon lamp also can be traced to NIST standards. In a primary calibration, done mostly by NIST itself, filament current is used to balance standard lamp brightness against the goldpoint temperature in a blackbody furnace, in accordance with the ITS-90. Typical uncertainties range from ±4°C at the gold point to ± 40°C at 4000°C. In secondary standard calibration, the output of a primary pyrometer, i.e., one calibrated at NIST, is compared with the output of a secondary pyrometer when sighted alternately on a tungsten strip lamp. Many systematic errors cancel out in this procedure and make it more practical for routine calibration. References and Further Reading Handbook of Temperature Measurement & Control, Omega Press, 1997. New Horizons in Temperature Measurement & Control, Omega Press, 1996. Temperature Measurement in Engineering, H. Dean Baker, E. A. Ryder, and N. H. Baker, Omega Press, 1975. The Detection and Measurement of Infrared Radiation, R.A. Smith, F. E. Jones, and R. P. Chasmar, Oxford at Clarendon Press, 1968. Handbook of Temperature Measurement & Control, Omega Engineering Co., 1997. Infrared Thermography (Microwave Technology, Vol 5), G. Gaussorgues and S. Chomet (translator), Chapman & Hall, 1994. Instrument Engineers' Handbook, Third Edition, B. Liptak, Chilton Book Co. (CRC Press), 1995. Process/Industrial Instruments and Controls Handbook, 4th ed., Douglas M. Considine, McGraw-Hill, 1993. Theory and Practice of Radiation Thermometry, David P. DeWitt and Gene D. Nutter, John Wiley & Sons, 1988.

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