Extended temperature range for calibration of two-color pyrometers - News Briefs - Brief Article

Journal of Research of the National Institute of Standards and Technology, Sept, 2001

Properties of metallic and ceramic coatings produced by the thermal spray process depend strongly on processing conditions. Two-color pyrometry is the most common technique used to determine particle and substrate temperatures, but incorrect calibration procedures can result in significant errors. One error source of is the use of the radiance temperature rather than the absolute temperature to calibrate the pyrometer. The radiance temperature of a test surface is equivalent to the temperature of a perfectly radiating surface (e.g., blackbody, [epsilon) = 1) with the same radiant intensity as the test surface at a specified wavelength. Tungsten ribbon lamps, the most readily available temperature standards, are usually supplied with calibrations to radiance temperature since they are normally intended for calibrating single-color pyrometers, where radiance temperature is appropriate. Another error occurs if the deviation of tungsten from gray body behavior (wavelength independent radiance) goes uncorrected. In addition to these errors, extrapolation beyond the range of the NIST calibrated tungsten ribbon lamps, will further degrade the accuracy of these temperature measurements.

NIST researchers have developed calibration procedures for two-color pyrometers that improve the accuracy of thermal spray particle temperature measurements. The errors associated with the use of radiance temperature rather than true temperature in calibrations were quantified, and the relative accuracy of determining true temperature from literature emissivity data versus direct measurement are investigated by comparing both corrections to a blackbody calibration. Errors associated with the deviation of tungsten from gray body behavior were also investigated in this manner. In addition, the calibration range has been extended beyond that which can be achieved with lamp standards to the melting point of tungsten using facilities in the NISTs Subsecond Thermophysics Laboratory. More accurate particle temperature data will lead to an improved ability to predict coating characteristics from spray processing conditions.

CONTACT: Steve Ridder, (301) 975-6175; stephen. ridder@nist.gov.