Development of a six-wavelength optical pyrometer for non-contact surface temperature measurement

A Six Wavelength Pyrometer was designed, developed and tested for non-contact surface temperature measurement of opaque materials in the temperature range of 500 to 1000 °C. The system included a rotating filter wheel containing six narrow band optical band pass filters and a liquid nitrogen cooled HgCdTe infrared detector. A digital data acquisition system was used to record the detector output and for subsequent data reduction. A least squares technique was used to determine the temperature and the normal spectral emissivity of the sample surface. The system was calibrated using a blackbody radiation source.

The pyrometer system was tested using copper and type 316 stainless steel sample materials. For the copper sample, surface temperature measurements to an accuracy of 3% were obtained. The accuracy of temperature measurements for type 316 stainless steel was 0.27%. Surface temperature errors as large as 18.1% and 9.5% were encountered for the copper and type 316 stainless steel materials respectively. Additionally, the normal spectral emissivity results for both sample materials were found to be inadequate.

Two possible sources of the temperature measurement errors were identified. The first was the separation distance between the six wavelengths chosen. With the analysis technique used, a small wavelength separation could cause significant errors in the calculation of the surface temperature. The second source was the assumed logarithmic variation of the directional spectral emissivity with wavelength and temperature. This profile would cause any small errors in the reduction of the data to result in large errors in the normal spectral emissivity and temperature values obtained. Recommendations for V further improvements are presented and discussed.