INCREASED ACCURACY FOR PARTICLE TEMPERATURE MEASUREMENT.

NIST scientists have obtained emissivity data that make possible more accurate measurements of the temperature of rapidly moving incandescent particles in thermal spray plumes. Thermal spray is used for many purposes, including deposition coatings for protection against heat, corrosion, and wear. However, it is difficult to obtain reproducible results or to determine the best instrument settings for processing new materials. One approach sought by industry is to use instrument control based on measured temperatures and velocities of particles in the spray plume.

The only method currently available to measure the temperatures of such rapidly moving particles (hundreds of meters per second) is two-color pyrometry, which derives a temperature from measurement of the light emitted in two different bands of wavelengths. It is normally assumed, due to lack of any specific knowledge, that the emissivity of the material does not depend on wavelength, so the intensity of the light in the two wavelength bands can be calculated from a standard Planck function. NIST researchers use a special facility, which uses electrical resistance heating to heat wires and holds the wires at an elevated temperature during optical meas-urements. The researchers were able to make specific measurements of the wavelength dependence of emissivity of tungsten and molybdenum at temperatures up to those materials melting points. The measurements allowed NIST to derive a correction factor for the two-color pyrometry of these materials, which was several hundred degrees at the melting point of tungste n. Additional factors, such as oxidation, which can affect the accuracy of two-color pyrometry, will be the subject of future investigations.