In order to attain exact and reliable results, the emissivity of the measured surface has to be known. It can only be determined precisely in complex measuring facilities. The Physikalisch-Technische Bundesanstalt (PTB) has developed a modern emissivity measuring facility for industry-oriented calibrations.
Today, the accuracy of industrial temperature measurements carried out with contact-free radiation thermometers, is often no longer limited by the quality of the radiation thermometers, but rather by insufficient knowledge of the emissivity of the surface observed. Industrial radiation thermometers can furnish a resolution of up to 20 mK, with an uncertainty of 1 K for temperature measurements of 100 °C. In contrast to this, the directional spectral emissivities of surfaces can often only be specified with standard measurement uncertainties of 5 %. When measuring a temperature of 100 °C in the spectral range by around 10 µm, this corresponds to a temperature uncertainty of typically 5 K.The emissivity is not a constant, but rather changes in general with changes of the surface (roughness, oxidation, impurities etc.), the observation angle, the observation wavelength as well as the temperature. Furthermore, it is often distributed inhomogeneously over the surface. Precise temperature measurements therefore demand exact knowledge of the emissivity. To determine the variety of dependencies of the emissivity on the above-mentioned parameters, complex measuring facilities are necessary.
The spectral emissivity is measured in the PTB by comparing the radiances of a cavity radiator of high quality – resembling an almost ideal black body – with the sample to be investigated by means of a Fourier transform spectrometer, whereby the radiation of the environment and the inherent radiation of the spectrometer are taken into consideration. Holding the sample in a temperature-regulated hemisphere hereby guarantees a constant radiation exchange with the environment. The apparatus allows the determination of the directional spectral emissivity as well as of the total emissivity of opaque samples under ambient conditions in a temperature range from 80 °C to 250 °C and a wavelength range from 4 µm to 40 µm under emission angles of 5° to 70° with a relative standard measurement uncertainly of better than 2 %. The extrapolation of the measured values of the directed spectral emissivity for emission angles above 70° then allows the hemispherical spectral emissivity, which is especially important for calculations of the net radiation exchange, as well as the total emissivity to be calculated. The homogeneity of the directional spectral emissivity at 4 µm is determined with the help of a thermography camera.
The results of the first orders from customers have served, for example, to optimise the paint drying process in the automobile industry, the thermal design of furnaces as well as the monitoring of glass forming processes.
Another measuring facility is currently being set up in the PTB which will allow emissivity measurements to be performed under vacuum conditions in an extended temperature and wavelength range – in particular for space applications.
Erika Schow | alfa
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