The development of emissivity measurements under vacuum at the PTB
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THERMAL MEASUREMENTS THE DEVELOPMENT OF EMISSIVITY MEASUREMENTS UNDER VACUUM AT THE PTB
A. R. Adibekyan,1 C. Monte,1 M. Kehrt,1 S. P. Morozova,2 B. Gutschwager,1 and J. Hollandt1
The consistency of the values of the spectral radiance of two standard vacuum absolutely black bodies (low and medium temperature) in the 80–170°C range, forming part of the equipment for measuring emissivity under vacuum, is considered. It is shown that, using this equipment, one can measure spectral radiance at low radiation temperatures (down to –100°C) in the 4.5–20 µm wavelength band. The characteristics of the inner coating of the sample enclosure, required in order to make the measurements in the far infrared band and compatible with the vacuum conditions and low temperatures, are presented. Keywords: emissivity, vacuum, spectral radiance, absolutely black body.
Measurements of the dimensionless quantity, directional spectral emissivity, are being carried out at the PTB at the present time using equipment which operates in air at a temperature of 80–500 °C in the 4–40 μm spectral band, and are a routine metrological service. New apparatus for measuring emissivity under a vacuum, which is part of a low-background calibration facility (LBCF), considerably extends the possibilities of these measurements. This equipment enables samples to be investigated at temperatures of 0–600°C in an angular range of ±75° and in the spectral range from 0.4 μm to 1400 μm. Its use also reduces the uncertainty of emissivity measurements, since two main components of the uncertainty, which arise when finding the temperature of the sample being investigated, are removed. These losses are due to convection of the air at high temperatures (this value of the uncertainty is necessary for an accurate determination of the surface temperature of the sample in air), and as a result of atmospheric absorption when the partial pressures of H2O and CO2 change. The LBCF equipment, shown in Fig. 1, is described in detail in [1]. It contains several reference sources: two vacuum absolutely black bodies (ABB) with a regulated radiation temperature, situated in the radiation-source chamber – a low-temperature black body (LTBB) [2] operating in the –173 to +170°C temperature range, and a medium-temperature (MTBB) [3], for use in the 80–430°C temperature range. The third (cold) reference source – a liquid-nitrogen cooled ABB – is mounted in the upper part of the optical-mechanical unit. The chamber containing the radiation sources also has additional space to accommodate a test source for calibration, or a spherical enclosure with a sample for measurement of emissivity inside it (see Fig. 1b). The radiation emerges through a circular opening in the front part of the sphere. Two standard radiators – the MTBB and the LTBB – are mounted on the right on a mobile table. Two schemes for recording the radiation are currently being used: 1) a Fourier transform infrared spectrometer which records spectral radiance in the 0.4–1400 μm band. In this case, 1 2
Physikalisch-Technisch
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