Vacuum Variable Medium Temperature Blackbody

This article describes the vacuum variable medium-temperature blackbody (VMTBB) constructed to serve as a highly stable reference source with an aperture diameter of 20 mm in the temperature range from 150 A degrees C to 430 A degrees C under medium-vacuum conditions (10(-3) Pa) and in a reduced background environment (liquid-nitrogen-cooled shroud). The VMTBB was realized for the calibration facility at the PTB in the field of reduced background radiation thermometry under vacuum. This facility is intended for performing radiometric and radiation thermometric measurements under vacuum conditions in the temperature range from -173 A degrees C to 430 A degrees C and spectral emissivity measurements in the temperature range from 0 A degrees C to 600 A degrees C without atmospheric interferences. It is difficult to realize a precision blackbody with high emissivity for temperatures above 400 A degrees C. Cavities of such blackbodies are normally made of copper and coated by a paint with high emissivity. But any paint put on copper does not survive several cycles of heating to temperatures up to 450 A degrees C. As a result of investigations at PTB, a special procedure of coating the surface of the cavity by paint with high emissivity has been developed. The cavity surface is coated by chemical nickel plating before covering it by a paint with high emissivity. The general concept and the design of the VMTBB are given. For realization of good temperature uniformity along the complete radiating cavity, a three module design is used consisting of a heat exchanger and two stages of temperature control of the cavity, based on two precision PID controllers. The temperature of the cavity is determined by 15 precision Pt resistance thermometers. Six of them are used for the VMTBB cavity and heat exchanger temperature control, and the others are used for the cavity temperature measurement and correction. A description of the temperature control and measurement system of the VMTBB is presented. Optical ray tracing with a Monte Carlo method (STEEP 3) indicated that the effective emissivity of this blackbody cavity is not worse than 0.9994. Tests of the VMTBB were carried out at the PTB facility, and the radiation of the VMTBB was measured in comparison to the vacuum variable low-temperature blackbody (VLTBB) in the temperature range from 150 A degrees C to 170 A degrees C with the vacuum infrared standard radiation thermometer (VIRST). The temperature uniformity of the blackbody from the bottom to the front of the cavity is better than +/- 100 mK in the whole temperature range. The stability of the temperature of the blackbody is within 50 mK in the whole temperature range.