MEASUREMENT OF T-T_(90) DOWN TO ZINC POINT TEMPERATURES WITH ABSOLUTE FILTER RADIOMETRY

摘要

In 1995, measurements of T-T_(90) for the temperature range 660°C to 962 °C have been performed at the Physikalisch-Technische Bundesanstalt (PTB) using a large area blackbody (LABB) and absolute filter radi-ometry at 676 nm and 800 nm. Although the inner cavity of the LABB is formed by a sodium heat pipe, it has been shown that the LABB can be used as an accurate source of blackbody radiation at temperatures as low as 400°C. Applying absolute interference filter radiometry with center wavelengths of 800 nm, and for the first time, 900 nm, and 1000 nm, the temperature range for thermodynamic temperature measurements of the LABB has been extended to temperatures as low as the zinc freezing point temperature of 419°C. The achieved standard uncertainty was about 20 mK for thermodynamic temperatures of 419°C. The measured thermodynamic temperatures have been compared with the temperature of the blackbody cavity measured by three high temperature standard platinum resistance thermometers (HTSPRT) located at the bottom of the radiating cavity. These HTSPRT were calibrated according to the International Temperature Scale of 1990 (ITS-90). Comparing the thermodynamic and the ITS-90 temperatures it was found that the former are always higher than the latter, with decreasing difference towards lower temperatures.