GENERAL PROCEDURE FOR UNCERTAINTY EVALUATION OF A TEMPERATURE CALIBRATION BATH

Abstract

ABSTRACT The objective of this paper is to develop a general procedure for evaluation of various temperature calibration baths or furnaces. Since the uncertainties of transfer measurement and temperature standards, such as standard platinum resistant thermometers in particular, are decreasing, metrological characteristics of calibration baths and furnaces are becoming a limiting factor in temperature calibration by comparison. Due to the fact that a time invariance of metrological characteristics of calibration baths (gradients, stability, repeatability, etc.) represents a major contribution to the total calibration uncertainty, the exact knowledge about a calibration bath is of an utmost importance. A general procedure for baths with different types of a media is proposed, which enables continuous monitoring of bath parameters in order to evaluate uncertainty contributions to a measured (calibrated) thermometer. There is no ideal calibration system, i.e., without inhomogeneity or gradient. Basic gradients that could be observed in calibration baths are vertical and horizontal gradients. They may also be recognized as axial and radial gradients in a case where cylindrical equalizing blocks are used inside a calibration bath. For the proper use of such a system in a process of calibration and measurement, gradients should be precisely determined. Gradients appear as a change of a temperature reading of a thermometer due to a change of its position inside a calibration bath. By this method, a realistic uncertainty contribution could be defined as opposed to under-or overestimated values of uncertainties, thus enabling an optimal use of a particular calibration bath. Examples of real data are taken from cryostat measurements. Otherwise, seven other temperature calibration baths and furnaces in the laboratory have been evaluated in the same way.