General procedure for uncertainty evaluation of a temperature calibration bath

The objective of this paper is to develop a general procedure for evaluatio n of various temperature calibration baths or furnaces. Since the uncertain ties of transfer measurement and temperature standards, such as standard pl atinum resistant thermometers in particular, are decreasing, metrological c haracteristics of calibration baths and furnaces are becoming a limiting fa ctor 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 tot al 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. Basi c gradients that could be observed in calibration baths are vertical and ho rizontal gradients. They may also be recognized as axial and radial gradien ts in a case where cylindrical equalizing blocks are used inside a calibrat ion bath. For the proper use of such a system in a process of calibration a nd 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 re alistic uncertainty contribution could be defined as opposed to under-or ov erestimated values of uncertainties, thus enabling an optimal use of a part icular calibration bath. Examples of real data are taken from cryostat meas urements. Otherwise, seven other temperature calibration baths and furnaces in the laboratory have been evaluated in the same way.