A general procedure for evaluation of calibration baths in precision temperature measurements

Abstract

The objective of the proposed paper is to develop a standardized procedure for evaluation of various calibration baths in precision temperature measurements. Since the uncertainties of transfer measurement and temperature standards such as SPRTs in particular are decreasing, metrological characteristics of calibration baths are becoming limiting factor in temperature calibrations 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 (providing known uncertainty of a standard thermometer), the exact knowledge about calibration bath is of an utmost importance. A general procedure for baths with different types of media (alcohol, water, silicon oil, fluidised alumina powder, etc.) is proposed, which enables continuous monitoring of bath parameters in order to properly evaluate uncertainty contributions to a measured (calibrated) thermometer. There is no ideal calibration system without inhomogeneity or gradient. Basic gradients that can be observed in calibration baths are axial and radial gradients. For the proper use of such a system in a process of calibration and measurement, gradients should be evaluated. Gradients appear as a change of the temperature reading of a thermometer due to a change of its position inside a calibration bath. A uniform profile and pre-defined points of calibration are under investigation. By this method a minimum uncertainty contribution could be defined as opposed to the maximum limit, thus enabling an optimal usage of particular calibration baths.