Zinc-Filled Multi-entrance Fixed Point

Abstract

Comparison calibrations comprise by far the majority of all thermometer calibrations, and the improvement of the calibration speed and quality is of significant importance. In general, these calibrations are performed by comparing a thermometer under calibration with a reference thermometer, with both thermometers being placed in a zone of controlled temperature that is in thermal equilibrium. The time invariance of metrological characteristics of temperature regulated zones (gradients, stability, repeatability) adds a significant contribution to the total calibration uncertainty. To improve spatial thermal homogeneity and temporal stability of ordinary equalizing blocks (commonly used in combination with calibration baths and furnaces), a new concept is developed under the name multi-entrance fixed point (MEFP). With the MEFP, performances are improved by utilizing a phase transition of matter, similar to the fixed-point cells that are used for calibration of thermometers at the primary level. While fixed-point cells represent temperature standards alone, the MEFP concept requires the use of a standard thermometer as the calibration reference. This paper discusses the advantages, design possibilities, and performance of the concept and gives results of the investigation of the zinc-filled MEFP cell. The performance of the new MEFP cell was examined by calibrating a thermometer against a standard. Two calibrations were performed in the same furnace, first using an MEFP cell as an ordinary equalizing block (without a phase transition) and then at the melt plateau of zinc. The investigation results confirmed the usefulness of the MEFP concept, as the uncertainty contributions from temperature gradients and the stability of the furnace were decreased by an order of magnitude, in comparison to the ordinary equalizing block.