Standard Guide for Use of Fixed-Point Cells for Reference Temperatures

Importancia y uso:

5.1 A pure material has a well defined phase transition behavior, and the phase transition plateau, a characteristic of the material, can serve as a reproducible reference temperature for the calibration of thermometers. The melting or freezing points of some highly purified metals have been designated as defining fixed points on ITS-90. The fixed points of other materials have been determined carefully enough that they can serve as secondary reference points (see Tables 1 and 2). This guide presents information on the phase transition process as it relates to establishing a reference temperature.

(A) Defining fixed point for ITS-90.(B) Realized as melting point.(C) Based on recommendation of International Bureau of Weights and Measures (BIPM) Working Group 2 of the Comité Consultatif de Thermométrie (CCT-WG2); published as: Bedford, R. E., Bonnier, G., Maas, H., and Pavese, F., "Recommended Values of Temperature on the International Temperature Scale of 1990 for a Selected Set of Secondary Reference Points", Metrologia, Vol 33, 1996, pp. 133. DOI: 10.1088/0026-1394/33/2/3. (A) Values for cells of good design, construction, and material purity used with careful technique. Cells of lesser quality may not approach these values.(B) Realized as melting point.

5.2 Fixed-point cells provide users with a means of realizing melting and freezing points. If the cells are appropriately designed and constructed, if they contain material of adequate purity, and if they are properly used, they can establish reference temperatures with uncertainties of a few millikelvins or less. This guide describes some of the design and use considerations.

5.3 Fixed-point cells can be constructed and operated less stringently than required for millikelvin uncertainty, yet still provide reliable, durable, easy-to-use fixed points for a variety of industrial calibration and heat treatment purposes. For example, any freezing-point cell can be operated, often advantageously, as a melting-point cell. Such use may result in reduced accuracy, but under special conditions, the accuracy may be commensurate with that of freezing points (see 6.3.10).

5.4 The test procedure described in this guide produces qualification test data as an essential part of the procedure. These data furnish the basis for quality control of the fixed-point procedure. They provide for evaluation of results, assure continuing reliability of the method, and yield insight into the cause of test result discrepancies. The test procedure is applicable to the most demanding uses of fixed-point cells for precise thermometer calibration; it may not be appropriate or cost-effective for all applications. It is expected that the user of this guide will adapt the procedure to specific needs.

Subcomité:

E20.07

Referida por:

E0644-11R19, E2593-17R23, E0344-23, E0879-20

Volúmen:

14.03

Número ICS:

17.200.20 (Temperature-measuring instruments)

Palabras clave:

calibration; cryoscopic constant; fixed point; freeze; freezing curve; freezing plateau; freezing point; freezing range; ITS-90; melt; melting curve; melting plateau; melting point; melting range; precision; qualification; recalescence; reference material reference temperature; resistance thermometer; thermocouple; thermometer; uncertainty; undercool ;