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NIST Certification of ITS-90 Fixed-Point Cells From 83.8058 K to 1234.93 k: Methods and Uncertainties



Gregory F. Strouse


The certification of ITS 90 fixed-point cells from the Ar triple point (83.8058 K) to the Ag freezing point (1234.93 K) is an integral part of the calibration services offered by the NIST Thermometry Group. This service is used to qualify new fixed-point cells for internal use within the NIST Thermometry Group and by those facilities seeking traceability to NIST for the ITS-90 calibration of SPRTs. The method of certifying an ITS-90 fixed-point cell is a multiple step process that is designed to evaluate the intrinsic repeatability of the fixed-point cell, evaluate the realized temperature difference between the test fixed-point cell and the NIST reference fixed-point cell, assess the interaction of the fixed-point cell with the furnace and SPRT, and estimate the uncertainty of the certification process. This paper gives results of the reproducibility of direct comparison measurements over time, furnace effects on the realized fixed-point temperature, the certification uncertainty budget, and an uncertainty budget showing the required measurements that an end user must perform to assign an uncertainty in the use of the fixed-point cell.


direct comparison, fixed-point cell, fixed-point cell certification, International Temperature Scale of 1990, ITS-90, SPRT, standard platinum resistance thermometer


Strouse, G. (2005), NIST Certification of ITS-90 Fixed-Point Cells From 83.8058 K to 1234.93 k: Methods and Uncertainties, TEMPMEKO, [online], (Accessed July 21, 2024)


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Created January 1, 2005, Updated February 19, 2017