Best Practices for Properly Using Thermometric Fixed-Point Cells as Calibration Reference Standards

Abstract

This paper discusses thermometric fixed-point cells and considerations for their use as International Temperature Scale of 1990 (ITS-90) defining standards for the calibration of Standard Platinum Resistance Thermometers (SPRTs). Designation as an ITS-90 defining standard is dependent on the chemical composition (e.g., purity) of the sample material, which in turn predicts the melting, freezing, or triple-point realization temperature of the material. However, sole knowledge of the chemical composition and purity of the sample used to construct a fixed-point cell does not guarantee a particular realization temperature. To establish traceability to the International System of Units (SI), the fixed-point cell must be compared to other fixed-point cells that are traceable to the SI - through National Metrology Institutes (NMI's) with internationally-accepted measurement capabilities. ISO/IEC 17025 outlines a method for establishing reference standard traceability, and accreditation bodies have required qualification of thermometric fixed-point cells for over a decade. However, the authors have observed that most fixed-point cells are not qualified, and many users do not realize the implications of using them without first establishing traceability. To better clarify these implications, this paper presents the main sources of fixed-point cell realization temperature errors and uncertainties, along with suggestions for handling these issues. References to current quality standards and industry best practices will also be presented, to underscore the current requirements for proper use of ITS-90 defining thermometric fixed-point cells.