Standard Platinum Resistance Thermometer Calibration Laboratory (SPRTCL)

Summary

The Standard Platinum Resistance Thermometer Calibration Laboratory (SPRTCL) is responsible for the development, maintenance, realization and dissemination of the national standards of temperature in the range from 83.8058 K (-189.3442 °C) to 1234.93 K (961.78 °C) through the calibration of Standard Platinum Resistance Thermometers (SPRTs). The SPRTCL supports industry, commerce and science by providing traceability to the International Temperature Scale of 1990 (ITS-90) and to the SI unit kelvin.

Description

SPRT Calibration Laboratory with fixed point realization systems and measurement bridges.

Credit: NIST

The SPRTCL provides an extensive list of calibration services that cover the whole temperature range for which long-stem SPRTs, including high-temperature SPRTs, are the ITS-90 interpolation artifacts. This range spans from the triple point of argon (-189.3442 °C) to the freezing point of silver (961.78 °C). The laboratory also provides calibrations of capsule-type SPRTs in a limited temperature range, from the triple point of argon up to the freezing point of tin (231.928 °C). The SPRTs are calibrated by employing fixed-point cells, artifacts that realize the ITS-90 by enclosing high-purity materials (99.9999% or more) for which the phase transition temperatures (melting, freezing or triple points) are known with extreme accuracy. Such temperatures are the defining ITS-90 fixed points, and SPRTs are calibrated at combinations of these fixed points that correspond to the desired ITS-90 subrange. The fixed points realized by the SPRTCL are:

Fixed point	T, K	T, °C	Uncertainty, mK (k=2)
Triple point of argon	83.8058	-189.3442	0.14
Triple point of mercury	234.3156	-38.8344	0.15
Triple point of water	273.16	0.01	0.09
Melting point of gallium	302.9146	29.7646	0.13
Freezing point of indium	429.7485	156.5985	0.25
Freezing point of tin	505.078	231.928	0.27
Freezing point of zinc	692.677	419.527	0.55
Freezing point of aluminum	933.473	660.323	TBD
Freezing point of silver	1234.93	961.78	TBD

Standards

The SPRTCL keeps ITS-90 fixed-point cells as the national temperature standards. NIST manufactures their own standards, made of materials of the highest purities (99.9999% or greater) and builds their own realization apparatus (furnaces and cryostats) for realizations of the ITS-90 with state-of-the-art accuracy. A number of commercial devices are also adopted to support regular calibration services, which are traceable to the national standards. The traceability of resistance measurements to the ohm is ensured using calibrated standard resistors in combination with carefully characterized resistance ration bridges.

(Left) Indium, tin, and zinc fixed point cells. (Center) Water triple point cell. (Right) Water triple point cell maintenance bath with inserted SPRTs.

Credit: NIST

Measurement Services

The measurement services offered by the SPRTCL includes:

• Long-stem SPRTs from -189.3442 °C to 961.78 °C
• Capsule-type SPRTs from -189.3442 °C to 231.928 °C
• Certification of fixed-point cells and realization apparatus
• Characterization of thermometry bridges
• Special tests on prototypes and non-standard artifacts
• The complete service list offered by the SPRTCL can be found on: https://shop.nist.gov/ccrz__ProductList?categoryId=a0lt00000013aDBAAY&cclcl=en_US

Research Projects

The SPRTCL is continuously working on the development of better temperature standards to improve the accuracy and reduce the uncertainties of their temperature measurements. Current efforts include:

• The development of new metal fixed-point cells
• The development of novel fixed-point cells as alternatives to the mercury triple point on the ITS-90. This includes the triple points of the carbon dioxide (~216.592 K) and sulphur hexafluoride (~223.556 K)
• The development of customized furnaces for optimal realizations of metal fixed points
• The development of a novel closed-cycle apparatus for the realization of the argon triple point
• Fixed Point Development: Non-Metals