Certification and implementation of the argon triple point in the fluke SPRT calibration laboratory

This paper summarizes the project to certify and implement an argon triple point system into the Fluke SPRT calibration laboratory. For more than twenty years Fluke has provided ISO/IEC 17025 accredited SPRT calibration by fixed point over the range of -197 °C to 962 °C. Fixed point cells were used at all points except for -197 °C where a comparator device using the boiling point of liquid nitrogen and a NIST-calibrated capsule SPRT was used in place of the argon triple point. To improve upon this setup the laboratory acquired an argon triple point system, calibrated it, and implemented it into the SPRT calibration process. This paper gives the results of the project including heat flux (immersion) testing, well-to-well temperature uniformity testing, plateau testing, realization temperature verification, interlaboratory comparison results, and the certification uncertainty analysis. A description of the argon triple point system is provided along with methods of dealing with non-ideal results that can occur when measuring temperature in the argon triple point system with metal-sheath SPRTs.