Development and Validation of an Argon Triple Point Apparatus with a Novel Automatic Pressure Control System

This paper describes the development and validation of an apparatus for the realization of the triple point of argon (83.8058 K), with a novel automatic pressure control system for the liquid nitrogen cryostat. The automatic pressure controller, together with custom-made software, was developed and tested in the Laboratory for Process Measurement at the Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb (FSB-LPM). Performance testing and characterization of the automatic pressure controller confirmed its suitability for precise and reliable control of gauge pressure in the cryostat. The characteristics and uncertainty of the measurement setup for the realization of the triple point of argon were validated through a bilateral hybrid comparison with the Laboratory of Metrology and Quality at the Faculty of Electrical Engineering, University of Ljubljana (MIRS/UL-FE/LMK). A long-stem quartz-sheathed standard platinum resistance thermometer was used as a transfer standard. The realizations of the International Temperature Scale (ITS-90) were compared in the subrange from the triple point of argon to the triple point of water. The comparison results show that resistance ratio (<i>W</i>) values determined by FSB-LPM at the fixed points of argon and mercury deviate from the MIRS/UL-FE/LMK values, within the determined combined uncertainty of the comparison.