Argon Triple-Point Device for Calibration of SPRTs

This paper presents an apparatus for the calibration of long-stem platinum resistance thermometers at the argon triple point (T90 = 83.8058K), designed at the Institute of Low Temperature and Structural Research, Poland (INTiBS). A hermetically sealed cell filled at the Istituto Nazionale di Ricerca Metrologica, Italy with high purity gas (6N) is the main element of this apparatus. The cell is placed in a cryostat fully immersed in liquid nitrogen. A temperature-controlled shield ensures the quasi-adiabatic condition needed for proper realization of the phase transition. A system for correcting the temperature distribution along the thermometer well is also implemented. The cell cooling and argon solidification is carried out by filling the thermometer well with liquid nitrogen. A LabVIEW computer program written at INTiBS automatically controls the triple-point realization process. The duration of a melting plateau in the apparatus lasts for about 24 h. The melting width for F between 20% and 80% was <0.3mK. The reproducibility of the plateau temperature is better than ±20 μK.