Study On The Reproducibility And The Assignment Methods For SF₆ Triple Point

Mercury triple point（234.3156 K）is the only non-gas fixed point below273.16 K defined by International Temperature Scale of 1990（ITS-90）.Mercury triple point plays a very important role in temperature value traceability.The use of mercury has been restricted in metrology field because of the inception of the Minamata Convention on Mercury.SF₆ triple point has been regarded as the potential substitute for Hg triple point.In this paper,research on SF₆ triple point cell and its reproducibility were carried out.At the same time,research on its assignment were carried out.In order to evaluate the possibility of SF₆ triple point replacing mercury triple point and improve the assignment level of SF₆ triple point,the study of non-uniqueness was also included in this paper.The specific research work and achievements are as follows:（1）The SF₆ triple point reproducibility apparatus was designed as an immersion type apparatus with an“opened”cell.The cell has four thermometer wells that can accommodate the simultaneous measurements of four standard platinum resistance thermometers（SPRTs）.A high-precision reproducibility system was used to carry out the reproduction research on SF₆ triple point.Good quality melting plateaus were obtained with a duration of about 25 h,and the triple point temperature of SF₆ was determined to be 223.55603 K±0.54 m K（k=1）.（2）For the temperature range from 83.8058K to 273.16 K,the replacement of the mercury triple point by SF₆ or CO₂ triple point was theoretically analyzed.Without changing the form of interpolation equation of ITS-90,difference of the temperature deviation and the propagation uncertainty before and after replacement was discussed.The results show that the equations defined by ITS-90 still has the applicability in this temperature range even after SF₆ triple point and CO₂ triple point replace mercury triple point.Under the same level of the reproducibility uncertainties of the three fixed points,the propagation uncertainty of Hg,SF₆ and CO₂ triple points decreases in turn over the whole temperature range.It is predicted that the corresponding temperature primary standard level of NIM has no change when the realization uncertainty of SF₆and CO₂ triple points is less than 0.25 m K（k=1）and 0.27 m K（k=1）,respectively.（3）Five standard platinum resistance thermometers with good long-term stability were used to measure the non-uniqueness of Type3 by“instantaneous comparison method”.The measurement results show that the non-uniqueness of Type3 is about±0.7 m K in the temperature range of-55℃～30℃.