Reliability of Argon Triple Point Cells Materializing the ITS-90 Temperature Fixed Point

The triple point of argon is one of the defining fixed-points of the International Temperature Scale of 1990, ITS-90. Its value was assigned to be 83.8058 K by ITS-90. The thermal metrology laboratory of the National Institute for Standards, NIS has chosen, several years ago, to realize this fixed point through batch of thermometric cells. In the present work, three cells of different designs, composing the batch of reference, are inter-compared. The results of these inter-comparisons, over a period of time that reaches 17 years, showed the excellent reliability of these cells. No malfunction has arisen for the oldest cell "Ar-INM-42-NIS" over this period, it has 0.24 mK as the maximum variation among the other cells. For the other cells variations were found to be 0.17 and 0.18 mK for "Ar-NIS-QA" and "Ar-LNE-NIS-MC" respectively. The recent estimations of uncertainties for the batch of cells that include all the factors affecting the measurements showed values of 0.52, 0.33 and 0.31 mK for "Ar-INM-42-NIS", "Ar-NIS-QA" and "Ar-LNE-NIS-MC" respectively, and that cell "Ar-INM-42-NIS" has the maximum expanded uncertainty of 0.52 mK.