Long Term Performance of Mercury Triple-Point Cells Realized at NIS-Egypt

The triple point of mercury is one of the defining fixed-points of the International Temperature Scale of 1990 (ITS-90). Its value was assigned to be 231.3456 K (?38.8344 °C) by ITS-90 and has a unique importance since it is the only fixed-point suggested by ITS-90 between 0.01 °C and ?190 °C. The thermal metrology laboratory of the National Institute for Standards has chosen, several decades ago, to realize this fixed point through batch of thermometric cells. In the present work, four cells, composing the batch of reference, are inter-compared. The results of these inter-comparisons, over a period of time that may reach a dozen of years, showed the excellent reliability of these cells. One cell of the batch, that is constructed thirty years ago, is still able to materialize a fixed point with an expanded uncertainty of 0.44 mK.