NIST SP 260-127 : 1997

The freezing points of Sn (231.928 degrees C) and Zn (419.527 degrees C) are defining fixed points of the International Temperature Scale of 1990 (ITS-90). Realization of these freezing points is performed using fixed-point cells containing high-purity (>=99.9999% pure) metal. The five Sn and five Zn freezing-point cells constituting Standard Reference Material (SRM) 1747 and SRM 1748, respectively, have been constructed and certified as suitable for use in the realization of the freezing-point temperatures of Sn and Zn for the ITS-90. The expanded uncertainties (k=2) assigned to the freezing-point temperatures of the fixed-point cells SRM 1747 and SRM 1748 do not exceed 0.40 m degrees C and 1.1 m degrees C, respectively. In this document, the methods used for the construction of the SRM Sn and SRM Zn freezing-point cells and their certification are described.