Previously utilized only as a secondary fixed point, the Triple Point of Mercury has been adopted as a defining fixed point of the International Temperature Scale of 1990 (ITS-90). Virtually all standards-grade cryogenic thermometer calibrations include the Triple Point of Mercury. Used with the Triple Point of Water and the Melting Point of Gallium, the Mercury Triple Point is particularly important to users in oceanographic, medical, biological and industrial process thermometry.
A rugged, hermetically sealed Type 304 Stainless Steel cell envelope filled with 2.5 kg of .999 999 pure Mercury provides stable, long-lasting temperature plateaus. Since triple point conditions can only exist at the liquid-to-vapor interface, hydrostatic head corrections must be applied to the theoretical triple point temperature. The user manual includes tables and charts presenting a measured melt plateau profile and the hydrostatic head corrections applied to each cell.