INVESTIGATING THE MERCURY TRIPLE POINT AT THE NATIONAL RESEARCH COUNCIL OF CANADA

摘要

Recent work on the mercury triple point at NRC is reported. To consolidate the calibration capabilities in our laboratory for both long-stem and capsule-style standard platinum resistance thermometers (SPRTs), we have recently filled three new cells and compared their performance against the primary mercury cell that has been in use for several years. The first of the new cells is virtually identical in design to the primary cell: it is constructed from glass and has similar dimensions, the only significant modification being a larger bore re-entrant thermowell suitable for use with both types of SPRT. This new cell contains approximately 2.75 kg of Hg with a purity of 7N5 as specified by the manufacturer; the primary cell uses 2.5 kg of 7N5 purity. The new cell was filled by vacuum distillation, rather than by pouring mercury into the cell and then pumping off the excess gas as was done for the primary cell. In addition to the new large cell, two small stainless-steel sealed cells, similar in design to typical cryogenic sealed cells, were fabricated and filled with substantially smaller amounts of 7N5 purity mercury. The first of the small cells contains only a few grams of mercury, and is essentially empty. The second contains about 200 g of mercury, which raises the liquid level to a height closer to the length of the sensing element inserted in the thermowell. These two cells, which are useable only with capsule-style SPRTs when installed in the NRC low-temperature cryostat, have been melted using pulsed heating in an adiabatic, calorimetric fashion. Melting curves and comparative temperature analysis for all four cells are presented, along with results obtained using both long-stem and capsule-style CSPRTs. The performance of the two small sealed cells during the melting experiments is summarised, including data illustrating the very different behaviour for returning to equilibrium following a heat pulse.