System for Realizing the Triple Point of Mercury to Calibrate Industrial-Type Platinum Resistance Thermometers

Most platinum resistance thermometers (PRTs) sent to CENAM for calibration are of the industrial type (IPRT). The cells used to calibrate both standard PRTs (SPRTs) and IPRTs form part of the national standard of temperature. In order to reduce their use, we built a set of fixed-point cells, furnaces, and baths to calibrate IPRTs using fixed points, including an apparatus and cells to reproduce the Hg triple point. To realize the triple point of mercury (TP-Hg), an apparatus that operates from room temperature to  − 45 ^◦C using a CENAM-constructed heat pipe was designed and built. With the heat pipe, it is unnecessary to use a recirculation bath to provide a temperature-controlled environment, the temperature gradient is reduced, and the system is more efficient because its thermal mass is reduced. In this way, it is possible to reproduce the TP-Hg for long periods of time to facilitate IPRT calibration, using cells of smaller size than is conventional and without using expensive liquid baths. The system was tested with cells having 400 g and 800 g of mercury, and a reproducibility of about 0.1 mK was obtained.