Realization of the ³He Vapour‐Pressure Temperature Scale at NMIJ/AIST

An apparatus for realizing the ³He vapour‐pressure temperature scale has been constructed at NMIJ/AIST. The apparatus is designed to be operated for a long time by adopting a continuously operating ³He cryostat. The cryostat has an experimental access port, which is a top‐loading type. The diameter of the access port is 50 mm. An insert for ³He vapour‐pressure measurements is placed in the access port. The preliminary measurement results confirm that the cryostat system stabilizes the temperature of a ³He vapour‐pressure cell to better than 50 μK over several hours over the temperatures range of the ³He vapour‐pressure scale. The ³He vapour pressure is measured with a diaphragm differential pressure gauge, which has a silicon‐resonant sensor. The uncertainties of this system are estimated in the case of calibrating a rhodium‐iron resistance thermometer measured by an automatic DC bridge.