Gas-Controlled Heat Pipes for Accurate Liquid-Vapor Transition Measurements

At IMGC several gas-controlled heat pipes have been used to realize liquidvapor transitions of pure fluids, such as water, potassium, mercury, and sodium. A very uniform and stable temperature zone has been realized in all cases, at the millikelvin level. Therefore, very accurate measurements of the phase-transition temperature are possible, using platinum resistance thermometers. Water and potassium heat pipes have been developed for the calibration of standard platinum resistance thermometers (SPRTs). Mercury and sodium heat pipes are used for vapor-pressure curve studies for primary thermometry. The temperature stability inside the heat pipes is maintained at the millikelvin level by means of a very accurate pressure control system specially developed at IMGC. The heatpipe features and capabilities are summarized, together with a description of the pressure control systems. The most important results achieved using the gascontrolled heat pipes in primary thermometry and in liquid-vapor phase-transition curve measurements are reported here.