Apparatus for measuring thermodynamic properties at low temperatures

An apparatus has been constructed to provide thermodynamic data for models of planetary atmospheres. Often these data are needed at low temperatures, especially for the outer planets and their satellites, but are not readily available in the literature. The vapor pressure of propane was measured from 85 to 240 K to demonstrate one application of the apparatus for the acquisition of these types of data and to assess the performance of the system. This molecule was chosen because it is available in high purity, it has a well-established vapor-pressure curve, and it exhibits only one phase change over this temperature range. Our results compare favorably with the values available in the literature. The major components of the system include several types of pressure measuring instruments (1000 and 1 Torr capacitance manometers, spinning-rotor gauge), a residual gas analyzer to monitor sample purity in situ, and a helium closed-cycle refrigerator for cooling. The gas-handling manifold was constructed using materials and techniques adapted from the semiconductor production industry to minimize sample impurities which constitute a significant source of error in these types of measurements. Several unique design features were also incorporated in the construction of the sample cell to facilitate proper correction for thermal transpiration—an important factor for pressure measurements at low temperatures—and to ensure that the temperature sensor accurately reflected the sample temperature. The operational temperature and pressure limits are 62–240 K and 3×10 −6 –10 3 Torr , respectively. The lowest achievable temperature is governed by the no-load temperature of the first stage of the refrigerator and vertical thermal gradients along the sample cell walls, while the minimum obtainable pressure is set by the base pressure of the manifold and a slight outgassing rate.