Dynamic Behavior of a Pure Fluid at and Near Its Critical Density Under Microgravity and 1g

A new sample cell allowing accurately measurable density quenches was developed for further and systematic investigation of dynamic temperature propagation, or the piston effect. Several experiments were performed under 1g and microgravity during and in preparation for the Perseus mission in 1999. The starting temperatures ranged in the one-phase state between 1 and 1000 mK above Tc , while the density varied between 0.7rc < r < 1.3rc . The method for the determination of the isentropic difference coefficients (Dr/DT)S, (Dr/Dp)S, and (DT/Dp)S is explained. The coefficients are in reasonable agreement with the equation of state for SF6 , and the difference between the ground and the microgravity experiments is discussed. The advantage of the density quench method in contrast to the temperature quench method is demonstrated, particularly with regard to the influence of convection.