Evaluation of the isochoric heat capacity measurements at the critical isochore of SF6 performed during the German Spacelab Mission D-2

The evaluation of the isochoric heat capacity (c(upsilon)) measurements on the critical isochore of SF6 performed with the newly developed scanning-radiation-calorimeter during the German Spacelab Mission D-2 is being presented. During cooling in the single-phase region under mu g conditions the "piston effect" avoids significant temperature and density inhomogeneities in the fluid. In the two-phase region both phases are continuously subcooled into the metastable region by the ''piston effect'' causing a permanent nucleation of small droplets and bubbles, which keeps the system near its thermodynamic equilibrium. For the slowest cooling run of dT(0)/dt = -0.06 K h(-1) at T-c, the c(upsilon) data are distorted by ramp rate effects only for \(T-T-c)/T-c\<3 x 10(-6). Using a range shrinking procedure for the determination of the asymptotic region yields that the simple power law is valid for \(T-T-c)/T-c\ < 1.6 X 10(-4). For the fitting procedure the theoretical constraints alpha = alpha' and B = B' are applied Fitting the data in the asymptotic region to the simple power law yields for the exponent alpha = 0.1105(-0.027)(+0.025) and the amplitude ratio A(-)/A(+) = 1.919(-0.27)(+0.24) in good agreement with values of the renormalization-group theory and other experiments for the 3,1-universality class. The validity of the power law extended by the first Wegner correction is found to be \(T-T-c)/T-c\ < 10(-3), giving similar values for the fitting parameters. Testing the two-scale-factor universality by combining the critical amplitude with the correlation length gives R-x = 0.284+/-0.018, in agreement with theoretical estimates and other experimental values for fluid systems. [S1063-651X(99)09502-1].