Evaluation of the isochoric heat capacity measurements at the critical isochore of SF 6 performed during the German Spacelab Mission D-2

The evaluation of the isochoric heat capacity ${(c}_{v})$ measurements on the critical isochore of ${\mathrm{SF}}_{6}$ 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 \ensuremath{\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 ${\mathrm{dT}}_{0}/dt=\ensuremath{-}0.06{\mathrm{K}\mathrm{}\mathrm{h}}^{\mathrm{\ensuremath{-}}1}$ at ${T}_{c},$ the ${c}_{v}$ data are distorted by ramp rate effects only for $|(T\ensuremath{-}{T}_{c}{)/T}_{c}|l3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}.$ Using a range shrinking procedure for the determination of the asymptotic region yields that the simple power law is valid for $|(T\ensuremath{-}{T}_{c}{)/T}_{c}|l1.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}.$ For the fitting procedure the theoretical constraints $\ensuremath{\alpha}={\ensuremath{\alpha}}^{\ensuremath{'}}$ and ${B=B}^{\ensuremath{'}}$ are applied. Fitting the data in the asymptotic region to the simple power law yields for the exponent $\ensuremath{\alpha}{=0.1105}_{\ensuremath{-}0.027}^{+0.025}$ and the amplitude ratio ${A}^{\ensuremath{-}}{/A}^{+}{=1.919}_{\ensuremath{-}0.27}^{+0.24},$ 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 $|(T\ensuremath{-}{T}_{c}{)/T}_{c}|l{10}^{\ensuremath{-}3},$ giving similar values for the fitting parameters. Testing the two-scale-factor universality by combining the critical amplitude with the correlation length gives ${R}_{x}=0.284\ifmmode\pm\else\textpm\fi{}0.018,$ in agreement with theoretical estimates and other experimental values for fluid systems.