The dielectric constant of SF6 near the critical point

Abstract

Measurements are reported of the dielectric constant of SF₆ at the critical density (±1%) over a temperature range of −10⁻³〈(T−T_c)/T_c〈+10⁻². The experimental cell was especially designed to suppress spurious effects resulting from temperature gradients and gravity. Temperature stability was better than ±20 μK and capacitance stability better than 0.5 ppm over several days. The critical temperature was determined as 318.718±0.002 K and was completely free of drift. Capacitance data were reproduced within a few ppm over the duration of the experiment. No direct evidence of a (T−T_c)^1−α anomaly was found; an upper bound of 5 ppm is established. Effects of fluid stratification and phase separation were clearly seen and are well understood. We find a value of 0.325±0.005 for the critical exponent β.