Accurate density measurements on a binary mixture (carbon dioxide + methane) at the vicinity of the critical point in the supercritical state by a single-sinker densimeter

Pressure, density, and temperature measurements on a binary mixture (carbon dioxide + methane) with the composition 0.950 mol fraction carbon dioxide were carried out at the vicinity of the critical point in the supercritical state using a single-sinker magnetic suspension densimeter. The expanded measurement uncertainties (k = 2) were 35 mK for temperature, 3.4 kPa for pressure, and 0.033% for density. The mixture sample was prepared gravimetrically with the expanded uncertainty in the composition less than 0.001 mol fraction. Relative deviations of the experimental densities at temperature 301.15 K with pressures from (2.0–12.0) MPa from the values calculated by the GERG-2008 equation of state were generally within approximately 1.0%, and these experimental data agreed well with the experimental results from other groups. The relative deviations from the results calculated by the GERG-2008 equation of state for the experimental data measured at temperatures from (300.15–313.15) K with pressures from (8.0–10.0) MPa were within 5.0%. The absolute values of the relative deviations first increased and then decreased with increasing pressure for each isotherm, and the pressure of the peaking point was close to the pressure of the maximum value of (∂ρ/∂T)p, calculated using the experimental data, on the same isotherm.