Accurate experimental (p, ρ, T) values and virial coefficients for the (methane and helium) binary system

• Accurate density data for three binary mixtures of methane with helium are presented.
• The second and third virial coefficients are determined.
• Experimental data are compared with the densities calculated from the GERG-2008 and AGA8-DC92 equations of state.
• The relative deviations are clearly higher for GERG-2008 than for AGA8-DC92.

This work provides accurate experimental (p, ρ, T) values for three binary mixtures of methane with helium: (0.95 (amount-of-substance fraction) CH4 + 0.05 He) and (0.90 CH4 + 0.10 He) at temperatures of (240, 250, and 260) K and (0.50 CH4 + 0.50 He) from (240 to 400) K. This work is a continuation of a previous one which reported accurate experimental (p, ρ, T) values for the (0.95 CH4 + 0.05 He) and the (0.90 CH4 + 0.10 He) binary mixtures over the temperature range from (250 to 400) K. All density measurements were performed by using a single-sinker densimeter with magnetic suspension coupling at pressures up to 20 MPa. Experimental values were compared with the corresponding densities calculated from the GERG-2008 and the AGA8-DC92 equations of state, respectively. Deviations from the GERG-2008 are much larger than from the AGA8-DC92 (up to −6.5%). These deviations increase with decreasing temperature, with increasing pressure, and with increasing helium fraction. In contrast, deviations from the AGA8-DC92 are within the 0.5% band. The experimental values were also used to calculate the second and the third virial coefficients, B(T,x)B(T,x) and C(T,x)C(T,x), as well as the second interaction virial coefficient B12(T)B12(T) for this mixture.