Accurate density measurements on a binary (argon + carbon dioxide) mixture were carried out at temperatures T = (273.15, 283.15, 293.15, 308.15, and 323.15) K with pressures up to the dew-point pressure or 9.0 MPa, whichever was lower. A two-sinker magnetic suspension densimeter was utilized for the measurements. The composition of the gravimetrically prepared mixture was 0.94951 mole fraction carbon dioxide. Taking the measurement uncertainties in temperature, pressure, density, and composition into account, the relative combined expanded uncertainty (k = 2) in density was estimated to be less or equal 0.043% of the measured density value. Relative deviations of the new experimental data from the GERG-2008 equation of state (EOS) and from the EOS-CG (a recently developed multiparameter EOS optimized for combustion gases) were within 0.95% and 0.18%, respectively. Third-order virial equations were fitted to the new experimental data. The correlated molar masses at different isotherms agreed with the gravimetrically determined one within 0.02%. Values and uncertainties of the second and third virial coefficients and the second interaction virial coefficient for the binary mixture were determined.

A two-sinker magnetic suspension densimeter was used to carry out accurate density measurements on binary mixtures (argon + carbon dioxide) with carbon dioxide mole fractions of 0.50000, 0.49975, and 0.75093. The measurements cover a temperature range from T = (273.15 to 323.15) K with pressures up to 9.1 MPa or the dew-point pressure, whichever was lower. The mixtures were prepared gravimetrically. With all measurement uncertainties in temperature, pressure, density, and composition taken into account, the relative combined expanded uncertainty (k = 2) in density was estimated to be less or equal to 0.033%. Relative deviations of the experimental densities from the GERG-2008 equation of state were within 0.31% for the mixtures with 0.50000 and 0.49975 mole fraction carbon dioxide and up to 1.90% for the mixture with 0.75093 mole fraction carbon dioxide. In contrast, the relative deviations from a recently developed multiparameter equation of state (EOS) optimized for combustion gases (EOS-CG) were less than 0.30% for the three mixtures under study. Third-order virial equations were fitted to the measured density values. The correlated molar masses at different temperatures agreed within less than 0.01% with the molar masses determined from the gravimetric mixture preparation. Values and uncertainties of the second and third virial coefficients as well as of the second interaction virial coefficient were calculated.

Accurate density measurements on a binary (hydrogen + carbon dioxide) mixture with a hydrogen mole fraction of 0.05362 were carried out at temperatures T = (273.15, 293.15, and 323.15) K with pressures up to the dew-point pressure or 6.0 MPa, whichever was lower. The gas mixture was prepared gravimetrically. A well-proven two-sinker magnetic suspension densimeter was utilized for the measurements, and a preheating device for the gas sample was specially designed and integrated in order to avoid condensation when filling and flushing the densimeter. Considering all measurement uncertainties in temperature, pressure, density, and composition, the combined expanded uncertainty (k = 2) in density was estimated to be less than or equal to 7.4 × 10–4ρ. The relative deviations of the experimental densities from the GERG-2008 equation of state were less than 0.4%, which is clearly within the uncertainty of this equation. Sorption effects were carefully investigated, and a large impact on the reproducibility of the density measurements on the order of 6 × 10–4ρ (k = 2) was observed. Values and uncertainties of the second and third virial coefficients were determined by fitting a third-order virial equation to the experimental results. The second interaction virial coefficient was determined as well.

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Accurate (p, ρ, T, x) measurements of three hydrogenenriched natural-gas mixtures were carried out at T = (273.15, 283.15, and 293.15) K with pressures up to 8 MPa using a two-sinker densimeter. The relative expanded combined uncertainty (k = 2) in density was 0.02 %. To make up the mixtures under study, a 21-component high-calorific natural gas taken from a pipeline in Germany was blended with hydrogen to compositions of approximately (0.05, 0.10, and 0.30) mole fraction hydrogen. Comparisons of the measured densities with values calculated from the GERG-2008 and the AGA8 equations of state for natural gas mixtures are presented. The densities calculated with the equations of state are generally in very good agreement with our measured values and the limited available literature data. For the natural-gas mixtures containing hydrogen mole fractions of up to 0.10, relative deviations in density are essentially smaller than 0.05 %. Relative deviations of up to 0.1 % were observed for the natural-gas mixtures containing a hydrogen mole fraction of 0.30. This result suggests that both equations of state are suitable for hydrogen-enriched natural-gas mixtures in the investigated temperature-and pressure-range.