Extended calibration of a vibrating tube densimeter and new reference density data for a methane-propane mixture at temperatures from (203 to 423) K and pressures to 35 MPa

In this work, the lowest operational temperature of a commercial vibrating tube densimeter (VTD) calibrated using a robust physically-based model was carefully extended down to 203 K. The VTD was calibrated under vacuum and filled with pure gaseous methane and liquid propane as calibration reference fluids at temperatures from (203 to 423) K and pressures up to 35 MPa. The calibration parameters were obtained by linear and nonlinear regression of the calibration data (period of oscillation, temperature and pressure) to the physically-based calibration model. The relative deviations between the fitted densities of methane or propane and those predicted by the corresponding default reference equations of state were between (−0.56 to 0.60) % and the relative root mean square error was 0.30%. This wide-ranging calibration model was then employed for single-phase density measurements of a 0.95 methane + 0.05 propane mixture at a temperature of 203 K and pressures up to 35 MPa. Additionally, two sets of measurements at pressures approaching the bubble point at (203 and 208) K were completed. The measured densities ranged from (243 to 369) kg·m^{− 3}, with combined standard relative uncertainties between (0.31 and 0.42) %. The relative deviations of the measured densities from those calculated using the GERG-2008 equation of state spanned between (−0.5 to 0.4) %. This work demonstrates the potential of using the physically-based calibration model for VTDs at significantly low temperatures than previously attempted.