A vibrating-tube densimeter for fluids at high pressures and temperatures

A vibrating U-tube apparatus has been developed for determining the densities of pure fluids and fluid mixtures at 10-200 MPa and 323-773 K. Measured parameters are P, T, and {tau} (period of vibration). Fluids are injected into the U-tube at constant P and T. Three or more reference fluids are used to calibrate the response of the instrument. Fluid mixtures are produced by pumping pure fluids into T-junctions on the upstream side of the U-tube using high accuracy syringe pumps. An automated syringe pump is used to maintain P at setpoint {plus_minus}0.01 MPa. T is controlled to {plus_minus}0.01 K using a closed-loop, electronic signal amplification/feedback system. For mixtures, a statistically significant number of measurements of {tau} are obtained to account for the effects of small heterogeneities in fluid composition (generally