A VIBRATING-TUBE DENSIMETER FOR FLUIDS AT HIGH-PRESSURES AND TEMPERATURES

A vibrating U-tube apparatus has been developed for determining the de nsities 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 a re injected into the U-tube at Constant P and T. Three or more referen ce fluids are used to calibrate the response of the instrument. Fluid mixtures are produced by pumping pure fluids into T-junctions on the u pstream side of the U-tube using high accuracy syringe pumps. An autom ated syringe pump is used to maintain P at setpoint +/-0.01 MPa. T is controlled to +/-0.01 K using a closed-loop, electronic signal amplifi cation/feedback system. For mixtures, a statistically significant numb er of measurements of tau are obtained to account for the effects of s mall heterogeneities in fluid composition (generally <0.005 X(i)). Typ ically, density data for 15 fluids can be obtained in a 6- to 8-h peri od. Considering all of the potential sources of error in the experimen tation, conservative estimates of uncertainty are as follows: P, +/-0. 02 MPa; T, +/-0.05 K; rho (pure fluids),+/-0.0005 g . cm(-3); and rho v(fluid mixtures), +/-0.0005 - 0.0010 g . cm(-3)