A physics-based model of the electrical impedance of ionic polymer metal composites A methodology to formulate equivalent electric circuits to vibrating-wire sensors is presented, as well as examples of its application. Vibrating-wire sensors have been used in a number of instruments built for measurement of the density and viscosity of fluids up to high pressure and in wide temperature ranges. These instruments are based on a rigorous theoretic model describing both the mechanics of oscillation and the hydrodynamic effects arising from the presence of the sample fluid surrounding the vibrating wire. The equivalent circuits proposed in this work are essential in order to interpret the output signals of the sensor in terms of its mechanical parameters. Design choices dictate the type of pertinent electromechanical analogy. The use of equivalent circuits made possible the simultaneous measurement of the density and viscosity of fluids using one single sensor, which is a demonstration of a complete understanding of its behavior.

Total vapor pressures for liquid mixtures of xenon + ethane at 161.40 and 182.34 K and of xenon + propane at 161.40, 182.34, and 195.49 K have been measured. Both systems show negative deviations from Raoult's law at all temperatures. The corresponding excess molar Gibbs energies ( ) have been calculated from the vapor pressure results. Liquid molar volumes have also been measured for both mixtures at 161.40 K, leading to calculated excess molar volumes ( ) which are negative in all cases. Additionally, the excess molar enthalpies ( ) for the xenon + ethane system have been determined directly using a batch calorimeter and found to be negative. Xenon + ethane is thus the simplest system which exhibits negative values for all three major excess molar functions. The results were interpreted using the statistical associating fluid theory for potentials of variable attractive range (SAFT-VR). The theory is able to predict the phase behavior of both systems in close agreement with the experimental results. It was found that the xenon + n-alkane mixtures obey Lorentz−Berthelot combining rules, so that no unlike interaction parameters are fitted to experimental mixture data. The theory is therefore totally predictive. It was also found that the parameters calculated for xenon using this model lie within the average values of the parameters obtained for the n-alkanes. This implies that, in contrast with the anomalous behavior of methane, xenon can be treated as the first member of the n-alkane family. Furthermore, the xenon + n-alkane mixtures can be thought as a particular case of mixtures of n-alkanes.

A new vibrating-wire instrument for the measurement of the density of fluids at high pressures was described in a previous paper. The technique makes use of the buoyancy force on a solid sinker and detects this force with a vibrating wire placed inside the measuring cell. Owing to the simple geometry of the oscillating element there exists a complete theoretical description of its resonance characteristics, enabling the calculation of the density of the fluid from their measurement. In the present paper a new method for the determination of the cell constants is outlined which permits the operation of the densimeter essentially as an absolute instrument. Furthermore, it is shown that the viscosity of the fluid can be measured simultaneously with the density. New results for three fluids are presented: for cyclohexane at temperatures from 298 to 348 K and pressures up to 40 MPa, for 2,2,4-trimethylpentane between 197 and 348 K at 0.1 MPa, and for l,l,l,2-tetrafluoroethane from 197 to 298 K close to saturation. The sets of measurements were chosen with the intention of testing the performance of the apparatus, complementing previous work at higher pressures. The densities and viscosities measured exhibit the same accuracy for all of the three fluids over the entire temperature and pressure ranges and were obtained using the same set of cell parameters The precision of the densities is _+0.03% and their estimated accuracy is +0.05% . The viscosities have a precision of +0.6% and an estimated accuracy of _ 2 %.

New density and viscosity results for 2,2,4-trimethylpentane at temperatures from 198 K to 348 K and pressures up to 100 MPa are reported. The measurements of both properties were performed simultaneously using a vibrating-wire instrument operated in the forced mode of oscillation. The present set of measurements extends those found in the literature to the high-pressure and low-temperature region. Correlations for both the density and the viscosity are presented, and comparisons are established with various sets of results from the literature. The estimated uncertainties of the results are ±0.05% for the density and ±2.5% for the viscosity.

The total vapor pressure of liquid mixtures of (xenon + n-butane) has been measured at 182.34 and 195.49 K, and of (xenon + isobutane) at 195.49 K. The liquid molar volumes have also been measured at 182.34 K for both systems. The mixtures follow the behavior already found for other (xenon + alkane) mixtures, i.e., negative deviations from Raoult's law, negative excess molar Gibbs energies ( ) and negative excess molar volumes ( ). The excess molar enthalpy ( ) is approximately zero in the case of (xenon + n-butane). The results were interpreted using the statistical association fluid theory for potentials of variable attractive range (SAFT-VR). This study provides further evidence that the (xenon + n-alkane) mixtures can be thought of as a particular case of mixtures of n-alkanes.