João M. N. A. Fareleira

Electromechanical model for vibrating-wire instruments

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.

Simultaneous Measurement of the Density and Viscosity of Compressed Liquid Toluene

A vibrating-wire densimeter described previously has been used to perform simultaneous measurements of the density and viscosity of toluene at temperatures from 222 to 348 K and pressures up to 80 MPa. The density measurements are essentially based on the hydrostatic weighing principle, using a vibrating-wire device operated in forced mode of oscillation, as a sensor of the apparent weight of a cylindrical sinker immersed in the test fluid. The resonance characteristics for the transverse oscillations of the wire, which is also immersed in the fluid, are described by a rigorous theoretical model, which includes both the buoyancy and the hydrodynamic effects, owing to the presence of the fluid, on the wire motion. It is thus possible, from the working equations, to determine simultaneously, both the density and the viscosity of the fluid from the analysis of the resonance curve of the wire oscillation, the density being related essentially to the position of the maximum and the viscosity to its width. New results of measurements of the density and viscosity of toluene in the compressed liquid region are presented, and compared with literature data. The density results extend over a temperature range 222 K≤T≤348 K, and pressures up to 80 MPa. The viscosity results cover a temperature range of 248 K≤T≤348 K and pressures up to 80 MPa. The uncertainty of the present density data is estimated to be within ±0.1% at temperatures 298 K≤T≤350 K, and ±0.15% at 222 K≤T≤273 K. The corresponding overall uncertainty of the viscosity measurements is estimated to be ±2% for temperatures 298 K≤T≤350 K, and ±3% for 248 K≤T≤273 K.

Impedance spectroscopy of a vibrating wire for viscosity measurements

Many routine viscosity measurement systems suffer from low accuracy and some are also operator dependant which creates repeatability problems. This paper describes the ongoing developments of a viscosity measurement system based on a vibrating wire cell. The proposed improvements include the simultaneous fitting of the magnitude and phase of the measured impedance and the test of different equivalent impedance circuits. Since the fitting function has a high number of search variables and contains many local minima, a hybrid method involving genetic algorithms as well as a traditional search algorithm is used. The system is tested with liquid diisodecyl phthalate (DIDP) and the viscosity results are compared to reference values available in the literature. A new equivalent impedance circuit is also suggested as one of the possible circuits to model the cell behavior in the future, when used with ionic liquids.