Vibrating-wire viscometers for liquids at high pressures

Abstract

The design and operation of two independent vibrating-wire viscometers are described. The instruments are intended for operation in the liquid phase at pressures up to 300 MPa and have been designed specifically for this purpose using the detailed theory of the device. Extensive evidence is adduced to demonstrate that the operation of the viscometers is consistent with the theory. Although the instruments attain a precision in viscosity measurements of • when used in an absolute mode the accuracy that can be achieved is no better than __+3%. However, if the instrument is calibrated for two welldefined instrumental parameters, the uncertainty in the reported viscosity is improved to _+0.5%. The results of measurements of the viscosity of normal heptane in the temperature range 303 to 348 K at pressures up to 250 MPa made with one of the viscometers are reported. The results are shown to be totally consistent with measurements reported earlier using the instrument designed for lower pressures.

Viscosity Measurements of Liquid Toluene at Low Temperatures Using a Dual Vibrating-Wire Technique

“…At its lower end an assembly composed of two weights, through a pulley system, tensions the wire vertically. The main aim of this assembly is to inhibit any motion of the wire lower end, as discussed previously [11].…”

“…The theoretical model for the forced mode of operation has been published by Pádua et al [9], and, for the free decay mode of operation, the model equations are essentially analogous to the ones previously described by Oliveira [10] and Assael et al [11]. Hence, only a brief description of the working equations for both modes is given here.…”

The viscosity of R32 and R125 at saturation

“…Since neither can be measured directly with sufficient accuracy for the sample of material available, it is preferable to perform relative measurements. A calibration of each instrument must therefore be carried out with respect to standard reference viscosities [4].…”

“…The entire assembly is supported from the cap of a pressure vessel, and in particular, the two samarium-cobalt magnets (2) are held symmetrically with respect to the wire axis in a cage. The magnets serve the same purpose as before [3,4]: to provide a means of initiating the wire motion through the interaction of the field with a pulse of current through the wire and to induce a measurable emf between the ends of the wire during its free motion.…”

“…Furthermore, compared with the liquid phase viscometer, the changes to the wire radius and vibration frequency reduce the amplitude of the electrical signal to be observed. For that reason, the length of the samarium-cobalt magnets has been increased to retain a similar signal-to-noise ratio to that obtained in the liquid phase system [3,4].…”