VIBRATING-WIRE VISCOMETERS FOR LIQUIDS AT HIGH-PRESSURES

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 +/- 0.1 %, 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 well-defined 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.