PNNL

Abstract

An on-line sensor to measure the density of a liquid or slurry, based on longitudinal wave reflection at the solid-fluid interface, has been developed by staff at Pacific Northwest National Laboratory. The objective of this research is to employ shear-wave reflection at the solid-fluid interface to provide an on-line measurement of viscosity as well. Both measurements are of great interest for process control in many industries. Shear wave reflection measurements were carried out for a variety of liquids. By analyzing multiple reflections within the solid (only 0.63 cm thick - similar to pipe-wall thickness) we increased the sensitivity of the measurement. At the 6th echo, sensitivity was increased sufficiently and this echo was used for fluid interrogation. Shear wave propagation of ultrasound in liquids is dependent upon the viscosity and the shear modulus. The data are analyzed using the theory for light liquids (such as water and sugar water solutions) and also using the theory for highly viscous liquids (such as silica oils). The results show that for light liquids, the shear wave reflection measurements interrogate the viscosity. However, for highly viscous liquids, it is the shear wave modulus that dominates the shear wave reflection. Since the density is known, the shear wave velocity in the liquid can be determined from the shear wave modulus. The results show that shear wave velocities in silica oils are very small and range from 315 cm/s to 2389 cm/s. Shear wave reflection measurements are perhaps the only way that shear wave velocity in liquids can be determined, because the shear waves in liquids are highly attenuated.