A Model-Based Method for In Situ Viscosity Measurement With Continuous-Wave Ultrasound

Abstract Robust methods for viscosity measurement in situ are needed in many industrial applications. In this work, we developed a method to capitalize on the strong signal-to-noise ratio imparted by continuous-wave (CW) ultrasound echoed through a medium between two piezoelectric transducers. An optimization algorithm was developed to solve the inverse problem of extracting medium viscosity by optimizing the parameters of a model for the compound wave sensed by one of the transducers. Numerical constraints and parameter initialization techniques were designed to enhance algorithm convergence. The resulting measurement method was validated with a variety of materials, including lubricants at various temperatures, and compared against standard viscometers. The measurement precision was <1% and correlation coefficient (r) with standard reference or instruments was >0.95. Room temperature measurements were accurate within 5% across a broad range (101–105 cP) of viscosities. Furthermore, it showed advantage over another acoustic method in accuracy for high viscosity measurement. The methodology allows continuous measurement of viscosity in situ with simple rugged hardware, which is needed for process monitoring and control in numerous applications.