An in-line viscometer is valuable to the control of a flow process, particularly a non-Newtonian flow because its viscosity is shear-rate dependent and thus requires continuous monitoring. Currently available in-line viscometers are mostly of the probe design [1] and may obstruct the flow pattern, causing plugging problem in flows with suspended particles. A probe-type viscometer generally measures a response of fluid density-viscosity product. To separate mass loading and viscous effects, Kim and Bau [2] used a dual-probe design based on torsional wave speed and attenuation measurements. However, no commercial system has been developed from their concept. Another limitation of existing probe-type viscometers is the range of application, primarily in the low-viscosity range.