Viscosity management is a missing capability of many process control systems because of the absence of reliable digital instrumentation that can be incorporated into the process stream. Acoustic wave viscometers can fill this need for OEMs and process manufacturers as they can detect changes in the process stream to provide real-time quality control, eliminating need for decisions based on intermittent 'snapshot data' acquired from periodic sampling.

Viscometers developed for laboratory use were never intended for real time measurements and integration into the control loop. Such traditional mechanical and electro-mechanical instruments were generally ill-suited for high-flow-rate in-line applications in the manufacture of coated papers, paints, inks, injection molded plastics, pastes, creams, gels, and other non-flowing products.

Building on acoustic wave research funded by the U.S. Department of Defense, Environmental Protection Agency, and National Institutes of Health, Biode Inc. recently developed the first commercially available, solid-state viscometer for integration into in-line, real-time monitoring and process control systems.

As a solid-state device, the Biode acoustic wave viscometer provides continuous real-time measurements. It has no moving parts and, with an operating shear rate several orders of magnitude higher than fluid flow characteristics, it is unaffected by static, laminar, or turbulent flow streams.

Capable of operating at temperatures of -20 to 135 ºC, the sensor can measure viscosity from 0 to 10,000 cP with ±3% repeatability. Measuring approximately 1.3 x 1.1 x 0.3 in. and weighing 4 oz, the hermetically sealed sensor can be completely immersed and can be incorporated into a small chamber or process piping to serve as an 'early warning system' to signal changes in product consistency.

The Biode sensor measures viscosity by placing a quartz crystal wave resonator in contact with liquid. This can be a high flow-rate stream or a sample as small as 100 µl. Particulates in the flow stream do not affect viscosity measurement. The quartz sensor's surface is in uniform motion; the electronics determine frequency and amplitude. As the shear wave penetrates the adjacent fluid, power is transferred from the quartz to the viscous liquid. Viscosity is calculated by measuring the power loss from the resonator into the fluid.

Solvent loss and/or polymerization increases with time, demonstrating how an acoustic wave viscometer can provide reliable and accurate in-line, real time measurements in a production environment.

On board-electronics then transmit viscosity and temperature readings in real-time using Profibus, CanBus, or other communications protocols to provide a continuous audit trail to help control operating costs and maintain quality standards.

A proprietary foaming resin application demonstrates the capabilities (graphic) of the device. The sensor was installed in-line to measure viscosity in real time as the material was subjected to a hot/cool pattern over a temperature range of 25 to 200 ºC. Viscosity is seen to reproducibly vary with temperature, tracking the fine detail of the temperature curve. The test shows how the acoustic viscometer can track changes in the characteristics of the resin as a function of temperature.