Controlling viscosity - the measure of a fluid's consistency - is crucial to maintaining consistent quality and controlling costs in the application of paints and coatings in production environments. Viscosity can determine the blending characteristics of multi-formulation fluids and can provide an indirect measure of product consistency relative to properties such as molecular weight and molecular weight distribution. A change in viscosity can indicate solvent loss or other quality problems.
Many facilities rely on off-line viscosity measurements. However, off-line methods can't provide accurate real-time data because viscosity is directly affected by temperature, shear rate and other variables that can be different off-line from what they are in production. Today's finishing managers need the ability to detect changes as they occur in real-time, making measurements from a baseline rather than simply measuring absolute values.
Having real-time, on-line viscosity data could eliminate the need to make decisions based on intermittent "snapshot" data acquired from periodic sampling. Acquiring this real-time data would require a sensor that can be embedded directly into the process stream and interfaced to a computer platform or control system through a standard USB port. Ideally, the sensor would be a solid-state device. It would be small enough so it could be embedded in process piping and, when combined with a hand-held reader, could be used as a portable, self-contained instrument by quality control and laboratory personnel to spot check ongoing operations in dispensing, mixing, pumping and coating systems.
Figure 1. A cutaway view of a quartz crystal sensor with the propagated waves.
Image courtesy of BiODE, Inc.
Current Viscometer Technology
Attempts to meet the need for in-line viscosity measurement by developing miniature versions of the familiar rotational viscometers used throughout the paint and coatings industry were largely unsuccessful due to issues with clogging, fouling and vibration. Development efforts focused on micro-electromechanical systems (MEMS), either as micro-machined silicon or as hybrid systems with silicon and single crystal piezoelectric sensors.At the same time, there were a number of efforts to develop solid-state viscosity sensors. For example, Sandia National Labs demonstrated prototypes of two acoustic wave (AW) sensors. One of these approaches used a popular thickness shear mode (TSM) quartz crystal device and tracked resonance damping and frequency shift in an advanced electronic circuit to measure changes in viscosity. The second approach used a crystal quartz plate as a waveguide and combined the sensing mechanism of a TSM device with the wafer scale manufacturing process used to build surface acoustic wave (SAW) sensors.
In addition, there was a concerted commercial effort to overcome challenges in design, reproducibility and measurement range. The monolithic piezoelectric sensor (MPS) offers the simplicity of a TSM device while having distinct input and output ports for differential measurements, thereby aiding reproducibility and overcoming circuitry effects. The multi-reflective acoustic wave device (MRAWD) blends the features of resonators and delay lines to offer a wide dynamic range (air to several thousand centipoise [cP]) in a single sensor, overcoming the major pitfalls of earlier prototype designs.
The Acoustic Wave Viscometer
The recent commercial introduction of an acoustic wave viscometer provides a viable solid-state solution for simultaneously measuring viscosity and temperature on-line in real-time. The acoustic wave sensor combines solid-state surface chemistry with ultra-sensitive acoustics. And, since it does not contain any moving parts, the sensor is immune to shock and capable of withstanding vibrations of 30Gs or more. With an operating shear rate several orders of magnitude higher than fluid flow characteristics, the acoustic wave sensor is unaffected by static, laminar or turbulent flow, so it can be used in high-flow-rate, on-line applications to measure viscosity instantaneously from 0 to well beyond 10,000 cP with ±3% repeatability at temperatures from -20 to +135°C.The most familiar viscosity measurements are kinematic viscosity (centistokes) and dynamic or absolute viscosity (centipoise). These two measurements are related, as centistokes equals centipoise/specific gravity. Acoustic sensors measure viscosity in units of centipoise • specific gravity. This measurement is based on the transfer of acoustic shear wave energy from a quartz crystal or other solid waveguide with characteristic impedance. The square of the power loss of the acoustic wave passing through the process fluid is proportional to the product of frequency, density and viscosity. Since the frequency is known, the sensor measures viscosity • density.
Knowledge of specific gravity allows conversion from one measurement to another when shear rate and temperature are equal. Thus, the digital output of the acoustic wave sensor can be automatically displayed in cup seconds or centipoise units if the user knows the specific gravity (density) of the fluid.
Figure 2. Monitoring changes in viscosity as a function of process conditions. Image courtesy
of BiODE, Inc.
Solid-Sate Technology
The solid-state acoustic wave sensor measures viscosity when the quartz crystal wave resonator is immersed or placed in contact with a liquid. The resonator supports a standing wave. The wave pattern interacts with electrodes on the lower surface - hermetically sealed from the liquid - and interacts with the fluid on the upper surface. The bulk of the liquid is unaffected by the acoustic signal, and a thin layer - on the order of microns or micro-inches - is moved by the vibrating surface (see Figure 1). The liquid's viscosity determines the thickness of the fluid hydrodynamically coupled to the surface of the sensor.The sensor surface is in uniform motion at frequency,
