Viscosity is a measure of the resistance of a liquid to flow, and is an important measurement requirement in industrial process control and OEM applications. Viscosity describes the retarding force that is proportional to the rate of deformation. This so-called shear rate has units of s-1 and describes the crossstream gradient of the flow speed.
The definition of viscosity based on rates of motion leads to one of the most undesirable aspects of traditional viscosity measurement methods — either the sensor mechanism or the liquid must be in motion in order to accomplish the measurement. This is seen in flow-based systems such as capillary tubes, coriolis force tubes, efflux cups, falling balls, moving pistons, and rotating spindles.
While MEMS (microelectromechanical systems) technology and specifically "lab on a chip" capabilities may miniaturize this class of measurement device, fundamental physics and engineering factors complicate the use of these devices for embedded process control monitoring, where flow rates, particulates, and pressures play crucial roles.
There has been a long-standing effort to introduce solid-state technology for viscosity measurement. Sandia National Laboratories demonstrated prototypes of two acoustic wave (AW)-based sensors. One used a popular thickness shear mode (TSM) quartz crystal device and tracked resonance damping and frequency shift in an advanced electronic circuit. The other used a less well-known waveguide mode of a quartz plate and combined the TSM device's sensing mechanism with the wafer-scale manufacturing capability of surface acoustic wave (SAW) sensors.
Commercial manufacturers refined both sensor structures, overcoming challenges in design, reproducibility, and measurement range. The monolithic piezoelectric sensor (MPS) offers the simplicity of the TSM, while having distinct input and output ports for differential measurements, aiding reproducibility and overcoming circuitry effects. The multi-reflective acoustic wave device (MRAWD) blended the features of resonators and delay lines to offer a wide dynamic range (air to several thousand cP) in a single sensor, overcoming the major pitfalls of the earlier prototype designs (see Figure 1). These technologies led to the development of robust, reliable, and cost-effective commercial acoustic wave solid-state viscometers for integration into in-line, real-time monitoring and process control systems (see Figure 2).
The sensor has a number of shared advantages over the aforementioned devices. It has no moving parts other than the atomic scale vibration of the surface and, due to the high frequency of the vibration (several millions of vibrations per second), is independent of flow conditions of the liquid and immune to vibration effects of the environment.
The importance of these acoustic sensors lies in a distinctly different measurement principle. Whereas one class of mechanical devices measures kinematic (flow) viscosity and the other class measures intrinsic (friction) viscosity, acoustic wave (AW) sensors measure acoustic impedance (ωρη)1 ⁄2, where ω is the radian frequency (2πF), ρ is the density, and η is the intrinsic viscosity.
The viscosity measurement is made by placing the quartz crystal wave resonator in contact with liquid. 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 ω =2πF, with amplitude U. The frequency is known by design, and amplitude is determined by the power level of the electrical signal applied to the sensor. As the shear wave penetrates into the adjacent fluid to a depth d, it is determined by the frequency, viscosity, and density of the liquid as d=(2η/ωρ)1 ⁄2. Acoustic viscosity is calculated using power loss from the quartz resonator into the fluid. The unit of measure is acoustic viscosity (AV) and is equal to ρη, (g/cm3 • cP).
The acoustic wave resonator supports a standing wave through its thickness. 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.
Shear-rate-dependent effects are significant in acoustic wave sensors. These sensors exhibit shear rates from a few thousand to several million inverse seconds. By comparison, rotary viscometers cover a range of <1 s-1 to ~10,000 s-1. In contrast, lubricating oils, for example, experience shear rates of hundreds of thousands to tens of millions in today's engines.
In conclusion, the development and commercial availability of solid-state viscometers provides OEMs with ready-to-run, inline, solid-state devices that can be designed into process machinery with the promise of increased yields for their customers' applications. They can also provide a continuous audit trail to control operating costs and extend the life of machine transmissions, motors, and other lubricant-protected assets.