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Viscosity is a broad term that can be used to describe various aspects of modern process control. For example, viscosity can describe a material’s behavior, as it relates to the viscous nature of liquids. Viscosity can also define a set of various material properties as they correspond to dissipative losses and attenuation following elastic wave propagation within fluids.
Considered to be one of the most complex physical fluid parameters, viscosity plays a central role in rheology and various other material applications. Furthermore, biological researchers also utilize viscosity measurements to determine diffusion rates, energy transduction and the active transport of biologically active molecules through cellular membranes.
Two fundamental types of viscosity measurements must be considered before determining which sensor is more appropriate for a given application. Newtonian fluids state that both shear and longitudinal viscosity coefficients1 determine the relationship between a stress (tensor) and its displacement (vector) within a viscous fluid. Longitudinal viscosity is responsible for the viscous damping of pressure waves in liquids and gasses.
Shear Viscosity Sensors
Simply put, shear viscosity refers to the resistance to flow that exists between two flat plates that are parallel to each other2. When a system is immersed in a sample liquid, the adjacent liquid is drawn in by the flow of the fluid as a result of shear movement that is occurring on the surface of the system. A change in the resonance frequency then arises, which can then be used to determine the viscosity-density product of the liquid.
Shear viscosity sensors that are capable of maintaining dominant shear vibrating liquid, surfaces and interfaces are ideal. Some commonly used shear viscosity sensors include thickness-shear-mode (TSM) resonators and micromachined cantilevers1.
Advancing Shear Viscosity Sensor Technology
A great deal of research has been devoted to developing shear viscosity sensors that are equipped with online measurement capabilities. This research has been primarily driven by the limitations of traditionally used standard laboratory equipment, which is often bulky, unsuitable for online measurements and usually requires an extensive amount of maintenance. Advancements in shear viscosity sensor development have led researchers to develop viscosity sensors that are capable of simultaneously measuring other fluid properties, such as mass density or speed of sound3.
Longitudinal Viscosity Sensors
The primary mechanism behind longitudinal viscosity monitoring tools is based on the ability of these sensors to detect the viscous attenuation of pressure waves. As compared to shear viscosity sensors, longitudinal viscosity sensors are capable of achieving a more considerable penetration depth of pressure waves in fluids. As a result, these sensors can directly probe bulk materials, rather than just coming into contact with a thin surface layer1. Longitudinal viscosity sensors typically utilize ultrasonic and resonant pressure waves to measure the viscosity of sample fluids.
Since 2011, when the first longitudinal viscosity sensor prototype was developed, various sensor variations have been introduced. For example, a 2013 study discussed the development and success of an acoustic transmission longitudinal viscosity sensor. In their design, a piezoelectric lead zirconate titanate (PZT) transmitter and receiver were used to excite and detect pressure waves3, respectively, within a small test chamber that contained the sample liquid. These researchers determined that their sensor was a useful tool for providing measurements that could be used for condition monitoring applications that require any changes in the observed fluid to be monitored and recorded.
References
- “Viscosity Sensors” – Danube University Krems
- “Viscosity” – The Free Dictionary by Farlex
- Antlinger, H., Clara, S., Beigelbeck, R., Cerimoic, S., Keplinger, F., & Jakoby, B. (2013). An acoustic transmission sensor for the longitudinal viscosity of fluids. Sensors and Actuators A: Physical 202; 23-29. DOI: 10.1016/j.sna.2013.03.011.
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