Real-time monitoring of viscosity

Dr. Neil Canter, Contributing Editor | TLT Tech Beat June 2012

A sensor device obtains data from a liquid that can be used to calculate viscosity.

KEY CONCEPTS
• A new technique can constantly monitor a liquid’s viscosity.
• By using a technique known as an inverse method, various flow parameters are measured in a sensor device that contains a T-junction. The data produced can be used to calculate viscosity through the use of a mathematical relationship.
• Viscosities ranging from 0.0001 pascal seconds up to 10 pascal seconds can be measured using the inverse method technique.

VISCOSITY REMAINS THE MOST IMPORTANT PARAMETER THAT IS USED TO CATEGORIZE A LUBRICANT. When an end-user is looking for a specific lubricant to be used in a particular application, undoubtedly the first property examined is viscosity.

Kinematic viscosity and dynamic viscosity have been used to determine the viscosity of a fluid. In the former case, a sample is introduced into a viscosity tube that is equilibrated to a specific temperature (typically 40 C or 100 C) in a bath. The kinematic viscosity is obtained when the fluid moves from a reservoir through a small capillary under the force of gravity.

Dynamic viscosity is measured by rotating a specific spindle in a jar containing the samples. The viscosity is determined by the fluid’s resistance to the spindle movement. These two figures are related to each other by the fluid’s specific gravity. Kinematic viscosity can be determined by the ratio of dynamic viscosity to specific gravity.

In a previous TLT article, a technique known as acoustic viscometry is used to continuously monitor the viscosity of a fluid (1). A shear-acoustic standing wave is generated through a sample placed in a solid-state quartz crystal cell. The viscosity is measured by determining the difference in power that originates from the wave, prior to and after contact with the fluid.

Knowing the viscosity of a lubricant operating in a specific machine at a specific time can be critical because it might predict premature failure. Dr. Julia Rees of the School of Mathematics and Statistics at the University of Sheffield in Sheffield, U.K., says, “Viscosity is an important property for many applications where it is important to determine how a liquid in a specific system behaves under different circumstances. Such behavior may affect how the liquid can fulfill its function.”

A new technique that can constantly monitor the viscosity of a liquid may have wide-reaching applications in lubricants because of the desire to extend the optimal performance for as a long a period as possible. Such a technique has now been developed.

INVERSE METHOD
Rees, in collaboration with H.C. Hemaka Bandulasena and William Zimmerman, has developed a new technique for monitoring the viscosity of a fluid in real-time. She says, “We designed a sensor device that a liquid can flow through and obtain data from the liquid, which we can use to calculate the viscosity.” The sensor does not have any effect on the properties of the fluid.

An image of the sensor device is shown in Figure 3. The researchers determined that it was best to measure parameters that did not directly generate a viscosity value but, rather, can be used in a mathematical relationship to calculate viscosity. This approach is known as an inverse method for obtaining a value for a specific property. Rees explains, “Often in engineering problems, values of quantities that we want to know cannot easily be measured directly. Instead we measure something else, then relate this measurement (or set of measurements) to the quantity (or quantities) that we need to know the value(s) of.”


Figure 3. This sensor device can be used to measure the real-time viscosity of a fluid through an inverse method. (Courtesy of the University of Sheffield)

The researchers designed the sensor to contain a T-junction so that various flow parameters such as pressure or flow rate can be measured. The geometry of the T-junction is important because it enables the researchers to effectively remove shear rate as one of the variables that needs to be measured. Rees says, “A range of shear rates coexist as the fluid turns the corner in a T-junction. The fluid nearest to the corner will flow more slowly than the fluid near the center of the channel.”

The viscous material xanthan gum was selected as the fluid to be used in evaluating the sensor. Rees says, “We selected xanthan gum because it is representative of a common class of viscous fluids and it is easy to handle in our experiments (non-toxic and stable at room temperature).”

Xanthan gum was exposed to a range of shear rates, and the researchers collected optical measure of the velocity distribution. The data was then plugged into an equation that relates viscosity to the flow consistency index (K) and the flow behavior index (n).

One technique used by the researchers to measure the velocity is to add microscopic fluorescent particles to the fluid. Rees says, “In micron-resolution particle image velocimetry, the fluorescent particles are illuminated using a laser and photographic images are taken at very close time intervals so that the motion of these seeded particles can be tracked. We then use image- processing software to calculate the velocity.”

The researchers are able to measure viscosities ranging from 0.0001 pascal seconds (water) up to 10 pascal seconds. Rees indicates that the accuracy of the technique is a function of the type of sensor used. She adds, “Currently, we are using cheap sensors as we are still proving the concept. Some of our intermediate parameters are estimated to within 0.05%, while viscous parameters are estimated to within 3%.”

Future work involves development of a range of portable devices to measure viscosity in different fashions. Rees says, “We are planning to develop portable devices over a range of scales from microscales to milliscales. We aim to develop rheometers that can be used in line for continuous processing. We are also planning to develop a version that will just analyze a small drop of fluid, which would be useful if only small quantities (e.g., just a few microliters) of a fluid are available.”

Additional information on the development of this inverse method is available in a recent article (2) or by contacting Rees at j.rees@sheffield.ac.uk.

REFERENCES
1. Canter, N. (2006), “Continuous Online Monitoring of Viscosity,” TLT, 62 (7), pp. 12-14.
2. Bandulasena, H., Zimmerman, W. and Rees, J. (2011), “An Inverse Method for Rheometry of Power-Law Fluids,” Measurement and Science Technology, 22 (12), doi:10.1088/09570233/22/12/125402.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.