Viscosity is a general property of fluids that measures how “thick” the fluid is. More precisely, viscosity measures a fluid’s response to physical stresses. The more a fluid resists being deformed by the stress, the greater its viscosity. The most basic mathematical definition for viscosity originated from Issac Newton, who postulated that the relationship between the stress and the strain in a fluid was linear and proportional. Viscosity is defined as the constant of proportionality, which can be determined experimentally. Fluids that obey this linear relationship are known as Newtonian fluids, and their viscosity can be measured with a viscometer. Non-Newtonian fluids exhibit more complicated behavior and are more suitably studied with rheometers.
For Newtonian fluids, there are several different techniques that can be used to measure viscosity. Some of these are based on simple manual techniques. These have been automated to some extent for greater convenience. The falling ball viscometer, for instance, makes use of the Navier-Stokes equation for a ball falling through a viscous medium. The ball reaches a terminal velocity, which can be measured by the time it takes the ball to fall a certain distance. Using the terminal velocity and other information about the size and density of the sphere, one can calculate the viscosity of the fluid. Manual measurement of the terminal velocity can be made using a simple stopwatch, but modern automated systems use electronic measurement for greater accuracy and precision.
Another manual method for measuring viscosity involves U-shaped glass tubes, which measure time as the liquid in the tube flows from one level to another. Adding some automation not only allows the measurements to be made more accurately, but makes it easier to introduce a controlled temperature bath, which increases measurement precision.
Perhaps the most common viscometer is the rotational viscometer, in which a rotating cup, plate or cone is in contact with the sample fluid. The relationship between the applied torque and the response of the fluid can be transformed into a measurement of viscosity.
Industrial applications for viscometers exist for many fluid products, and are typically oriented toward quality control. Among the products that are commonly tested are lubrication oils, beverages and other foods, inks, paints, toiletries, and other household products. Another important application is in the monitoring of used oils from industrial engines.
The market leader in viscosity is Brookfield Engineering, which expanded its presence in this area with the acquisition of RheoTec Messtechnik in November 2008. In addition to the leading suppliers, other competitors include Thermo Fisher Scientific, Schott Instruments (Nova Analytics) and Malvern Instruments (Spectris), which acquired Viscotek in January 2008 (see IBO 1/31/08). The total market for viscometers was about $80 million in 2008.
Viscometry at a Glance:
• Brookfield Engineering
• PAC (Roper)
• Cannon Instruments
• Paint and Ink
• Food and Beverage