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Product Intelligence Viscometers: The Science of Measuring Fluid Flow by Katriona Scoffin


iscometers measure the viscosity and flow properties of fluids. Viscosity arises from the internal friction of a fluid and is defined as a liquid’s resistance to flow or shear stress.


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Applications of viscometers Viscometers have wide and varied applica-


tions in the materials science and chemical industries for materials such as oil, asphalt, plastics, wax, paints, coatings, and adhesives. They are also used for food and beverages and personal-care products such as cosmetics, shampoo, and toothpaste.


Measuring viscosity is important when con- sidering process conditions for materials that need to be pumped or piped. Viscosity also af- fects dipping and coating performance, which is particularly relevant to paints and inks.


Viscosity is a very useful indirect measure of material properties including molecular weight and density, both of which affect flow behavior. Viscometers can therefore be used to monitor batch consistency and quality control.


Example applications include:


• Measuring the flow of engine oil under different temperature conditions


• Analyzing the flow properties of milk to design suitable piping systems


• Checking the viscosity of jams and syrups to ensure batch consistency.


Since changes in viscosity reflect changes to molecular weight, viscometers are used to characterize plastics. They are particularly use- ful in polymer synthesis since the flow rate can be used to determine the change of polymer length through different processes.


In many industrially useful cases, viscos- ity decreases as enzymatic activity increases.


Viscometers are used to determine the activity of enzymes such as cellulase, protease, amy- lase, and pectinase.


Types of viscometers There are many ways to measure viscosity,


but they break down into two basic methods. Either an object, such as a sphere or a rotor blade, moves through a stationary material, or the material flows through or past a stationary object. In either case, the resistance to flow is measured.


Capillary viscometers Also known as u-tube or glass viscometers,


capillary viscometers are often used in labora- tory settings. Viscosity is measured by timing how long it takes for a transparent or trans- lucent fluid to flow between two points of a capillary tube.


For opaque liquids, it is hard to determine when the sample has passed a point in the tube, so it is necessary to use reverse-flow viscometers that only wet the timing section of the viscometer capillary during the actual measurement. The Cannon-Fenske Opaque, Zeitfuchs Cross-Arm, and BS/IP/RF Viscometers from CANNON Instrument Co. (State College, PA; www.cannoninstrument.com) are all reverse-flow types. Reverse-flow viscometers must be cleaned, dried, and refilled before a repeat measurement can be made.


Capillary viscometers can be manual or au- tomatic. Automatic instruments use infrared optical sensors for transparent samples, or thermal sensors for opaque samples.


Rotational viscometers Simple rotational viscometers, also known


as Brookfield type viscometers, use a torsion spring to measure the torque required to rotate


AMERICAN LABORATORY • 29 • SEPTEMBER 2013


Figure 1 – The SVM 3000 Stabinger Viscometer from Anton Paar measures the dynamic viscosity and density of oils and fuels.


a spindle in the material. Changing rotor speed and size allows for the measurement of differ- ent ranges of viscosity.


The SVM 3000 (Figure 1) from Anton Paar (Ashland, VA; www.anton-paar.com) uses the Stabinger measuring principle. A lightweight magnetic rotor floats in a liquid-filled tube, which rotates at constant speed. The sample fluid’s viscous forces drive the rotor, leading to an equilibrium rotor speed, which is recorded as a measure of the fluid’s viscosity.


Different spindle geometries such as cylin- drical, coaxial cylinder, parallel plate, and cone-plate are used in rheometers to create different shear forces. Rheometers are more complex instruments than viscometers and are used to measure flow rate-dependent viscosity in non-Newtonian fluids.


Rolling ball, falling ball, sphere,


and piston viscometers The Lovis 2000 M/ME from Anton Paar is a micro- viscometer based on the rolling ball principle. A ball rolls through a closed, sample-filled capillary that is inclined at a defined angle. Inductive sen- sors determine the ball’s rolling time between two marks.


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