Viscometers measure viscosity, the resistance of fluids to flow or stress. In common terms, viscosity is related to a fluid’s “thickness”—a physical property of great interest to manufacturers of liquids, slurries, and pastes. Viscosity is a critical characteristic of foods, paints, cleaners, adhesives, polymers, fuel oils, and pharmaceuticals. Many industries use viscosity as an endpoint in the manufacture of liquid-formulated products.
Dozens of viscometer types are in use in academia, industry, and basic research, and cost from about $100 for simple mechanical viscometers to $15,000 automated instruments. Rheometers, which measure viscosity and related properties, may cost as much as $200,000.
At the low end of the price range are U-tube and Cannon-Fenske tube viscometers found in schools, colleges, and some industries. Other simple designs include falling sphere, falling piston, rolling ball, and oscillating piston viscometers. These are not instruments in the truest sense; some require pourable fluids.
More sophisticated are vibrational viscometers used in process industries, and rotational viscometers. The latter operate on the principle that energy must be applied to a spindle or disk rotating within a liquid to overcome the resistance of that liquid. That energy is proportional to the fluid’s viscosity. Rotational viscometers do not require that the testing sample be pourable.
Higher-end viscometers may be connected to and operated through a computer, or readouts may be taken directly off the front panel display. Some industries favor one viscometer type. Paint and pigment industries prefer the Stormer viscometer, which uses paddles on a rotor submerged into the product. Similarly, resin labs use bubble viscometers, which measure the time it takes a bubble to emerge from varnishes, and petrochemical companies prefer the Stabinger viscometer, which employs a rotating cylindrical tube.
Viscometry is an old technique, but while manufacturers continue to fine-tune more-sophisticated electromechanical rotational viscometers, the underlying technology hasn’t changed much.
What is different, says Steven Colo, president at ATS RheoSystems (Bordentown, NJ), is user expertise. “Twenty years ago most users had an academic specialization in viscometry or rheology.” Today, specialists have been replaced by generalists.
In biotech and pharmaceuticals, the viscosity of protein and buffer solutions constitutes one of numerous quality checks; in beverages, viscosity is an indicator of sugar content and overall quality. Polymer scientists use viscosity to determine the concentration of plastics in acid solutions.
Viscosity measurements are usually conducted on dilute solutions and at varying concentrations. Measurement of the viscosity of polymer solutions at different strengths, for example, provides estimates of secondary properties such as intrinsic viscosity, molecular weight, and chain length.
Viscosity is related to solute concentration, making it one of the most useful physical measurements in research and product development. The viscosity of a liquid tends to rise with the concentration of solid solutes, as with sugar in water. But the viscosity of a blend of fully miscible and non-reacting liquids is usually somewhere between the viscosities of the components.
Useful viscometer/rheometer features that buyers should be aware of are temperature control, spindle rotational speed control, a range of sample holders, and ease of use.
- Measures dynamic viscosity and density of oils and fuels according to ASTM D7042
- Automatically calculates the kinematic viscosity and delivers measurement results which are equivalent to ISO 3104 or ASTM D445
- Processes up to 30 samples in an hour
Black Pearl Rotational
- Performs both routine rheological tests and complex rheological evaluation
- Comes standard with built-in Peltier temperature control for all measuring systems
- Measuring systems employ novel “Quick Capture” mounting technology
- Features an angular velocity range from 0.01 to 200 rad/s, a torque range from 0.005 to 20 mNm and a temperature range from -10° to 120° C
µVISC™ (micro VISC)
- Supports testing of higher viscosity ranges up to 20,000 mPas or higher
- Provides for easy and rapid viscosity measurements and now supports a wider range of applications
- Features a simple 1-2-3 step operation, which allows tests to be completed within a minute
- The world’s first truly portable, solvent-free, temperature-controlled kinematic viscometer
- Allows the user to measure kinematic viscosity from only a few drops of oil
- Requires no solvents for cleaning
- Entire sampling and testing process takes only a few minutes
- Weighs just under 1.8 kg