Viscometers

Viscosity is a material's resistance to flow under an applied stress, expressed as shear stress divided by shear rate.

Viscometers are instruments used to measure viscosity. Since viscosity cannot be measured directly, viscometers are instead used to measure an experimental parameter that is related to viscosity in a known manner. Although there are many different kinds of viscometers, this module will focus on the three most common types.

Capillary Viscometers

Capillary viscometers measure viscosity by monitoring fluid flow through very narrow glass tubes.

(Copyright of Koehler Instrument Company, Inc., Bohemia, NY)

General Information

In a capillary viscometer, a liquid is drained or forced through a fine-bore tube , and the viscosity is determined from the measured flow rate, applied pressure, and tube dimensions. A variety of capillary viscometers is shown below.

(Copyright of Cannon Instrument Company, State College, PA)

For a given pressure drop, the time it takes for a fluid volume to flow a certain distance through a tube is a function of the fluid's resistance to flow. A more viscous fluid will take a longer time to travel that distance.

Equipment Design

To determine viscosity, the time for a given volume of fluid to travel a distance through the capillary tube (either due to gravity or a driving force) is measured .

Capillary viscometers are calibrated with standard fluids of known viscosities. The viscosity of a fluid can be computed by multiplying the time required for flow by a calibration constant .

Usage Examples

Capillary viscometers are used to measure the viscosity of a wide range of diverse fluids. Common examples include petroleum products, lubricants, adhesives, and sealants. Different models of capillary viscometers are used to measure specific types of fluids, such as opaque viscometers for darker fluids, or high-viscosity viscometers for fluids with particularly high viscosities.

(Copyright of Metro Tech Systems Ltd., Calgary, Alberta)

Advantages

Disadvantages

  • Measure precise viscosities for many diverse fluids
  • Small and portable
  • Can use a wide variety of capillary tubes on the same viscometer
  • No single tube is suitable for all viscosities
  • Basic models can only be used for translucent fluids
  • Difficult to clean the capillary tubes

Rotational Viscometers

Rotational viscometers are made up of two parts: One that rotates, and another that remains stationary.

(Copyright of Brookfield Engineering Laboratories, Inc., Middleboro, MA)

General Information

Rotation of the inner cylinder generates shear on the fluid, causing the fluid to flow within the viscometer. The torque required to produce a given angular velocity , or the angular velocity resulting from a given torque, are measures of the viscosity of the fluid.

Equipment Design

The rotational viscometer consists of two basic parts separated by the fluid being tested. The two parts may be: concentric cylinders (cup and bob), parallel plates, a low angle cone and plate, or a spindle inside of a cylinder. Common spindle shapes, disk, T-bar, cylinder, and vane, are shown below.

(Copyright of Brookfield Engineering Laboratories, Inc., Middleboro, MA)

One part rotates while the other remains stationary. Pictured and illustrated below is the parallel plate type of viscometer, with the top plate rotating above the stationary bottom plate.

(Photograph copyright of Chemical Engineering Department,

University of Michigan, Ann Arbor, MI)

Another type of rotational viscometer is the cone and plate model, shown below. This instrument consists of a low angle cone (usually about one to three degrees) set above a flat plate. Either the cone or the plate can rotate while the other part is held stationary.

(Photograph copyright of Chemical Engineering Department,

University of Michigan, Ann Arbor, MI)

Usage Examples

Rotational viscometers are used to measure the viscosity of a wide range of fluids from different industries. Some specific examples include sauces, lotions, shampoos, and paints, as shown below. Rotational viscometers are also used to measure the viscosity of liquid-like raw materials used in the chemical industry.

(Copyright of Zenith Pumps, Monroe, NC)

There are many possible uses of rotational viscometers because of the various different configurations.

Advantages

Disadvantages

  • Can measure viscosities of opaque, settling, or non-Newtonian fluids
  • Useful for characterizing shear-thinning and time-dependent behavior
  • Speed of the rotating part easily adjusted
  • Often linked to computers for semi-automated measurement
  • Can be relatively expensive
  • Often large and not portable

Falling Ball Viscometers

General Information/Equipment Design

Falling ball viscometers determine the viscosity of a Newtonian fluid by measuring the velocity of a ball moving through the fluid. A lower fluid viscosity results in less resistance to flow, which corresponds to a faster velocity of the moving object.

A falling ball viscometer is shown below. The fluid is placed in a container, such as a graduated cylinder. The motion of a ball, bubble, plate, needle, or rod through the fluid is monitored. The velocity of the moving object is then used to calculate the viscosity of the fluid.

(Copyright of Brookfield Engineering Laboratories, Inc., Middleboro, MA)

The balls used in a falling ball viscometer can be made from various materials. Pictured below are balls made of boron silicate glass, nickel-iron, and steel.

(Copyright of Brookfield Engineering Laboratories, Inc., Middleboro, MA)

Usage Examples

Falling body viscometers are commonly used for low-viscosity substances in the pharmaceutical , food, chemical and mineral oil industries. Specific examples include liquid hydrocarbons, sugar solutions, solvents, and polymer solutions. Quality control of a milk processing system such as the one shown below would rely on such viscosity measurements.

(Copyright of Emerson Process Management, St. Louis, MO)

Advantages

Disadvantages

  • Small and portable
  • Easy operation
  • Limited to Newtonian fluids
  • Restricted to translucent fluids (need to be able to see the object's movement)

Acknowledgements

Brookfield Engineering Laboratories, Inc. , Middleboro, MA

Cannon Instrument Company , State College, PA

Chemical Engineering Department, University of Michigan , Ann Arbor, MI

Koehler Instrument Company, Inc. , Bohemia, NY

Metro Tech Systems Ltd. , Calgary, Alberta

Zenith Pumps , Monroe, NC

References

Kamrich, Jr., Peter, and Clifford K. Schoff. "Rheological Measurements" Kirk-Othmer Concise      Encyclopedia of Chemical Technology . 4th ed. Vol. 2. Hoboken, NJ: Wiley-Interscience,      1999. 1760-764. Print.

Wilkes, James O. Fluid Mechanics for Chemical Engineers . Upper Saddle River, NJ:      Prentice-Hall PTR., 1999. Print.

Developers

Marc Schneidkraut

Kelsey Kaplan

Henry Chen