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1 June 2002

How do you like your mashed potatoes?

Viscosity is an internal property of a fluid that offers resistance to flow. For example, pushing a spoon with a small force moves it easily through a bowl of water, but the same force moves mashed potatoes very slowly.

In fact, one of the major differences among styles of mashed potatoes is the viscosity of the starchy mass.

There are people out there who like their potatoes running and teeming with milk and butter-they are fans of low-viscosity potatoes. Others like their potatoes drier and stickier so they almost crack rather than flow-these people are devoted to high-viscosity potatoes.

There are many ways to measure viscosity, including attaching a torque wrench to a paddle and twisting it in a fluid, using a spring to push a rod into a fluid, and seeing how fast a fluid pours through a hole. One of the oldest and easiest ways is to simply see how fast a sphere falls through a fluid. The faster the sphere falls, the lower the viscosity.

This makes sense: If the fluid has a high viscosity, it strongly resists flow, so the sphere falls slowly. However, if the fluid has a low viscosity, it offers less resistance to flow, so the ball falls faster.

The measurement involves determining the velocity of the falling sphere. To do this, one drops each sphere through a measured distance of fluid and times how long it takes to traverse the distance. Thus, one knows distance and time and therefore velocity, which is distance/time.

The formula for viscosity is:

where

= difference in density between sphere and liquid

g = acceleration due to gravity

a = radius of sphere

v = velocity = d/t = (distance sphere falls through fluid)/(time it takes to fall)

This equation makes sense in that spheres that fall slowly have low velocities. This makes the denominator small, so the answer (viscosity) is large. Viscosity is measured in units of pascal•seconds (Pa•sec), which is a unit of pressure times a unit of time. This is not especially intuitive.

How does it relate to flowing liquids? One way of looking at it is to realize pressure is force per square area. This makes a little more sense: force (Pa) applied to the fluid, acting for some length of time (sec), Pa•sec, which are viscosity's units.

THOUGHTS IN CHOOSING METER

A viscosity measurement can be of value for one of the following reasons. First, viscosity is a direct measurement of fluid characteristics and behavior when in motion. It is very difficult to size a pump, pipeline, orifice meter, or agitator without knowing the viscosity of the process fluid.

In any operation where liquids are used, the viscosity of the fluid determines the effectiveness of the process and the quality of the finished product. In short, viscosity is one of the most important process properties.

Secondly, viscosity detection can be a sensitive indirect measurement of other properties. Molecular weight and its distribution in polymers, lubrication oils, and other substances, as well as the concentration, specific gravity, color, size, shape, and distribution of solids in a slurry or in an emulsion can all be reflected in viscosity variations.

Some viscometer designs are batch type and provide intermittent reading (falling ball or piston, oscillating blade, and others), while others are continuous.

In closed loop control, the continuous ones are preferred. Some viscometers necessitate the taking of a sample from the process (capillary, float, rotational), while others insert directly into the process pipe or vessel. Because sampling systems usually increase installation cost and maintenance and add to the dead time (transportation lag) of the measurement, the in-line designs are generally preferred.

Viscous process fluid usually also contains solids. For that reason the designs, which cannot tolerate solids (capillary, float), are less desirable, and the self-cleaning designs (oscillating, vibrational, double cylinder) are preferred.

In some applications the viscosity can change over a wide range. If that is the case, one can reduce measurement error by using designs that are capable of automatic range changing, such as the torsional oscillation design.

The torque or shear force measured by many viscometers will vary with changes in temperature and density.

Therefore, designs that provide automatic temperature compensation and allow for manual or automatic density correction are likely to provide more valuable information. In addition, one must also be concerned with viscosity range, design pressure, and temperature, as well as precision of the measurement.

—Source: Hawaii Space Grant College and Instrument Engineers' Handbook, 3rd edition

InTech senior technical editor Nicholas Sheble edits the Sensors department. IT