SUBJECT : A primer on viscosity 7-8

Viscosity is defined as resistance to pouring, with higher viscosity liquids affecting centrifugal pump performance in several ways:

Viscosity is a measure of the "thickness" of the liquid. Molasses and motor oil are thick or high viscous liquids. Gasoline and water are thin, low viscosity liquids. Do not confuse this viscosity with the specific gravity of the same fluid. Specific gravity is a measure of the weight of the liquid compared to an equal volume of 20° C (68° F) fresh water.

Motor oil has a low specific gravity (it floats on water), but a high viscosity of more than 500 centistokes. Mercury has a high specific gravity (13.7) but a low viscosity of only 0.118 Centistokes. It is important to note again that these two properties of a liquid are entirely independent of each other.

The viscosity of a liquid can change appreciably with a change in the temperature of the liquid, but seldom changes when the pressure is altered We all know that hot oil is "thinner" than cold oil, so we must always know the temperature of the fluid when the viscosity is to be measured. Without this information you will frequently select the wrong size pump.

Temperature is not the only variable when we look at viscosity. There are four classes of fluids that change their viscosity with agitation, and one that does not:

Viscosity is expressed in "absolute" or "kinematic" terms. Let's look at absolute first:

The two are related as follows:


Since the specific gravity of water at 68.4°F (20.°C) is almost one, it follows that the kinematic viscosity of water at 68.4°F is for all practical purposes 1.0 centistokes.

We measure viscosity with a viscosimeter and there are number of them available to chose from:

There are tables available that list the viscosities of many common liquids at various temperatures. It is very obvious that even small changes in temperature can affect viscosity greatly, which will change the friction losses in the pipe fittings and valves.

Unfortunately there is no acceptable analytical method of predicting pump performance when the liquid has a viscosity different than water. Many test have been conducted, and the data formulated into charts and nomographs with the result being that your pump performance can be reasonably estimated for liquids of just about any viscosity.

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