David W. Spitzer

Increasing the temperature of a liquid tends to:
A. Increase viscosity and decrease density
B. Increase viscosity and increase density
C. Decrease viscosity and decrease density
D. Decrease viscosity and increase density

Commentary
In general, liquids tend to get “thinner” when their temperature increases. For example, honey and oil tend to flow better at higher temperatures. Therefore, increasing temperature decreases viscosity. Answer A and Answer B are not correct.

In general, the liquids tend to expand when their temperature increases. For example, the same mass of boiling water occupies more volume at 100 degrees Celsius than at 20 degrees Celsius. Therefore, increasing temperature decreases density. Answer C is correct.

Additional Complicating Factors
The viscosity of liquids is affected by temperature. However some liquids are Non-Newtonian and their viscosity can be much more affected by how they are treated. For example, trying to pump certain slurries using more energy can cause the apparent viscosity to increase. This in turn causes the slurry to exhibit more resistance and can actually reduce the flowrate—even though more energy is used.

Related Content:
“Part II: The Expert Witness” By David W. Spitzer

While liquid density generally decreases with increasing temperature, this is not always the case. For example, the density of water between about 0 and 4 degrees Celsius will actually increase with increasing temperature.

David W. Spitzer is a regular contributor to Flow Control magazine and a principal in Spitzer and Boyes, LLC offering engineering, seminars, strategic, marketing consulting, distribution consulting and expert witness services for manufacturing and automation companies. He has more than 35 years of experience and has written over 10 books and 250 articles about flow measurement, instrumentation and process control.

Mr. Spitzer can be reached at 845-623-1830 or www.spitzerandboyes.com. Click on the “Products” tab to find his “Consumer Guides” to various flow and level measurement technologies.