David W. Spitzer, P.E.

Viscosity is an important parameter in flow measurement. Which of the following materials exhibit properties described by the concept of viscosity?

A. Water
B. Honey
C. Peanut butter
D. Glass
E. All of the above
F. None of the above

Commentary
Viscosity describes the ability of the fluid to flow over itself and it can be a determining factor in flowmeter selection. Some flowmeters simply will not measure viscous fluids while other flowmeters can introduce error when measuring nonviscous fluids.

In general, fluid viscosity is dependent upon the temperature of the fluid and (notwithstanding transient conditions) the flowing viscosity is important for flow measurement purposes.

Water (Answer A) is a relatively nonviscous fluid with a viscosity of approximately 1 cP. Honey (Answer B) illustrates the effect of temperature in that honey that is cooled to 5 C in a refrigerator is much more viscous than honey that is heated to 40 C.

There is also a viscosity associated with peanut butter (Answer C) that is highly temperature dependent. A published viscosity of 10,000 cP has been noted, but the question arises as to the temperature and composition of the fluid. In particular, purchased peanut butter likely has a much higher viscosity.

Is there a viscosity associated with glass (Answer D)? Have you ever been in an historic building that was over 150 years old? Did the building have its original windows? If so, did you notice that the glass in the windows was thicker at the bottom than at the top? They were not installed that way when the building was new. This is a result of the glass “flowing” down over time due to gravity. There is a viscosity associated with this glass, albeit large. Did you notice that the glass in the windows was wavy? These waves are likely caused because some areas had slightly different chemical compositions that resulted in different viscosities and different flowrates. As temperature increases, the glass will melt, so there is a viscosity associated with molten glass.

Newtonian fluids exhibit a viscosity that is a function of temperature. However, many fluids are non-Newtonian in that their viscosities vary not only with temperature, but also with how the fluid is mechanically treated. For example, ketchup generally does not flow freely from its bottle and may take some effort to get movement. However, once it begins to flow, it can flow quickly and put more than you want on your food.

David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 25 years of experience in specifying, building, installing, startup, and troubleshooting process control instrumentation. He has developed and taught seminars for almost 20 years and is a member of ISA and belongs to ASME, MFC, and ISO TC30 committees. Mr. Spitzer has published a number of books concerning the application and use of fluid handling technology, including the popular The Consumer Guide to… series, which compares flowmeters by supplier. Mr. Spitzer is currently a principal in Spitzer and Boyes LLC, offering engineering, product development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached at spitzer@spitzerand boyes.com or 845 623-1830.

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