David W. Spitzer

Increasing the temperature of a liquid causes the measurement in a positive-displacement flowmeter to:

A. Over-register
B. Under-register
C. Not be affected
D. Cannot be determined

Commentary
Positive-displacement flowmeters measure the volume of liquid flowing through the flowmeter. Increasing the temperature of the liquid will cause the flowmeter body to expand and cause it to under-register (Answer B).

However, in many applications, positive-displacement flowmeters are used to infer mass flow even though they measure volumetric flow. In these applications, increasing the temperature of the liquid will cause both the liquid and the flowmeter body to expand. These effects will offset each other if the fluid and flowmeter both expand by the same amount (Answer C). This is generally not the case, so the measurement will exhibit an error dependent upon the difference in the rates of expansion. Liquid will often expand more than the flowmeter body so the flow measurement will tend to over-register (Answer A).

Positive-displacement flowmeters have physical tolerances that allow some liquid to leak through the flowmeter. This is called slippage and typically tends to make positive-displacement flowmeters under-register. Increasing the temperature of the liquid will tend to decrease viscosity. Decreasing viscosity tends to increase slippage and allow more unmeasured liquid to pass through the flowmeter so the measurement can tend to under-register (Answer B).

Additional Complicating Factors
Increasing the temperature of high-viscosity liquids will tend to reduce the viscosity of the liquid. However, this (new) lower viscosity may be so high that slippage does not increase.

Given the vagueness of the problem statement and considering the many unknowns, perhaps Answer D is the pragmaticanswer.

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. David can be reached at 845 623-1830 or www.spitzerandboyes.com.