David W. Spitzer, P.E.

Differential-pressure flowmeters are readily available and are used extensively in industry. In particular, there are many orifice plate, Venturi flow tube and flow nozzle installations. Which of the following flowmeters use differential-pressure flowmeter technology in the sense that their operation is reflected by similar equations?

A. Target flowmeter
B. Variable-area flowmeter
C. Target and variable-area flowmeters
D. None of the above

Differential-pressure flowmeter primaries, such as the orifice plate, Venturi flow tube and flow nozzle present an obstruction to the flow. When the fluid is flowing, this obstruction generates a differential pressure across the flowmeter that is related to the flowrate through the flowmeter. The differential pressure is measured and the flowrate is calculated.

Target flowmeters are similar in that they are constructed to present an obstruction to the flow. Whereas an orifice plate allows flow in its hole and restricts flow in its annular area, target flowmeters restrict flow in its target and allows flow in its annular area. Instead of measuring differential pressure (as does an orifice plate), target flowmeters sense the force produced on the target by the differential pressure produced across the target. Therefore, the equations used for target flowmeters are similar to those used for differential-pressure flowmeters.

Variable-area flowmeters also present an obstruction to flow. Similar to target flowmeters, variable-area flowmeters restrict flow at the float and allow flow in the annular area. The differential pressure across the float produces an upward force that is counterbalanced by gravity such that the float reaches an equilibrium position that reflects the fluid flow. Therefore, the equations used for variable-area flowmeters are similar to those used for differential-pressure flowmeters.

Answer C is correct.

Additional Complicating Factors
There are other flowmeters and variants that are essentially differential flowmeters in disguise. Be aware of this possibility when flowmeter equations are similar to those for differential pressure primary elements. One clue is when the flowrate in the turbulent flow regime is related to the square root of its measurement.

David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 30 years of experience in specifying, building, installing, startup, and troubleshooting process control instrumentation. He has developed and taught seminars for over 20 years and is a member of ISA and belongs to the ASME MFC and ISO TC30 committees. Mr. Spitzer has written a number of books concerning the application and use of fluid handling technology, including the popular “Consumer Guide” 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 845 623-1830.