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

Commentary
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.