David W. Spitzer, P.E.

The subject matter for flow measurement related to testing for a teaching position at a technical school was recently circulated. It included mass flowmeters (thermal and Coriolis) and volumetric flowmeters (Annubar, nozzle, positive displacement, magnetic, turbine, vortex shedding, ultrasonic, Pitot, orifice plate, rotameter, Venturi, and V-cone). Which of the following groupings are correct for volumetric flowmeters?

A. Annubar, nozzle, Pitot, orifice plate, Venturi, V-cone
B. Magnetic, turbine, vortex shedding, ultrasonic
C. Positive displacement, magnetic, turbine, vortex shedding, ultrasonic, rotameter
D. Annubar, nozzle, positive displacement, magnetic turbine, vortex shedding, ultrasonic, Pitot, orifice plate, rotameter, Venturi, and V-cone
E. None of the above

Commentary
The equations for fluid flow in a pipe are:

Q = A * v
W = ρ * Q

where Q is the flowing volume, A is the cross-sectional area of the pipe, v is the velocity of the fluid in the pipe, W is the mass flowrate, and ρ is the fluid density.

Some flowmeters (volumetric) measure the flowing volume of the fluid while others measure the mass of the fluid. Other flowmeters measure the velocity of the fluid and infer the flowrate using the cross-sectional area of the pipe (A). Still other flowmeters infer the flow by measuring the velocity head (½ ρv2) across the flowmeter.

The flowmeters listed in Answer A are differential-pressure flowmeters. They present an obstruction to the flow that creates a differential pressure that is measured and used to infer the flow in the pipe. These flowmeters are not volumetric flowmeters, so Answer A is not correct.

The flowmeters listed in Answer B measure the velocity of the flowing fluid. They are not volumetric flowmeters. Thus, Answer B is not correct.

Regarding Answer C, positive-displacement flowmeters measure the volume of the fluid. However, the remaining flowmeters measure velocity. Therefore, Answer C is not correct. Similarly, Answer D is not correct. Thus, Answer E is correct.