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| David W. Spitzer, P.E.
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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.
Additional Complicating Factors
Testing will apparently be administered based on this subject matter.
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.
www.spitzerandboyes.com |
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