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| David W. Spitzer, P.E.
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Differential-pressure
producers, such as an orifice plate, Venturi, nozzle, and the like, are
commonly applied in conjunction with a differential-pressure
transmitter to comprise a flow measurement system. How might the
performance of this type of flow measurement system be stated?
A.
Percent of actual flowrate
B. Percent of set span
C. Percent of
maximum span
D. Percent of calibrated span
E. All of the above
F. None of the above
Commentary
The answer is more
involved than the question might indicate. It should be understood that
the flow measurement system is comprised of a flowmeter element and a
differential-pressure transmitter, where there are different
performance statements associated with each component.
Orifice plate, Venturi, and nozzle flowmeter elements have
performance statements that are expressed as a percentage of the actual
flowrate, so Answer A would seem to be correct. However, the
differential-pressure transmitter should also be considered when
evaluating the performance of the measurement system.
Depending on manufacture, differential-pressure transmitters can
have performance statements that are expressed as a percentage of
actual flowrate, percentage of set span, percentage of maximum span,
and/or percentage of calibrated span. So it might seem Answer E is
correct.
However, the performance of the flow measurement system is the
statistical combination of the performance of the flowmeter element and
the differential-pressure transmitter. Performing this calculation
involves converting individual performance statements into common
units, such as percentages of actual flowrate. Because the performance
of most differential-pressure transmitters is not expressed as a
percentage of actual flowrate, the flow measurement system exhibits
different performance (expressed as a percentage of actual flowrate) at
different flowrates. Therefore, performance would not be expressed
using the parameters stated in Answers A, B, C, and D, so Answer F
would be correct.
Not so fast … there is at least one differential-pressure
transmitter that has a performance statement expressed as a percentage
of the actual flowrate. Combining this statement with the percentage of
actual flowrate statement associated with the flowmeter primary would
result in a percentage of actual flowrate statement that describes the
performance of the flowmeter measurement system, so Answer A would be
correct.
Additional Complicating Factors
Differential-pressure
transmitters are also affected by ambient temperature and by the
operating pressure of the process. These effects degrade the
performance of the differential-pressure transmitter and are typically
expressed as a percentage of maximum span. In this case, even the
transmitter with a percentage of flowrate statement would make Answer F
correct.
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 845 623-1830.
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