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| David W. Spitzer, P.E.
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Despite claims that
may seem to the contrary, not all flowmeters are equal. In particular,
not all flowmeters of the same technology are equal. Further, not all
flowmeter models of the same technology made by the same manufacturer
are equal.
There are both subjective and objective
differences between flowmeters. Subjective differences include ease of
configuration, poor factory service, familiarity with the salesperson,
and the like. Objective differences include
flowmeter performance, flowmeter geometry issues, cost, and the like.
Flowmeters are purchased and installed to produce data that provides
measurements of the flowrate of the fluid over time. Flowmeter
performance specifications describe how well the data reflects the
actual flowrate. Comparing the flowmeter performance specifications of
different flowmeters can be like comparing apples and oranges.
Flowmeter performance specifications are typically expressed as a percentage
of flowrate, a percentage of full scale, a percentage of meter capacity, or a
percentage of calibrated span. These terms mean
completely different things and cannot be directly compared with one
another. For example, a flowmeter with 1 percent of rate performance
will measure 100 +/- 1, 50 +/- 0.5, 25 +/- 0.25, and 10 +/- 0.1 flow
units. Assuming a range of zero to 100 flow units, a flowmeter with 1
percent of full-scale performance will measure 100 +/- 1, 50 +/- 1, 25
+/- 1, and 10 +/- 1 flow units. Note that the errors are completely
different, even though both
performances are 1
percent. Similar calculations can be performed for the percentage of
meter capacity and the percentage of calibrated span.
Fortunately, with sufficient information, these specifications
can be converted from one to another for comparison purposes. However,
for proper analysis, all of the performance specifications should be
converted to the same type of error for comparison purposes. The
percentage of flowrate error is almost always used for comparisons
between flowmeters. In other words, if a performance specification is
expressed as a percentage of any other value, it is converted to a
percentage of flowrate performance to provide a fair basis for
comparison.
For example, at 100, 50, 25, and 10 flow units, the first
flowmeter in the previous example exhibits 1, 1, 1, and 1 percent of
rate performance. At 100, 50, 25 and 10 flow units, the second
flowmeter exhibits 1, 2, 4, and 10 percent of rate performance. Putting
the error into common units allows a valid comparison of apples and
apples. It can be clearly seen that the performance of the first
flowmeter is superior to that of the second flowmeter (all else being
equal).
Note that the percentage of rate statement is different at
different flowrates. This means one percentage of flowrate number does
not necessarily represent the performance of the flowmeter.
These comparisons are not always this easy. Some flowmeters have
different performance statements when calibrated and operated at
different flowrates. Other flowmeters have performance statements that
are the sum of two or more types of statements. Each should be
carefully converted to a percentage of flowrate statement for
comparison purposes. In addition, some flowmeters have more than one
component that should be considered in the calculation. An example of
this type of flowmeter is an orifice plate and differential pressure
transmitter, where each component has a different performance statement
and different statement type.
Setting up the calculations to compare performance statements can
involve a significant amount of time, even when using a spreadsheet.
Having done this for Coriolis mass flowmeters, differential pressure
flow transmitters, magnetic flowmeters, ultrasonic flowmeters, and
vortex shedding flowmeters, I can guarantee that you will find
significant differences between flowmeters. Not all flowmeters are
equal and, further, not all flowmeters within a given technology are
equal.
David W. Spitzer,
P.E., is a regular contributor to Flow Control. He has more than 25
years of experience in specifying, building, installing, startup, and
troubleshooting process control instrumentation. He has developed and
taught seminars for almost 20 years and is a member of ISA and belongs
to ASME, MFC, and ISO TC30 committees. Mr. Spitzer has published a
number of books concerning the application and use of fluid handling
technology, including the popular The Consumer Guide to… 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.
For More Information: www.spitzerandboyes.com
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