|By David W. Spitzer, P.E.
The debate over
repeatability and accuracy of flow measurement seems to replay itself every so often. I
admit having an opinion, but there are valid arguments on both sides
that should be heard.
the ability of a flowmeter to reproduce a measurement each time a set
of conditions is repeated. Flow measurements taken using a flowmeter
exhibiting poor repeatability would be chaotic. For example, if
measurements were taken of a known flow rate of 100 units per minute, a
flowmeter with poor repeatability might measure 85, 101, and 93 units
per minute on three consecutive days. If these measurements were used
to feed material to a process at a given flow rate, different amounts
of material would be fed to the process at each of these times. The
operator would be at a loss to determine what the flow setting should
be to obtain a flow of 100 units per minute. This amount of variation
could be detrimental to the operation of the plant.
In a similar test,
a flowmeter with better repeatability might measure 96, 94, and 95
units per minute. Note the difference between the measurements is
smaller (i.e., the measurement is more repeatable). From experience,
the operator will find setting the flow rate at 95 units per minute
results in the desired plant operation of 100 units per minute. As
such, one could make an argument that to operate the plant in a steady
manner it is desirable to use flowmeters that are repeatable. Note the
flowmeter setting does not correspond to the desired flow.
Accuracy is the
ability of the flowmeter to produce an output that corresponds to the
characteristic curve of the flowmeter. A flowmeter that is not
repeatable cannot be accurate. Stated differently, if the output of the
flowmeter is chaotic, it cannot correspond closely with the
characteristic curve. Therefore, in order for a flowmeter to be
accurate, it must be repeatable.
In a test similar
to the one described above, an accurate flowmeter might measure 101,
99, and 100 units per minute, and the operator will find that setting
the flow rate at 100 units per minute results in the desired plant
operation of 100 units per minute. Note the flowmeter setting does
correspond to the desired flow.
will be able to function with a flowmeter that is repeatable. If this
is all that is necessary, maybe a repeatable flowmeter should be
installed. For example, if a tank level increases, its level controller
will increase its effluent flow setting to maintain the level setting.
In this example, it is seemingly unimportant to accurately measure the
flow rate, but it would be beneficial if the flowmeter were repeatable
so that equal flow setting changes result in equal flow changes.
Repeatable flowmeters are often less expensive than accurate
flowmeters, providing another incentive for use.
overlooked is the opportunity — and sometime necessity — of performing
process calculations to ensure proper economic process operation or
process improvement. For example, if the effluent flowmeter cited above
is the overhead take-off of a distillation column, the flow measurement
may be used to calculate the column reflux flow rate. The original
plant design may not have contemplated this, but operating experience
may have determined that this control strategy will provide better
control of the column. Many such applications exist, and many of these
are “discovered” after the plant is operating. Sometimes, repeatable
flowmeters must be replaced with accurate flowmeters to obtain
operating data and/or accommodate these improvements.
are desirable because they are repeatable and yield measurements that
closely reflect the true flow rate. Repeatable flowmeters may not yield
accurate measurements, but they will perform in the same manner under
the same conditions. Which is appropriate depends in part on the
application and budget.
About the Author
David W. Spitzer,
P.E., is a regular contributor to Flow Control. He has more than 25
years of experience in specifying, building, installing, start-up, 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