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Accuracy vs. Repeatability

September 26, 2010
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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.

Repeatability is 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.

Many applications 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.

However, often 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.

Accurate flowmeters 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
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