QUIZ CORNER: Which Is More Important for Flow Measurement — Repeatability or Accuracy?

Feb. 4, 2015

Good measurement requires both repeatability and accuracy — but good accuracy generally means good repeatability.

David W. Spitzer

Commentary
Page 53 of my book Industrial Flow Measurement (ISA) states that “repeatability is the ability of the flowmeter to reproduce a measurement each time a set of conditions is repeated. It is not implied that the indicated flow is correct, but rather that the indication is the same each time.”

Page 56 states that the “accuracy of a flowmeter is its ability to produce an output that corresponds to its characteristic curve.” In other words, flowmeter accuracy is based on the ability of the flowmeter to measure correctly.

If a flowmeter is not repeatable, it cannot be accurate. For example, if a flowmeter randomly measures high and low by (say) 10 percent, how can its accuracy be (say) 1 percent? If a flowmeter is accurate, it must be repeatable. For example, if a flowmeter is accurate within (say) 1 percent, its repeatability must be better than 1 percent. On the other hand, a perfectly repeatable flowmeter can have an accuracy of (say) 10 percent.

In summary, good measurement requires both repeatability and accuracy — but good accuracy generally means good repeatability.

Additional Complicating Factors
The accuracy and repeatability of flowmeters are often based on Reynolds Number — not flow as implied above.

David W. Spitzer is a regular contributor to Flow Control magazine and a principal in Spitzer and Boyes, LLC offering engineering, seminars, strategic marketing consulting, distribution consulting and expert witness services for manufacturing and automation companies. Spitzer and Boyes is also the publisher of the Industrial Automation INSIDER. Mr. Spitzer has more than 40 years of experience and has written more than 10 books and 300 articles about flow measurement, instrumentation and process control. Mr. Spitzer can be reached at 845 623-1830 or www.spitzerandboyes.com. Click on the “Products” tab to find his “Consumer Guides” to various flow and level measurement technologies.