A flowmeter is calibrated to its full scale of 200 flow units but is capable of up to 400 flow units. The flowmeter can measure a flow of 100 flow units within ±1 unit and can measure 200 flow units within ±2 units. What is the accuracy statement associated with this flowmeter?

A. ±1 percent of rate
B. ±1 percent of full scale
C. ±1 percent of calibrated span
D. ±1 percent of meter capacity

The full scale and calibrated span are both 200 flow units. The accuracy at 100 flow units expressed as a percentage of these parameters would be ±1/200 or 0.5 percent of full scale or calibrated span. Answers B and C are not correct.

At 100 flow units, the accuracy expressed as a percentage of meter capacity (400 flow units) would be ±1/400 or 0.25 percent of meter capacity. Answer D is not correct.

Similarly, the accuracy expressed as a percentage of the flow rates (100 and 200 flow units) is ±1/100 (and ±2/200) or 1 percent of rate, which confirms that Answer A is correct.

Additional complicating factors

Note that “±1 percent” is not a complete specification. Be sure to ask, “Percentage of what?”

David W. Spitzer is a regular contributor to Flow Control magazine and a principal in Spitzer and Boyes LLC, which offers 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. He has more than 40 years of experience and has written more than 10 books and 350 articles about flow measurement, instrumentation and process control.

Spitzer may be reached at 845-623- 1830 or via spitzerandboyes.com. Click on the “Products” tab to find his Consumer Guides to various flow and level measurement technologies.