Which of the following forms describe how the accuracy of a differential pressure flowmeter would typically be expressed?
A. Percent of rate
B. Percent of full scale
C. Percent of measured value
D. Percent of calibrated span
E. Percent of span
This question is one that would be asked in a plant but it is not precise enough to elicit a definitive answer. The primary element (such as an orifice plate) has an accuracy expressed as a percentage of rate (Answer A) or measured value (Answer C). However, the accuracy of its differential pressure transmitter can be expressed as a percentage of full scale (Answer B), calibrated span (Answer D), span (Answer E), upper range limit or a combination thereof.
Additional complicating factors
The accuracy of a differential pressure measurement system includes the performance of the primary element and its transmitter. The transmitter accuracy expressed as a percentage of rate increases as flow decreases, so the combined accuracy expressed as a percentage of rate will be different at different flow rates.
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.