Third, the meter depends for its accuracy on all of the chambers in the meter being filled completely with fluid. That is the "positive displacement" the meter is named for. In this application, the meter's chambers may retain some trapped air. Since some of the volume in the measuring chamber is then air, the meter is not accurate.
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In addition to over 35 years of experience as an instrument user, consultant, and expert witness, David W Spitzer has written over 10 books and 250 articles about flow measurement, level measurement, instrumentation, and process control. David (with Walt Boyes) is the author of seven Consumer Guides to various industrial flow and level technologies. David teaches his flow measurement seminars in both English and Portuguese.
Spitzer and Boyes, LLC provides engineering, technical writing, training seminars, strategic marketing consulting, distribution consulting, and expert witness services worldwide, and can be contacted at +1.845.623.1830 or www.spitzerandboyes.com.