David W. Spitzer

Variable area flowmeters tend to have their accuracy statements expressed as a percentage of:?

A. Actual flow
B. Full-scale flow
C. Meter capacity
D. Calibrated span

Commentary
Flowmeters that have an accuracy statement expressed as a percentage of actual flow usually have a well-defined output at zero flow. For example, with no flow, vortex shedders do not shed vortices, positive-displacement flowmeters do not rotate, and differential-pressure flowmeters exhibit zero pressure differential. All of these are well-defined conditions. This is not the case with variable area flowmeters where the float may fall, but it will not necessarily be at a well-defined position.

In addition, the float can stick during operation (and especially at low flow), causing measurement errors that would exceed a reasonable percentage of actual flow. Answer A is not correct.

The accuracy of variable area flowmeters is typically specified as a percentage of full-scale flow, so Answer B is correct. For variable area flowmeters, meter capacity is the same as the full-scale flow, so Answer C is also correct. Answer D could be correct if the variable area flowmeter is calibrated at full-scale flow.

Additional Complicating Factors
The percentage of full-scale statement could also be expressed as an absolute flow error (such as +/-1 liter per minute) by multiplying the full-scale accuracy statement by the full-scale flow.

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. He has more than 35 years of experience and has written over 10 books and 250 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 in the navigation menu to find his “Consumer Guides” to various flow and level measurement technologies.