|David W. Spitzer, P.E.|
There have been a number of articles written about flowmeter performance claims made by suppliers. The published specifications are generally expressed as a percentage of something — usually rate, full scale, calibrated span, or maximum span. Naturally, suppliers attempt to express their claims in a manner that describes its performance and puts their instrument in the best light. The result is an eclectic mix of terms and meanings that results from the ability of each supplier to express them in the manner that they desire. As confusing as this has become, let me add one more “simplification” to the mix.
It has been almost 10 years since I last taught a flow measurement seminar in Brazil. In a recent seminar there, it became apparent that suppliers were generally expressing performance as either a percentage of “valor medido” (measured value) or as a percentage of “fundo de escala” (end of scale). The percentage of measured value was readily recognizable as a percentage of rate. However, the meaning of a percentage of “end of scale” was not immediately apparent because the “scale” could be the full scale, calibrated span, or maximum span.
I questioned a number of people to try to determine the meaning of end of scale. Interestingly, I received different answers from different people. Typically, they would cite the concept of either full scale or maximum span. (Instruments with percentage of calibrated span statements are becoming less common.)
Most were perplexed when I presented the alternative answer(s). In my limited and unscientific survey, it seemed that each supplier had its own definition of end of scale, and (shockingly) those definitions suited their specific needs. Instrument users produced similar answers and seemed to be perplexed as well.
Needless to say, this “simplification” of using one term to denote three concepts resulted in debates and confusion during the seminar. Some discussions were particularly confusing because I presented the seminar in Portuguese using the same term to describe the various concepts before inventing Portuguese terms to describe the concepts — percentage of operating range, percentage of calibrated range, and percentage of maximum range.
This series of events, and the confusion that occurred, illustrates the advice that I have been giving students for years: When a supplier specifies performance as a percentage, be sure that you know to which parameter the percentage relates. A given percentage of full scale, calibrated span, and maximum span can (and usually are) completely different. It is up to the user to ferret this information out and apply it in order to clearly understand the performance of the supplier’s equipment.
This may seem somewhat esoteric, but the net result is that the people who attended the seminars now have a wider appreciation of the concepts involved. What has your supplier told you lately?
About the Author
David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 25 years of experience in specifying, building, installing, start-up, and troubleshooting process control instrumentation. He has developed and taught seminars for almost 20 years and is a member of ISA and belongs to ASME MFC and ISO TC30 committees. Mr. Spitzer has published a number of books concerning the application and use of fluid handling technology, including the popular The Consumer Guide to… series, which compares flowmeters by supplier. Mr. Spitzer is currently a principal in Spitzer and Boyes LLC, offering engineering, product development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached at 845 623-1830.
For More Information: www.spitzerandboyes.com