David W. Spitzer, P.E.
As intricate and confusing as some flowmeter performance specifications seem to be, do not forget to consider the conditions under which these specifications are developed. Flowmeter suppliers use flowmeter performance at reference conditions to describe the operation of their equipment. Reference performance is a measure of the quality of measurement at a nominal set of operating conditions. Typical reference conditions for liquids include the flow of water at 20 C through a properly installed flowmeter in an ambient environment of 20 C and 50 percent ambient relative humidity.
The flowmeter is installed in piping with sufficient lengths of upstream and downstream straight run (containing no fittings) such that flowmeter performance is not impacted by the piping configuration. Data is typically obtained utilizing the flowmeter pulse output.
In the context of the industrial world, reference accuracy reflects performance under controlled laboratory conditions. As such, reference performance is usually the best performance that can be expected of the flowmeter when it is in excellent working condition and properly installed in a stable environment.
Whereas reference accuracy represents performance in laboratory conditions, the actual performance in the field may be somewhat degraded. For example, locating the flow tube and transmitter outdoors potentially exposes both devices to wide temperature variations, humidity variations, direct sunlight, and precipitation of various forms. These factors can cause measurement variability that exceeds the stated reference accuracy.
Appropriate testing can quantify these parameters, however sufficient data is generally not available to allow meaningful comparisons. Note that even the reference accuracy of the analog output can be difficult to compare because some suppliers do not know or do not release sufficient information to determine reference accuracy. In some flowmeters, this can be due to a lack of testing. Surprisingly, in my research I have found many flowmeter suppliers said that few people ask about the accuracy of the analog output. Other suppliers innocently provided the resolution of the analog output instead of its accuracy.
A combination of the reference accuracy and the operating effects that describe actual performance would provide a meaningful dataset with which to compare flowmeters. However, the general lack of operating effect data makes the comparison of reference performance a pragmatic surrogate.
The lack of operating effect data can be interpreted in a number of ways. Some suppliers are diligent in checking these parameters, whereas others are likely not. For example, some suppliers maintain internal design standards for relative humidity whereby all designs are tested (perhaps pass/fail) as a matter of course, but the results are not published. Other suppliers admit to not having quantified or tested humidity effects. They typically state humidity limits based upon electronic component specifications that are not generally rated from 0 to 100 percent relative humidity. Note that as a matter of course, outdoor installations will be exposed to 100 percent relative humidity, so even humidity specifications should be considered to ensure that the transmitter is suitable for the environment, lest it be damaged.
Ideally, comparisons of flowmeter performance should be based upon the anticipated performance in an actual installation and should include operating effects. The general lack of operating effect specifications makes this comparison difficult — if not impossible.
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