|David W. Spitzer, P.E.|
Installing the best and the worst flowmeters in the same installation may seem contradictory, but it is quite possible. Flow measurement systems consist of a flowmeter primary and transmitter. The flowmeter primary may be the best available, while the transmitter could be the absolute worst.
For example, differential-pressure flowmeters consist of a flowmeter element and a differential-pressure transmitter. Consider a hypothetical flowmeter element that is perfect in its design, construction, and installation. Despite this perfection, purchasing an inexpensive differential-pressure transmitter with poor performance specifications will result in flow measurements that exhibit significant error. Likewise, a poorly designed, constructed, or installed flowmeter element will likely perform poorly despite the installation of the best differential-pressure transmitter.
This situation is not unique to differential-pressure flowmeters. Consider the case of magnetic flowmeters where the flowmeter element and the transmitter are more integrated to the point that magnetic flowmeter transmitters are installed almost exclusively with the same supplier’s magnetic flowmeter primary. Whereas common analog output specifications were generally better than 0.1 percent of full scale, the specification for at least one such transmitter was 0.3 percent of full scale. This flow measurement system would perform poorly even if it had a perfect flowmeter element.
Presupposing that the flowmeter technology has already been selected, components used in technologies with distinctly separate components, such as differential-pressure flow elements and transmitters, can be analyzed individually. Component selection could focus on purchasing and installing the best flow element and best transmitter possible given the constraints of the application. When the flowmeter components are integrated, the performance of the integrated system should be considered. It is important to note, presupposing the flowmeter technology was already selected means that flowmeter performance was (at best) only cursorily considered in the selection of technology. Such an approach might be shortsighted because the performance of the flow measurement system might be compromised if (all else being equal) another technology would perform better in the application.
Therefore, flowmeter selection with regard to performance could be considered a multi-step process where the individual flow measurement systems are evaluated and selected. Depending on the technology, this may entail evaluating individual components or integrated flowmeter equipment. Then, flow measurement systems using different technologies can be evaluated and selected using performance plus other criteria.
Remember, the best flowmeter component does not necessarily mean the flowmeter measurement system as a whole is the best. Like so many other multi-component systems, flow measurement solutions are only as strong as their weakest links.
David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 30 years of experience in specifying, building, installing, startup, and troubleshooting process control instrumentation. He has developed and taught seminars for over 20 years and is a member of ISA and belongs to the ASME MFC and ISO TC30 committees. Mr. Spitzer has written a number of books concerning the application and use of fluid handling technology, including the popular “Consumer Guide” 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 845 623-1830.