Flow Control: Technology information & new products for fluid handling engineers

October 2006

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Flow Fundamentals Revealed
When a Small Percentage Adds Up to Big Measurement Error

David W. Spitzer, P.E.

Every now and then it is important to revisit fundamentals to keep things in perspective. In general, an instrument performs within its stated accuracy when it is operated within its stated range of operation. This may seem straightforward, but how many times have you seen instruments operated outside of their stated range of operation?

For example, it is not unusual for a differential-pressure flowmeter to operate between 10 percent and 60 percent of its flow range in some applications. This may seem acceptable on the surface. However, these flowrates correspond to differential pressures of between 1 percent and 36 percent of the set span of the transmitter. Examination of the transmitter reference specifications can reveal transmitter measurement errors between approximately 2 percent and over 10 percent of flowrate at 10 percent of flow. Considering temperature and pressure effects, these errors can increase to over 100 percent of flowrate in many transmitters. Presuming the flowmeter primary hydraulics are such that the flowmeter primary operates properly between 10 percent to 60 percent of full scale, transmitter errors will tend to dominate the accuracy of the flow measurement system.

Poor accuracy at low flowrates is due to a combination of factors. First, the reference accuracy of differential pressure is usually a fixed value that can be a function of the transmitter, its set span, and its calibrated span. However, suppliers typically express the accuracy as a small percentage, such as 0.10, 0.075, 0.05, or 0.04 percent, so it appears to be small and OK to ignore.

Second, these differential-pressure flowmeters generate a differential pressure that is proportional to the square of the flowrate, so the differential pressure generated at 10 percent of flow corresponds to 1 percent of the differential pressure generated at 100 percent flow. Therefore, the performance of the flow measurement system (expressed as a percentage of the actual flowrate) associated with the transmitter is magnified at low flowrates.

Both of these effects are well known and understood. However, both are masked to the extent that they are neglected in many applications. The small percentage numbers associated with differential-pressure transmitter specifications generally cause many users to fail to read the detailed specifications that expose these measurement problems. Further, differential-pressure transmitter specifications are typically expressed in terms of differential pressure such that many users do not remember to take into account the effect of the quadratic relationship between flow and differential pressure.

Now imagine being asked to investigate the accuracy of a differential-pressure flow measurement system used for billing purposes that operates between 10 percent and 90 percent of flow where the transmitter accuracy statement is 0.075 percent. It sounds pretty good… but is it?

David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 25 years of experience in specifying, building, installing, startup, 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.

www.spitzerandboyes.com

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