David W. Spitzer, P.E.

Differential-pressure producers, such as an orifice plate, Venturi, nozzle, and the like, are commonly applied in conjunction with a differential-pressure transmitter to comprise a flow measurement system. How might the performance of this type of flow measurement system be stated?
A. Percent of actual flowrate
B. Percent of set span
C. Percent of maximum span
D. Percent of calibrated span
E. All of the above
F. None of the above

The answer is more involved than the question might indicate. It should be understood that the flow measurement system is comprised of a flowmeter element and a differential-pressure transmitter, where there are different performance statements associated with each component.

Orifice plate, Venturi, and nozzle flowmeter elements have performance statements that are expressed as a percentage of the actual flowrate, so Answer A would seem to be correct. However, the differential-pressure transmitter should also be considered when evaluating the performance of the measurement system.

Depending on manufacture, differential-pressure transmitters can have performance statements that are expressed as a percentage of actual flowrate, percentage of set span, percentage of maximum span, and/or percentage of calibrated span. So it might seem Answer E is correct.

However, the performance of the flow measurement system is the statistical combination of the performance of the flowmeter element and the differential-pressure transmitter. Performing this calculation involves converting individual performance statements into common units, such as percentages of actual flowrate. Because the performance of most differential-pressure transmitters is not expressed as a percentage of actual flowrate, the flow measurement system exhibits different performance (expressed as a percentage of actual flowrate) at different flowrates. Therefore, performance would not be expressed using the parameters stated in Answers A, B, C, and D, so Answer F would be correct.

Not so fast … there is at least one differential-pressure transmitter that has a performance statement expressed as a percentage of the actual flowrate. Combining this statement with the percentage of actual flowrate statement associated with the flowmeter primary would result in a percentage of actual flowrate statement that describes the performance of the flowmeter measurement system, so Answer A would be correct.

Additional Complicating Factors
Differential-pressure transmitters are also affected by ambient temperature and by the operating pressure of the process. These effects degrade the performance of the differential-pressure transmitter and are typically expressed as a percentage of maximum span. In this case, even the transmitter with a percentage of flowrate statement would make Answer F correct.

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.