David W. Spitzer

A flow measurement system consists of a flowmeter element, transmitter and indicator. If each of the three components has an accuracy of 1 percent, the performance of the flow measurement system is approximately:

A. 3 percent
B. 1.7 percent
C. 1 percent
D. None of the above

The performance of the flow measurement system should be inferior to the performance of the flow element itself. Answer C could be viable (given “approximate” in the wording of the question). However, the magnitude of the performance of the other two components (1 percent) causes Answer C to be incorrect.

It is not often that all of the components in a measurement system are in error by the maximum amount in the same direction at the same time. Therefore, the errors for the three components in this system would not be mathematically added to obtain 3 percent. Answer A is not correct.

The overall accuracy of measurement systems is typically calculated by taking the square root of the sum of the squares of the effect of each component. For this system, this would be calculated as the square root of (12 + 12 + 12), or 1.73 percent. It might appear that Answer B is correct, however further investigation would show that the flowmeter element has a percentage of rate error whereas the transmitter and indicator have percentage of full scale errors. This means that the transmitter and indicator exhibit flow errors in excess of 1 percent at flows below 100 percent of full scale flow. Therefore, the calculation that results in 1.73 percent performance is only correct at full scale flow. Answer D is correct.

Additional Complicating Factors
Definitive calculations to determine the overall accuracy of a measurement system entail analysis of many factors that affect the measurement including dimensional, physical, process, operational, calibration, ambient and measurement considerations.

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 at 845 623-1830.