How Peripherals Affect Analog Output Accuracy  

July 2008   
David W. Spitzer, P.E.     
What is the approximate accuracy of the analog signal for a flow measurement system where the output of a linear flowmeter with 1 percent of rate accuracy is fed to a control system input board that has an accuracy of 0.01 percent?

        A. 0.50 percent of rate
        B. 1.00 percent of rate       
        C. 1.01 percent of rate
        D. 2.00 percent of rate
        E. None of the above

The flowmeter portion of a flow measurement system can be thought of as consisting of a primary element, a transmitter, and a signal converter. The primary element is generally in contact with the flowing fluid (but may not necessarily be wetted). The transmitter processes the signal from the primary element to produce the flow measurement. The signal converter changes the flow measurement into an instrument signal (such as 4-20 mA) for transmission to another device.
The stated accuracy of the flowmeter (1 percent of rate) typically refers to the accuracy of the primary element and transmitter portions of the flow measurement system. Examination of the flowmeter specifications will likely reveal there is a signal converter accuracy that should be added to the stated accuracy to reflect the accuracy of the analog output. The signal converter accuracy is typically expressed as a percentage of full scale, so its value expressed as a percentage of flowrate will be different at different flowrates. Therefore, answers A, B, C, and D are not correct. Answer E is correct. Further, the accuracy of the control system input board should not be considered in calculating the accuracy of the analog output.

Additional Complicating Factors
The overall flow measurement system error should include all the components in the system. As such, it should include the accuracy of the control system input board and subsequent calculations. It should be noted that while some system inputs can exhibit accuracies on the order of 0.01 percent of full scale, other system inputs can exhibit accuracies as high as 0.4 percent of full scale. These poorer accuracies can profoundly degrade measurement accuracy when the flow is low in the flow range. 

David W. Spitzer, P.E., is a regular contributor to Flow Control. He can be reached at 845 623-1830.



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