What is the approximate resolution of a flowmeter that converts a 12-bit register value representing flow rate to a 4-20 mA analog output?
- 0.006 percent of span
- 0.025 percent of span
- 0.10 percent of span
- 0.40 percent of span
The analog signal represents the flowmeter span in 4,096 steps (2 to the 12th power). The resolution is one step, where each step represents 1/4,096 of the span, which is 0.024 percent of span. Answer B is the best answer.
Additional complicating factors
Resolution is not the same as accuracy. This digital-to-analog converter has 4,096 steps, but its accuracy might be +/- 4 steps, which is 4 times its resolution, or 0.10 percent of span.
Accuracy specifications for flowmeter analog outputs are commonly expressed as a percentage of flow rate plus the digital-to-analog converter accuracy expressed as a percentage of span, such as 0.75% rate plus 0.10% span. The accuracy of this flowmeter at 20 percent flow is 1.25 percent of the flow rate (0.75 + 0.10/0.20), which is almost 70 percent higher than the measurement accuracy of the flowmeter itself. Operating at lower flow rates will further degrade accuracy.
About the Author
David W. Spitzer
David W Spitzer’s new book Global Climate Change: A Clear Explanation and Pathway to Mitigation (Amazon.com) adds to his over 500 technical articles and 10 books on flow measurement, instrumentation, process control and variable speed drives. David offers consulting services and keynote speeches, writes/edits white papers, presents seminars, and provides expert witness services at Spitzer and Boyes LLC (spitzerandboyes.com or +1.845.623.1830).