Which of the following adjustments are applicable to a pulsed magnetic flowmeter with a 4-20 mA analog output?
A. Zero and full scale flow
B. Zero and full scale flow; 4 mA and 20 mA
C. Zero flow; 4 mA and 20 mA
D. Full scale flow; 4 mA and 20 mA
The analog output zero and span (4 mA and 20 mA) circuitry needs to be adjusted or trimmed to ensure accuracy so Answer A is not correct.
Pulsed magnetic flowmeters measure the induced voltage on the electrodes when the coil is turned on and then when it is turned off. The flowmeter measures flow signal plus noise when the coil is on. Noise is measured when the coil is off. The flow signal can be calculated by subtracting the two (flow signal plus noise minus noise). Therefore the noise cancels itself out at zero flow so there is no zero adjustment per se. Answer B and Answer C are not correct.
The span does need to be adjusted or trimmed so Answer D is correct.
On a side note, when pulsed magnetic flowmeters were first introduced, two engineers spent an entire morning (unsuccessfully) searching for the zero adjustment.
Additional Complicating Factors
Other flowmeter technologies that can exhibit a well-defined zero that cannot be adjusted include differential pressure, turbine and vortex shedding flow technologies.
David W. Spitzer is a regular contributor to Flow Control magazine and a principal in Spitzer and Boyes, LLC offering engineering, seminars, strategic, marketing consulting, distribution consulting, and expert witness services for manufacturing and automation companies. Spitzer and Boyes is also the publisher of the Industrial Automation INSIDER. David has more than 40 years of experience and has written over 10 books and 300 articles about flow measurement, instrumentation and process control. He can be reached at 845 623-1830 or via www.spitzerandboyes.com. Click on the “Products” tab to find his Consumer Guides to various flow and level measurement technologies.