Last month I mentioned being asked to audit the flowmeters used to bill a chemical plant for its steam consumption. Satisfied with a review of the overall design, calibration and installation of the various steam flow measurement systems, I was able to focus on the symptom at hand: One of the differential pressure transmitters would be calibrated to be within its “As Left” specification only to be consistently out of its “As Found” specification during its next calibration approximately three months later.
Given the symptom, a more detailed investigation into the calibration technique was warranted. An interview with the technician revealed that the transmitter was taken out of service, removed from the line, transported to the shop where it sat for a few hours for its temperature to equilibrate to the shop temperature, calibrated with a dead weight tester, reinstalled in the line, zeroed at pressure and then rezeroed at pressure the next day after the transmitter temperature equilibrated to the outside ambient temperature (that could reach 20 degrees below 0ºC in the winter).
This procedure appears to be correct from a measurement perspective but presents a problem with calibration. In particular, the “As Found” calibration is generally compared to the previous “As Left” calibration to determine if the transmitter has drifted since the last calibration. Note that in this installation the transmitter is rezeroed the next day. Therefore, the next “As Found” calibration would be expected to be different than the previous “As Left” calibration, as it was for this transmitter.
More next month.
David W. Spitzer is a regular contributor to Flow Control magazine and a principal in Spitzer and Boyes LLC, which offers 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. He has more than 40 years of experience and has written more than 10 books and 350 articles about flow measurement, instrumentation and process control. Spitzer may be reached at 845-623-1830 or via spitzerandboyes.com. Click on the “Products” tab to find his Consumer Guides to various flow and level measurement technologies.
