In previous articles, I mentioned being asked to audit the flowmeters used to bill a chemical plant for its steam consumption. One of the differential pressure transmitters was within its “As Left” specification only to be consistently out of its “As Found” specification during its next calibration approximately three months later. This was determined to be caused by rezeroing the transmitter on the day after reinstallation — after the transmitter temperature was allowed to equilibrate with the outside ambient temperature. While one particular installation consistently exhibited this behavior, it is likely that the other installations were likewise affected, albeit not as much.
Understanding that the calibrations are performed correctly and that they can affect the “As Found” calibration, one approach to solve the problem would be to widen the “As Found” calibration tolerance. However, recognizing that it takes almost a full day to remove, equilibrate, calibrate and reinstall one transmitter, it might be more prudent to calibrate the transmitter in-situ, thereby eliminating transmitter removal, equilibration, reinstallation, rezeroing the next day and the “As Found” shift. I suggest that the time necessary to calibrate a transmitter might be reduced to two hours instead of all day, and involve considerably less work.
This sounds good on paper, but the plant did not have a portable calibrator so the technician continued to calibrate these flowmeters in the shop. I suggest a day may come when one of the parties realizes that the transmitter shift generally benefits the other party. This tends to create acrimonious feelings between the parties, especially when the calibration shift cannot be used to make accurate corrections to the steam invoice. Understanding the problem, potentially undesirable consequences and unproductive legal expenses can be avoided by using appropriate calibration techniques.
Often you can lead a horse to water, but you cannot make it drink.
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.
