Calibration Concerns

Sometimes doing the correct thing with insufficient tools can get you into trouble. One client’s custody-transfer differential pressure transmitters are a case in point.

The transmitters are currently calibrated in the instrument shop at a comfortable temperature using a dead-weight tester. This requires the transmitter to be physically removed from the process and transported to the instrument shop. In addition, the transmitter needs to remain in the shop for a few hours, so its temperature approximates that of the shop before calibration. After calibration in the shop, the transmitter is physically reinstalled in the process and the zero is trimmed.

Not only can this process take approximately six-to-eight hours per transmitter, but it can potentially result in calibration shifts due to the movement and handling of the transmitter. In addition, the calibration is performed manually, so it is subject to human error. Approximately one-to-two days later, operating steam pressure is applied to both sides of the transmitter. The transmitter is trimmed under these conditions to compensate for the transmitter pressure effect (zero shift) at elevated pressure. The calibration process is repeated a few months later, and some transmitters routinely fail their calibrations. This has created some acrimony due to potential billing errors.

The problem appears to be that the transmitter calibrations are not documented after they are (potentially twice) trimmed in the field. Therefore, after the calibration interval, trimming the transmitter in the field can cause the shop calibration to disagree with the documented shop calibration prior to trimming — even if the transmitter calibration may not have changed since it was (correctly) trimmed.

This problem can be resolved by calibrating the transmitters in the field with a portable calibrator. This should reduce calibration time to approximately one hour per transmitter, include more calibration points, automatically document calibrations, eliminate the zero trim at reinstallation, reduce the probability of damaging the transmitter, and reduce human error. The transmitter pressure-effect trim adjustment could be performed immediately after the calibration. Documenting the “As Left” transmitter performance should eliminate billing issues when the transmitter has not shifted its calibration since being trimmed.

This problem has multiple negative results, including excessive labor requirements, multiple adjustments, increased potential for transmitter damage, and the potential for legal action. The proposed solution is straightforward, and it effectively resolves most of the negatives detailed here.

David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 30 years of experience in specifying, building, installing, startup and troubleshooting process control instrumentation. He has developed and taught seminars for over 20 years and is a member of ISA and belongs to the ASME MFC and ISO TC30 committees. Mr. Spitzer has written a number of books concerning the application and use of fluid handling technology, including the popular “Consumer Guide” series, which compares flowmeters by supplier. Mr. Spitzer is currently a principal in Spitzer and Boyes LLC, offering engineering, product development, marketing and distribution consulting for manufacturing and automation companies. He can be reached at 845 623-1830.

www.spitzerandboyes.com