Application: Water utility brings calibration in house
-- Control Engineering, 2/21/2008
The water department had two choices: Either keep paying a service contractor to calibrate flowmeters, or buy the same tool he used to do the job themselves. Their choice paid off.
After replacing aging turbine flowmeters with ABB magnetic flowmeters, the City of Mesa, AZ, water division had 36 new 12 inch devices to add to the calibration schedule. They had been calling an ABB service technician to calibrate other magnetic flowmeters, so the choice was either to expand the service contract or buy the necessary calibration tools and do all the magnetic units themselves.
Since those flowmeters are responsible for watching over 30 billion gallons of treated water annually, accuracy is critical and needs to stay around ±1% of rate month after month. Prior to installing the new units, the city had spent about $17,000 annually to bring in an ABB service technician every six months. When they added 36 more devices to the list, it was clear that they should be doing the work in house.
The maintenance team purchased a CalMaster calibration verification tool. It is a portable, PC-based system designed for use with MagMaster brand magnetic flowmeters offered by ABB Instrumentation. The purchase included factory training of a division staff member so they gained in-house capability to verify and re-certify the calibrated accuracies not only of the new well meters but also of existing MagMaster flowmeters distributed throughout the system.
Each MagMaster primary sensor and its transmitter are factory-calibrated as a matched pair. The two are shipped with a memory card that contains a "fingerprint" with all pertinent calibration data for this specific sensor, including the shielded cable that carries the small millivolt measuring signal from the sensor to its transmitter.
The primary sensor stays in operation on the water line and connects by shielded cable to its stand-mounted flow transmitter. The continuous flow measurement is never interrupted. The PC contains software to carry out a 10- to 14-minute, preprogrammed verification procedure based on the data contained on the memory card. When the operator initiates the test, the PC carries out a check of the entire system: meter primary sensor, transmitter, and connecting cable.
To confirm that the test results are within set standards, the test operator reviews them on the display. If the results are positive, a certificate of validation for the tested metering system can be printed out and filed in the maintenance records.
The CalMaster also provides valuable diagnostic tools for preventive maintenance. It stores a history of calibration data from successive tests, so that trend curves for selected system components can be plotted on the PC. Thus, a gradual degradation in a component, such as a magmeter coil, can be detected and the component scheduled for repair or replacement.
—Edited by Peter Welander, process industries editor, peter.welander@reedbusiness.com,
Process Instrumentation & Sensors Monthly
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