Photo courtesy of ABB |
Calibration is used to ensure that flowmeters (and other instruments) are functioning accurately. However, both wet and dry calibrations have their problems. Wet calibrations that check the entire flow measurement system are generally not an option in most applications because challenging the installed flowmeter with a known volume of water is impractical --- especially in large pipes in remote locations. Even removing the flowmeter to perform a wet calibration in a flow laboratory can be expensive and time-consuming. Further, it does not take the installed piping into account.
On the other hand, dry calibrations verify the operation of only part of the flow measurement system. In the case of magnetic flowmeters, a traditional calibrator checks the transmitter by verifying that a given (millivolt) input produces the correct flow rate. What if the wiring is faulty? What if the coil has changed its characteristic? What if the flow tube leaked and shorted an electrode? Dry calibrations do not necessarily check for these problems.
Greg Livelli, Product Manager for Magnetic Flowmeters at ABB (Warminster, Penn.) says, “that the CalMaster magnetic flowmeter calibration tool is a portable PC-based system that performs a pre-programmed verification process of the entire flowmeter to include the flowmeter primary, liner, electrodes, interconnection cabling, and transmitter. This process entails comparing the measured flowmeter characteristics with characteristics that were generated when the flowmeter was manufactured.”
“The verification process takes about 15 minutes and occurs without interrupting the flow measurement. This is a key benefit to the user because it allows the user ultimate operational flexibility in when the test can be performed. A certificate of calibration is generated if the test results are within acceptable limits. In addition, a history of test results is stored to allow predictive diagnostics to identify problems before failure occurs. This allows the user to conduct preventative maintenance and prevent costly downtime.”
Many water authorities struggle with these issues including the City of Mesa Water Division (Mesa, Ariz.). Jim McCarter, Controls Engineering Specialist, says that, “ensuring accurate flow measurement systems is a prime concern. We have about 10 magnetic flowmeters that are used for custody transfer of water and wastewater that result in billings totaling millions of dollars. Needless to say, ensuring that these flowmeters are functioning accurately is important not only to the City of Mesa, but also to the City of Phoenix that treats sewage from the City of Mesa.”
Jim found that “traditional dry calibration of our magnetic flowmeters did not adequately verify that the magnetic flowmeter was functioning accurately. Simulators could check the analog signals and totalizer, but could not check the wiring and the condition of the magnetic flowmeter primary. On the other hand, inline wet testing was not an option and removing the flowmeter to perform testing in a flow laboratory was simply too expensive. Removal was also not practical because it required that the flowmeter be taken out of service for a significant period of time. The ABB CalMaster was selected as a compromise because it performs a dry calibration plus it verifies that the magnetic flowmeter characteristics have not changed since the flowmeter was manufactured. We currently use the CalMaster to verify our custody transfer magnetic flowmeters that have accuracy specifications of 0.5 percent of rate.”
CalMaster Schematic
