Water utility does its own verifying of magmeter accuracies

The City of Mesa's Water Division in Arizona decided it was time to replace aging 12-inch turbine meters at 36 well sites. Involved was metering of this ground water being pumped into distribution mains. The existing turbine meters were getting difficult to maintain, which could affect their accuracy. They were replaced with a brand of magnetic flowmeters ("magmeters") that the division had used successfully for other metering services over 20 years.

Each year, the Water Division sells over 30 billion gallons of treated well and surface water. For operating efficiency and custody transfer, it needs high, sustained metering accuracy. Magmeters met this need well, with an inherent calibrated accuracy up to ± 1 percent of rate -- sustained for many months in service.

Calibration Tool
At the time the division purchased the well water meters, it found it could justify also purchasing a calibration verification tool, tradenamed CalMaster. This portable, PC-based system was designed for use with MagMaster brand magmeters offered by ABB Instrumentation and bought for the well metering service.

The CalMaster purchase included factory training of a division staff member. Thus the division gained in-house capability to verify and re-certify the calibrated accuracies not only of the 36 new well meters but also all its MagMaster meters that existed on a variety of applications.

Previously, the City had contracts with ABB to have a service specialist come in every six months to use his CalMaster to verify and re-certify nine MagMaster meters. Accuracy of these was especially important because they were being used for custody transfers, involving large wholesale purchases. The cost for this service was about $17,000 a year. Thus, when the City purchased the 36 well meters, the scale easily tipped in favor of doing its own in-house calibration checks.

Checking an Installed Meter
Each MagMaster primary and its transmitter are factory-calibrated as a matched pair. The two are shipped with a memory card that contains a "fingerprint" of all pertinent calibration data for this specific meter, including the shielded cable that carries the small millivolt measuring signal from the primary to its transmitter.

A typical setup for a CalMaster on site test is shown in Figure 1. The magmeter primary stays in operation on the water line and connects by shielded cable to its stand-mounted flow transmitter. Flow measurement is never interrupted.

Hookup of test components is shown in Figure 2. They comprise: (1) a PC laptop that contains the meter's memory card and (2) the CalMaster control box with its two cables to connect the transmitter to the control box, and the box, to the laptop. This PC contains software to carry out a 10- to 14-minute, preprogrammed verification procedure. When the operator initiates the test, the PC carries out a check of the entire system -- meter primary, its transmitter, and connecting cable.

To confirm that the test results are within set standards, the test operator reviews them on a PC display. If the results are positive, a certificate of validation for the tested metering system can be printed out. Providing the customer with a copy of the certificate authenticates the billing.

The CalMaster test can also provide a valuable diagnostic tool 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 screen. Thus, a gradual degradation in a component, such as a magmeter coil, can be detected and the component scheduled for repair or replacement.

Figure 1 - At one well site, Mesa's Jim McCarter prepares to start a magmeter calibration procedure at portable PC. Magmeter and its transmitter are at extreme left. CalMaster control box (arrow) rests on the ground.

Figure 2 - Schematic of CalMaster hookup for verification test. Shielded cable is the permanent link between the meter primary and its transmitter; thus, test does not interrupt flow measurement.

ABB (www.abb.com/) is a leader in power and automation technologies that enable utility and industry customers to improve performance while lowering environmental impact. The ABB Group of companies operates in around 100 countries and employs about 108,000 people.