A new approach to accurate water flow measurement


Water measurement and management are critical to conservation and sustainability efforts. Many existing water treatment and distribution systems that need to upgrade their technology cannot shut down or stop service to the communities they serve, resulting in unmetered lines or inaccurately metered systems, leading to costly water loss.

McCrometer’s solution is the FPI Mag®, a full profile insertion electromagnetic meter offering the same accuracy of traditional full-bore and flanged mag meters — but with added benefits. The FPI’s hot-tap design allows for installation while systems are running at normal capacity, preventing costly construction and shutdowns. When total ownership costs are considered, the FPI can save as much as 45% compared to other full-bore mag meters. Additionally, the long-term benefits of accurate flow measurement and critical flow data are unparalleled.

The claimed accuracy for a calibrated FPI Mag meter is ±0.5% for velocity from 0.3 m/s to 9.8 m/s, and ±1.0% from 0.1 m/s to 0.3 m/s.

Accuracy testing was done at Utah State Water Research Laboratory (USWRL), a well-known water laboratory with a wide range of line sizes and achievable flow rates. A 30″ FPI was tested due to the sheer volume of water a 30″ line carries. Accuracy matters more in larger pipe sizes; if measurements are off by 1% in a small line, the meter would be off maybe a thousand litres. In a 30″ line, the amount of water flowing is much greater, so a 1% error can mean many thousands of litres of water unaccounted for, leading to significant revenue loss.

Testing was done over a wide 22:1 turndown with 23D upstream piping and 14D downstream piping, simulating an infinite straight pipe section to establish baseline meter performance without obstructions. Flow rates reported by the FPI Mag were compared against the flow rates reported by a 20″ master Venturi flow meter owned by USWRL for the moderate and high flows. Flow rates reported by the FPI Mag were compared against a traditional 12″ full-bore mag master meter owned by USWRL for the low flow rates.

One of the pipe systems was reduced from 30″ to 20″ to employ the Venturi, and again reduced to 12″ to employ the full-bore mag. Not only did the FPI Mag measure the full range of the Venturi and full-bore mag, it did so without needing to reduce or change the pipe system. The FPI Mag allows operators to place the meter where they need it without making modifications to the piping system.

Tested accuracy of the 30″ FPI Mag compared against the master flow meter is better than ±1.0% for velocities between 0.1 m/s to 0.3 m/s. Tested accuracy above 0.3 m/s was better than ±0.5% within uncertainty of the system. For the test point at 1.58 m/s, the tested FPI Mag accuracy was 0.66%. This is less than the total accuracy of measurement of 0.71% for the combined system.

FPI size
Min flow
Max flow
Min velocity
Max velocity
Turndown Upstream
30" 1904 41262 0.07 1.58 22:1 23 14

Table 1: FPI Mag test protocol parameters.

Results of the 30″ FPI Mag test prove the FPI Mag meets the accuracy claims. The measured error of the 30″ meter was equal to or better than the specified accuracies for a wide range of flow. Not only was the meter accurate at low flow rates where accuracy is often difficult to achieve, but it was also very accurate at high flow rates where inaccuracy equals lost revenue.

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