Calibrating flowmeters

Getting the kind of accuracy you want isn't easy, but it can be done.

01/14/2011


Dear Control Engineering: I saw a photo of a flowmeter testing setup that seemed hopelessly complicated. What’s the point of that, and is it really necessary?

ABB’s “Monster” flow calibration rig in Shanghai The photo you saw may have been the new ABB testing facility in Shanghai, China. ABB has a large instrumentation manufacturing facility there, and that normally requires appropriate testing capabilities. In this particular case, they have built in support to test everything from ½ to 96 in. diameter flowmeters, so that means they need the ability to control flow precisely from 0 to 50,000 gpm.

Testing flowmeters is a difficult process, because there is no practical way to do it without simply pushing whatever volume of liquid you need through the device. To make matters worse, you have to calibrate one flowmeter with another one that is even more accurate. If you follow the old rule of thumb, you need a measuring device that is capable of ten-times the accuracy of the unit you want to check. In other words, if you have to calibrate a flowmeter to ±1 gpm, the flowmeter that is your standard has to be able to read ±0.1 gpm. Given the high accuracy of instrumentation these days, that can be a serious challenge. I have seen flowmeters that are used as calibrators that are capable of accuracy of ±0.015%. Once you get to this level, calibration involves measuring liquid volumes and other ways to boost the practical accuracy of a device.

But that’s just the start. The dynamics of liquid flowing through a pipe are messy with turbulence, velocity profiles, entrained gas, and the like. If you study that picture, you’ll see long sections of straight, horizontal pipe that minimize those effects. Each pipe joint will be smooth to avoid any obstacles that will disrupt laminar flow. I’ll even wager that the water itself is calibrated, in that its chemical composition is analyzed, its precise density known, and temperature regulated. There will also be critical weighing apparatus to measure specific volumes of water. All of these are there to eliminate any variability to the process.

The new lab in Shanghai is approved by the Shanghai Institute of Measurement and Test Technology (SIMT), which is itself accepted by a group of international testing bodies, including UKAS, NIST, DKD, NVLAP, NATA, and ILAC. All things considered, it is a very complex process but necessary to deliver the kind of accuracy that users demand.

Peter Welander, pwelander@cfemedia.com

Also read:

Custody transfer—flowmeter as cash register

Advances in flowmeter technology

Flowmeter challenge: Right size, right design



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