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Expert advice on flowmeters

Charles Wemyss lists 10 reasons why you should – and should not – calibrate your flowmeter


e use the word flowmeter to describe a device that measures the flow of a fluid. Mostly we’re considering gases or liquids in a closed pipe or conduit and we

need either the instantaneous flow rate or the total amount of fluid that has passed. There are many varieties of techniques dependent on the fluid being measured and

dependent on the flow rate, pressure, viscosity and more. The flowmeters range from miniature positive displacement devices to large electromagnetic or ultrasonic units used for pipes over 3m diameter. The way we garner confidence in the displayed value is through calibration. Most flowmeters are supplied by the manufacturer

with a ‘laboratory’ calibration. In other words, they have been tested in close to ideal conditions. Depending on the meter type, once installed in your process, that original calibration may be valid – or it may not be.

WHY SHOULD YOUR FLOWMETER BE CALIBRATED? There are a number of key reasons why it should be calibrated:

n To reflect the new, current conditions n Because some component has a wear factor n There is an accumulation of dirt or setting product, affecting the sensor.

n Because the calibration frequency states it has to be n Because the results don’t feel right compared to the rest of the process

n The process is producing poor quality product yet the flowmeter seems stable

The best calibration is that which is performed

in situ. Many of the variables are tuned out. The fluid is the same, as is the installation attitude, straight lengths, etc. That is the precise reason why engineers should re-calibrate; it gives them that confidence in the device. If in situ is not possible – for example, when the fluid is hazardous or at high pressure – then it has to be uninstalled and calibrated elsewhere.

Litre Meter’s latest rig is designed for calibrating ultra-low flowmeters


WHY SHOULDN’T IT BE CALIBRATED? Clean versus dirty is the first argument for not calibrating your flowmeter. If it comes out of the line dirty and is sent away for calibration then you would normally expect to ship it clean. The test laboratory calibrates it in the clean state.

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