Expert advice on flowmeters and calibration

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

We 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.

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

There are a number of key reasons why it should be calibrated:

* To reflect the new, current conditions

* Because some component has a wear factor

* There is an accumulation of dirt or setting product, affecting the sensor

* Because the calibration frequency states it has to be

* Because the results don’t feel right compared to the rest of the process

* 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’s the precise reason why you should re-calibrate; it gives you 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.

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’d normally expect to ship it clean. The test lab calibrates it in the clean state. However, as soon as you re-install it the process might be depositing dirt back on it. It has been calibrated for a perfect installation and is almost immediately imperfect.  In this scenario, calibration is pointless.

Next, it’s hard to compare installation to installation. All calibration laboratories pride themselves on making adequate provisions for calibration, especially good installation practice. If they’re testing a turbine meter, for example, then they should have a long length of correctly sized piping before the meter – and a length after, too. This eliminates swirl, if it’s long enough, to generate a flat flow profile and present optimum conditions to the meter. Most labs have this setup for horizontal installation – so if you have a vertical install, then watch out. Likewise, if you don’t have a long length of correctly sized pipe, or perhaps a connector that necks the diameter down a few percent, then don’t bother. The results they give you will be meaningless.

The Litre Meter low flow rig FlowLabPro delivers automatic calibration of flowmeters and instrumentation within a flow range of 0.0006 to 200 l/hr to an accuracy of ±0.2%

Next you should ask whether it is the right fluid. Unless your process is running clean water or, maybe a calibration fluid, then your average lab will not be able to calibrate with the same fluid. For some flowmeter types this may not be important. For example, if you fluid is a weak acid with a viscosity of 1.2cP and the meter is an electromag, then the calibration with water will be perfectly valid. Contrarily, if you have 10cP process fluid and it’s a turbine meter then it could be very important that the test fluid is in the 9 to 11cP range to adequately represent the effect of viscosity on meter performance at lower flows.

Traceability is next on the list. If you have been able to clear the hurdles above then it’s important you pick a lab that has the right traceability for you. If your process demands an indication of flow within +/- 4% then there’s little point on getting a UKAS-accredited laboratory with an uncertainty level of 0.22%.

We’re regularly asked ‘how often should it be calibrated?’ Recalibration periods of flowmeters are based on industry standards. In industrial applications, depending on the industry, periods of six to 12 months are recommended. We advise the user to seek out data relating to the process, other components within the process and the usage of the meter. If the measurement is critical then the recalibration should be more frequent than a non-critical, rarely used device. In the absence of any other data we advise an annual check and to vary the future calibration periods depending on results.

If it has remained unused then no recalibration may be necessary, depending on the meter type. It is wise to check that no fluid has settled in the meter that might alter the way the meter works or even cause corrosion. In the event of any doubt then the manufacturer is always your best source of advice.

{originally published in International Process Engineer in May 2016, www.engineerlive.com}

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