Calibrating flow meters and subsequent re-calibrations are crucial for assuring reliable flow-meter performance, over the longer term. The required frequency of the flow meter calibration will depend on the duty cycle that the flowmeter is being subject to. A new technical note discusses the differing calibration requirements for different types of flowmeter and offers 5 key considerations to help you determine the appropriate interval between calibrations for your flow system.
Calibrating Flow Meters
Oval geared flow meters are the flow-meter of choice for oils and lubricating fluids. The oval gear meter represented above was made for use with aqueous surface finish products. Without a lubricant, over time, the oval gear flow meter bearing and rubbing surfaces “polish in” which results in a lower coefficient of friction. This results in better efficiency, particularly at lower flows where the mechanical drag is a more significant factor – especially without the lubricating properties of oil. This is highlighted in the red curve which shows change in performance. At maximum flow the calibration shift is only around 0.25% – which will be insignificant in many applications. However, at minimum flow the change is nearly 2.5% which could result in a problem for the user. In this application the meter is only used for relative flows and not primary measurement, so the overall shift in the curve is not too important – hence the long flow meter re-calibration period. This application highlights some of the main considerations of the re-calibration period, flow-meter type, usage and how critical the meter’s performance is.
Typically, over time, the error curve will reverse it’s shift from positive to negative. As the flowmeter wears, clearances increase and the bearings are no longer optimum. The low end is likely to give a lower K factor and show a negative change from the original calibration. Eventually the flow-meter gears – or the walls of their cavity will start to wear, and the top end will start to show much lower efficiency. Consequently, in such a scenario the flow-meter would be ruined. Had the flowmeter been returned to the manufacturer for a “check-up” before this damage had occurred, the installation of a replacement set of gears may have been possible. However, once the cavity is damaged this option is not a possibility.
Theoretically, electronic flow meters offer the advantage of no mechanical parts to wear. This often ensures longer performance stability – but users must not be complacent – other factors can affect calibration. Electronic components can change characteristics over time and bores can change dimension due to corrosion or deposits. Obviously the smaller the flowmeter the more critical this last point is.
The above curves are for a miniature ultrasonic flowmeter on its annual calibration check. The shift in the curve is +0.15% and -0.3% at the very low end. This calibration obviously includes the calibration rig uncertainty as well as the flowmeter repeatability. This ultrasonic flowmeter can be seen to be considerably better than the mechanical flow meter above even considering the longer service interval for the oval gear meter.
Considerations for calibrating flow meters & intervals of calibration:
- Flowmeter purpose: Is this a process-critical measurement where a change in meter characteristics would compromise the process in some way, or result in increased expenditure?
- Application: Is the fluid and the process benign or aggressive? Is the flow meter working at its operational limits? Could deposits or corrosion affect calibration? Are there elements of the fluid that could compromise the measurement in some way e.g. particles in suspension?
- Flowmeter type: Is the meter a type which is likely to change performance for some reason? Is it mechanical, electronic or just a visual aid?
- Historic data: How accurate have previous re-calibrations been? Could/should the recalibration interval be reduced or increased with little risk to the overall process?
- Have there been any noticeable changes in measurements? Some modern flowmeters will monitor themselves and advise when something is not quite right. Most traditional types of flowmeter do not offer this facility but an astute operator may well notice a change and have it monitored for cause and effect.
Manufacturers will often try to dictate re-calibration periods. But as you can see from the tips above, the re-calibration period should be determined by the user – taking into account their unique conditions. Advice should still be sought from the manufacturer, as they will know what the longer-term limitations of their flowmeters are likely to be. However, the final decision is likely to be a moving target, at least initially until the whole system operation and reliability is understood.
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