Five considerations for flow meter re-calibration intervals
Flowmeter re-calibration and calibration is a regular topic of conversation between Titan and its customers. Calibration is crucial for assuring reliable performance over the longer term and the frequency of the calibration will depend on the duty cycle the flow meter is being subject to. The graph below shows the change in performance over 3 years for an oval gear flow meter used to meter water.
The characteristics of the changes are typical for this type of mechanical meter and the profile of the calibration shift will depend on the flowmeter type and the operating conditions.
Oval gears are the flowmeter of choice for oils and lubricating fluids, this oval gear meter was made for use with aqueous surface finish products. Without a lubricant, over time, the oval gear flowmeter 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 recalibration period. This application highlights some of the main considerations of recalibration period, flowmeter type, usage and how critical the meters performance is.
Typically, over time, the error curve will reverse its 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 flowmeter gears will start to wear or the walls of their cavity and the top end will start to show much lower efficiency. Consequently, in such a scenario the flowmeter would be ruined. Had the flowmeter been returned to the manufacturer for a “check-up” before this damage a replacement set of gears may have been possible but once the cavity is damaged this option is not a possibility.
Theoretically electronic flowmeters offer the advantage of no mechanical parts to wear. This often ensures longer performance stability but users must not be complacent as 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. Below is data from a 1mm bore flow device and the measurement method is fundamentally fluid velocity so a 0.05mm change in the tube diameter would change the area by over 9% with a corresponding change in flowmeter reading. Even on a 100mm tube it could still be a 0.1% shift depending on the meter type, Reynolds number etc.
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 flowmeter above even considering the longer service interval for the oval gear meter.
Considerations for flow meter re-calibration intervals:
- Flowmeter purpose: is this a process-critical measurement where a change of 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 best 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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