For Titan and its customers, the calibration and re-calibration of flowmeters frequently crops up as a topic of conversation. In order to ensure reliable performance over the long term, calibration is essential. The frequency of this calibration depends on the duty cycle to which the flowmeter is being subjected.
Importance of Flowmeter Calibration
Over a period of 3 years, the change in performance of an oval gear flowmeter used to meter water is shown in the graph below:
For this kind of mechanical meter, the characteristics of the changes are expected. The profile of the calibration shift depends on the operating conditions and the type of flowmeter.
Oval Gear Flowmeter
For lubricating fluids and oils, oval gears are the flowmeter of choice. This oval gear meter was designed to be used with aqueous surface finish products.
Over time, in the absence of a lubricant, the rubbing surfaces and bearing of the oval gear flowmeter “polish in”, leading to a lower coefficient of friction and improved efficiency (especially at flows in which the mechanical drug is a factor of greater significance – particularly in the absence of oil’s lubricating properties).
This is displayed in the red curve, which demonstrates change in performance. The calibration is only approximately 0.25% at maximum flow, which, for many applications, is insignificant. However, the change is almost 2.5% at minimum flow, which may cause issues for users.
The meter used here is not used for primary measurement, only relative flows. Consequently, the curve’s overall shift does not hold a great significance, which explains the extended recalibration period.
Key Considerations of Flowmeters
Highlighted in this application are some of the key considerations of flowmeter types, their usage, how important their performance is, and issues concerning the recalibration period.
Over time, the error curve generally reverses its shift from positive to negative as the flowmeter wears, causing clearances to increase. The result of this is that the bearings are no longer optimum. A lower K factor is likely to be given by the low end, displaying a negative change relative to the original calibration.
After a while, the gears of the flowmeter will begin to wear. Alternatively, their cavity walls and the top end will begin to exhibit significantly lower efficiency. As a consequence, the flowmeter would be ruined.
If the flowmeter had been sent back to the manufacturer to be checked up prior to this damage happening, it may have been possible to fit a replacement set of gears. However, after the cavity has suffered damage, this option is no longer possible.
In theory, electronic flowmeters provide the advantage of not needing any mechanical parts which are liable to wear. This can guarantee longer performance stability. However, other factors are still able to influence calibration, so users should not be complacent.
Over time, bores are liable to change dimension as a result of deposits or corrosion, and electronic components may alter their characteristics. Clearly, this last point is more critical if you are operating with a smaller flowmeter.
Data from a 1 mm bore flow device is shown below. Fluid velocity is fundamentally the measurement method used. Therefore, an alteration of just 0.05 mm in the diameter of the tube would change the area by more than 9%, leading to a corresponding alteration in flowmeter reading. Depending on the type of meter (Reynolds number, etc.), there could still be a 0.1% shift, even on a 100 mm tube.
The curves shown above represent the annual calibration check of a miniature ultrasonic flowmeter. The curve’s shit is +0.15% and, at the very low end, -0.3%. Clearly, included in this calibration is the calibration rig uncertainty together with the flowmeter repeatability.
The ultrasonic flowmeter is noticeably superior to the mechanical flowmeter, before the longer service interval of the oval gear meter is even taken into account.
Steps to be Considered for Flowmeter Calibration Intervals
Some things to consider regarding flowmeter calibration intervals:
- Flowmeter purpose: is this measurement process-critical, whereby the process could be compromised in some way by a change of meter characteristics, or expenditure could be increased?
- Application: is the process and fluid aggressive or benign? Does the flow meter have to operate at its operational limits? Could calibration be affected by corrosion or deposits? Could the measurement be in any way compromised by certain elements of the fluid (for example, particles in suspension)?
- Flowmeter type: is the type of meter used liable to change performance for any reason? Is this just a visual aid, or is it electronic or mechanical?
- Historic data: have previous re-calibrations been accurate, and if so, to what degree? Can or should the recalibration interval be increased or reduced without risking the overall process?
- Have the measurements undergone any noticeable changes? Certain flowmeters are able to self-monitor and advise when something is amiss. This facility is not offered by the majority of traditional flowmeter types, however, alert operators can notice changes and monitor these for cause and effect.
Re-calibration periods are often liable to be dictated by manufacturers. However, as the advice above shows, the user should be the one who determines the re-calibration period after considering their unique conditions.
The manufacturer knows best concerning their flowmeters’ long-term limitations, so they should still be consulted by the user. However, it is likely that the final decision will be a moving target – at first at least, until the reliability and entire operation of the system is fully understood.
This information has been sourced, reviewed and adapted from materials provided by Titan Enterprises Ltd.
For more information on this source, please visit Titan Enterprises Ltd.