Calibrating Flow Meters

There are numerous flow meter calibration systems. Each method has its own pros and cons and the best method to select will depend upon the size of the meter and the required outcome.

Flow Meter Calibration Techniques
Bucket and stopwatch

This is a commonly used calibration method for a quick flow meter check and for lower flows it is quite a viable method. It is less suitable for extremely low flows or volatile or aggressive liquids as evaporation will cause serious errors and safety must be paramount. Measuring the volume, or preferably the mass of a fluid, of known density, is crucial and the time spent highly relevant. If the steady rate flow can be started and stopped very quickly even better. Ideally a relatively large volume should be metered over a long time period as this reduces the errors. A set of calibrated scales with a suitable capacity for the speed of discharge over the time period can be used. For example a ½-inch meter running at 20 litres per minute of water would require a set of scales with a capacity of perhaps 50 Kg. This would allow for a two minute flow run and some “tare” for the weight of the water container. Ideally the scales should have a 0.1Kg resolution so that the recorded resolution would be ±0.1 in 40 Kg i.e. ±0.25%. Factoring in possible timing errors over a 2 minute run i.e. ±1 second in 120 the combined uncertainty becomes ±1.1% which may be acceptable for a quick check. From this crude example it is easy to see how inaccuracies and uncertainties build up even when care has been taken with the set-up and execution of the test.

Flow Meter Calibration Techniques
Flying start/stop

One of the oldest methods of flow meter calibration is an extension of the ‘bucket and stopwatch’ approach where a custom diverter is coupled to an electronic clock and counter system. A “knife” edged diverter cuts a narrow stream of fluid and simultaneously starts a counter and timer at the centre point of the diversion. It could also be a diverter valve which is specially manufactured to ensure no change in flow during diversion. In either case the flip-over must be quick and consistent to guarantee repeatability (see figure 1 below). The chart below (Figure 2) shows the importance of consistency during the changeover. Ideally the lost and gained volumes of fluid are equal and opposite. Using this method, the faster the diversion the less inaccuracy in the diverted volumes.

 

flow meter calibration flying start stop

Figure 1

 

flow meter calibration - diverter

Figure 2

Flow Meter Calibration Techniques
Volumetric measurement

For many applications a volumetric measurement can be made and compared to a total recorded on your flow meter. Such volumetric measurement vessels are usually calibrated with a narrow neck for better resolution and accuracy at around the vessels designated volume. Volumetric measurement vessels are calibrated at a given temperature and sometimes given a temperature coefficient for variations from the reference point. Figure 3 below shows an old weights and measures approved ½ pint flask showing the +3ml / -3ml acceptable tolerances. This particular flask is rated at 20°C.

flow meter calibration - volumetric flow measurement

Flow Meter Calibration Techniques
Piston provers

Piston provers are fundamentally as simple as they sound. They are not unlike a hydraulic ram with a linear encoder attached to it. The diameter of the piston is known and the encoder will give a certain number of pulses for every litre of liquid dispensed. The drive system for a piston prover can be pneumatic, hydraulic or even a stepping/servo motor. It does not matter as long as the output is continuous and controllable. The clever part is the way the output from the encoder and the flowmeter are dealt with. In a piston prover the two pulse counts may not be synchronous and one has to act as a gate for the second. The primary is usually the linear encoder and constant flow is essential for accurate interpolation. The lag between the first pulse from the flowmeter after the encoder “start” pulse, as well as the corresponding stop signals after the predetermined volume of fluid or number of pulses have to be recorded. The actual number of pulses can then be determined for a known volume even though the two signals are not synchronous. It is important in piston provers that the actual pulse widths are constant as irregularities will cause “jitter” in this double timing system. Corrections can be applied for temperature and pressure variations.

Flow Meter Calibration Techniques
Pipe/Ball provers

These are used where other calibration systems are impractical or high accuracy is required such as custody transfer stations handling oil products. They can be fixed installations or even skid mounted and driven to a site, perform a calibration and be driven away again. Like the piston prover a “displacer”, often a sphere, is introduced into the pipe and pushed by the fluid passed a start and stop detector positioned a set distance apart. The swept volume is determined by calibration. Pipe and ball provers can be uni- or bi-directional depending on the valve arrangements. Typically pipe and ball provers are extremely accurate (±0.05%) and often used in large diameter pipe applications.

Flow Meter Calibration Techniques
Transfer standards

One of the easiest ways to calibrate an inline flow meter is to use a transfer standard. However for best accuracy this must be operated under the precise conditions under which the meter was calibrated. One of the advantages of this method is the fact that the flow meter under test and the reference meter can be in series in the same line. The accuracy of such a system is one step further away from a traceable reference but this is often an acceptable factor. Several companies offer meter hire for such applications.

Flow Meter Calibration Techniques
Calibration of flow meters at Titan

Titan Enterprises have used piston provers for flow meter calibration since 1994. Our primary rigs are rated from 0.004 to 40 and 160 litres per minute. With recent software upgrades our low end flow calibration has been reduced to 2ml/min. All Titan flow calibrations are traceable to national standards and have real time temperature compensation to accommodate changes in the fluid properties. Our piston prover systems are calibrated regularly and are pneumatically driven to ensure a constant flow rate.

For lower flows we have a bespoke micro piston prover. This is driven by a very high resolution servo motor the speed of which can be controlled accurately with no pulsations in the axle speed. The ball drive to move the small bore piston is driven by a precision toothed belt through a pair of reduction sprockets. Like the main flow rigs the volumetric throughput is traceable to national standards via a water draw test. Flows as low as 0.1 ml/min may be calibrated using this rig.