consultant engineer at TÜV SÜD National Engineering

Laboratory, takes us through the various flow meter calibration methods available, and the factors that determine which one to choose


alibration is not an absolute operation, but instead a comparison between the

reading of a device and that of a reference standard. It is therefore necessary to consider what properties are required from a standard. For flow measurement, the standard is a system comprising a measure of quantity and the subsidiary measurements to determine the fluid conditions, properties and influence factors. To provide confidence that the measurement taken by the standard is accurate, all the measurements in the system must demonstrate traceability to higher level measurements and ultimately to national and international standards. To correctly express the ‘performance

accuracy’ of a standard or a calibration, the ‘uncertainty’ must be determined and quoted. For flow measurement the confidence in the result lying within the uncertainty is normally quoted with a ‘coverage factor’ of k=2, which is approximately 95 per cent confidence level. All calibration results should have a stated uncertainty and ‘coverage factor’ on the calibration report or certificate.

Calibration fluid and conditions All flow meters are affected to some degree by fluid properties and velocity profiles. The nature of how flow meters interact with the flowing fluid is affected by the properties of the fluid or the velocity distribution of the fluid passing through the device. When a fluid passes through a pipe, the distribution of velocity across the pipe alters, depending on the pipe’s internal diameter, roughness


and fluid Reynolds number. The presence of bends and valves can introduce asymmetry to

the velocity distribution and potentially swirl. All of these effects must also be considered in the calibration. Ideally, the calibration should be

completed using the same fluid and pipework configuration within which the meter will normally operate. In reality, this is seldom possible. Whilst a calibration laboratory will try and match process conditions as much as possible, some degree of disturbance to the meter is inevitable.

Frequency of calibration? Specific industry standards or a third-party (regulator or trading partner) dictate the calibration frequency. In this case the meter is calibrated at specific intervals and it is assumed to be accurate between calibrations. For most applications however, it is the

user who must define the calibration interval and the calibration methodology. The interval is normally chosen to minimise the risk of an incorrect meter reading having a significant impact on the process. For example, high flowrates of oil attract huge tax liabilities and could possibly require weekly in-situ calibrations of the meter. The methodology dictates whether the device is calibrated in-situ or at an accredited

The TÜV SÜD National Engineering

Laboratory is a global centre of excellence for flow measurement and fluid flow systems

laboratory and even whether the reference is mass or volume. Other factors affecting the decision are

the history of the meter and maintenance periods. Whatever the frequency, it is always good practice to keep calibration graphs and control charts of the meter performance as this will assist in determining calibration intervals and demonstrate meter performance over time.

Methods for liquids and gases Common flow calibration standards are usually classified as being ‘bucket and stopwatch’ systems. The ‘bucket’ is a container which is weighed or has a known volume. The ‘stopwatch’ is a method of measuring the time to fill the bucket. The ‘Standing start and stop’ calibrations

of flow meters are the simplest method available and can be used for both high and low accuracy calibrations. The quantity of fluid collected is measured and compared with the meter reading, and combined with the time to fill, to give the flowrate. The ‘flying start and finish’ method is

sometimes called the diverter method, where the flow through the meter is not stopped but continues uninterrupted and is physically diverted between a return path to the liquid supply tank and the collection container. A switch on the diverter mechanism starts and stops a timer and a pulse totaliser. Beyond ‘bucket and stopwatch’ systems,

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