Highest measurement accuracy despite difficult measurement conditions

When installing a flow meter, it is generally assumed that the measuring instrument will be fitted at a measuring point with a stable and undisturbed flow. In reality, however, it is often the case that achieving such ideal measurement conditions is difficult or even impossible. Disturbances in the flow profile can be caused by numerous factors, but are usually a consequence of changes to the pipe through which the fluid flows. This can be caused by a different slope or angle of the pipework due to bends, by a change in diameter caused by a reducer or diffuser, or by the installation of valves and pumps. These changes in flow conditions result in a shift in the flow profile, meaning that the point of maximum flow velocity is no longer located in the centre of the pipe.

Local changes to the flow profile resulting from these pipe modifications pose a problem for most flow meters installed in this area. The reason for this is that, regardless of the measurement technology used, it is assumed that the observed flow condition occurs at the point of fastest flow and that no other external influences are at play. This raises the question of how to recognise when flow profile disturbances occur and, secondly, how best to deal with them.

It should be noted at this point that certain measurement techniques are less susceptible to this effect than others. Coriolis flowmeters, for example, calculate flow rate and density based on the vibrations inside the pipe within the measuring element and, as such, are not affected by flow profile disturbances in the same way. Users of clamp-on flowmeters must – as with many other measuring instruments – take particular account of the flow conditions by ensuring that the instrument is installed in a suitable location.

To achieve reliable measurement results even under difficult flow conditions, dual-channel flowmeters are used. Care must also be taken to ensure the ultrasonic sensors are correctly positioned on the pipe. Guidance on two-channel flow calculations can be found on the right and in the download section of the website. All newly published operating manuals are also available for download there. The sections ‘Installation of the ultrasonic sensors’ and ‘Installation locations’ describe in detail what to bear in mind when selecting the section of pipe on which to mount the sensors, both in general and, in particular, in the vicinity of sources of interference.