In addition to various physical principles for determining the flow rate of the fluid in the line, preferably in a circular pipe, there is the use of ultrasound waves which are transmitted into the line. The flow rate of the flowing fluid and thus of the flow can be determined by means of the time of flight difference method.
In this respect, ultrasound waves are transmitted and received by a pair of ultrasound devices, in particular ultrasonic transducers, wherein the ultrasound devices are arranged mutually opposite at a wall of the line at the ends of a measurement path obliquely to the main direction of flow or to the flow of the fluid.
The ultrasound waves transported through the fluid are accelerated in the direction of flow and are decelerated against the direction of flow. The resulting time of flight difference is calculated using geometrical parameters to form a mean speed of the fluid from which the flow rate of the flowing fluid is determined.
An important and demanding area of application is represented by gas meters for natural gas pipelines in which, due to the huge gas volumes conveyed and to the value of the resource, even the smallest deviations in the measurement precision can result in highly noticeable value differences such as between actually conveyed volumes and measured volumes. This results in high monetary differences. The above-named measurement apparatus are used in this field of the measurement of large gas volumes due to their accuracy, freedom from servicing and self-diagnosis possibilities in gas transport and gas storage.
Since an ultrasound measurement path only samples the flow rate at defined positions, ultimately the mean flow rate over the total flow cross-section is approximated. High accuracies can therefore only be achieved if the flow is easily reproducible or has an undisturbed flow profile or if a plurality of measurement paths are able to resolve the irregularities.
On the use of a plurality of measurement paths at circular lines or pipes, the ultrasound devices can only be arranged at the wall of the circular lines such that diametrical measurement paths result. This means that the measurement paths intersect at the pipe center so that the measurement always takes place through the pipe center. A measurement signal hereby runs through a flow cross-section having the most speed gradients since a speed distribution of the fluid in a circular line is a parabola with a laminar flow. The measurement signal can thereby only deliver an imprecise average of the flow rate.
For this reason, non-round flow cross-sections in a measurement section of the measurement apparatus is of interest for a sampling of the flow profile.