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Average flow velocity and flow cross section are required to determine the flow Q. This leads to the general formula:
The flow cross section A is determined by continuously measuring the filling level taking the channel shape as well as the channel dimensions into account.
The average flow velocity within the flow cross section is determined by using the velocities found in one or more measurement paths. The higher the number of measurement paths used, the higher the measurement accuracy.
The flow velocity measurement is based on the determination of the transit time (time of flight) of ultrasonic signals between two sensors (A and B).
Here, the signal transit time t1 in flow direction is shorter than the signal transit time towards the flow direction t2. The difference between both transit times is proportional to the average flow velocity along the measurement path vm.
The system computes the average cross-sectional velocity vA from the path velocity vm and indicates it directly on the transmitter display.
To measure fully developed flow velocity profiles in pipes
a 1-path measurement (1L1P) is sufficient. Normally, however, there are distorted flow profiles which have negative effects on the measurement accuracy.
Such effects can be compensated by using up to 32 measurement paths. Moreover, measurements with NivuFlow 650 transmitters comply with IEC 60041 standard. Depending on the measurement site varying sensor types are used.
Measuring in open channels and rivers is very demanding regarding the measurement systems used. From indefinable cross-sectional profiles to moving river beds we can find conditions which complicate the use of standard measurement systems.
Our transit time measurement sytems have been developed particularly for such cases and provide reliable results even at very difficult measurement sites.
For use in open channels there are up to 4 measurement paths available as well (32 in combination with extension modules). Right as in full pipes, the number of used measurement paths will significantly increase the measurement accuracy.
Various types of sensor constructions are available for flow measurement in open channels too. Special mounting accessories facilitate installation on site.
Accurate flow measurement requires precise and reliable determination of levels under all hydraulic conditions which is achieved by using 0/4 - 20 mA level sensors. For such cases use either contactless ultrasonic sensors such as „i-Series“ sensors or hydrostatic sensors such as NivuBar Plus.