The Accuracy of Open Channel Flow Measurements in Industry
Risto Kuoppamäki
Abstract:
Open channel flow measurements are largely used in the process industry to measure waste water discharge. Most measurements use open channel venturi or weir as the measuring device. Demands for sound measurement accuracy from process control, environmental quality systems and environmental protection authorities are constantly increasing. Indmeas has for years been engaged in quality maintenance of the waste water flow measurements of the Finnish process industry. The basis of quality maintenance is repeated field calibrations. The reference value for an open channel flow is determined by using a radioactive tracer and the dilution method. The quality assurance of the method is obtained in the field by regular comparisons against the accredited transit time tracer method. The calibration results, altogether more than 700, give a representative picture of the accuracy of noncalibrated open channel flow measurement positions, on the critical parts of the measurement chains and on the accuracy level that can be reached by reasonable efforts. The most critical part in the open channel measurement chain is the water level measurement that is most often carried out by using either a pressure or an ultra sound measurement. Both have their own operational limits and maintenance problems. Measurement stability may also be disturbed by sludge sedimentation on the channel bottom. Other uncertainty components are most often constant with time and can therefore be removed by a field calibration. Experience has shown that a measurement uncertainty level of ± 5 % can be reached in a fairly straightforward manner by using field calibrations.
Keywords:
Flow measurement, waste water, accuracy, field calibration, quality assurance
