Malfunctioning field devices can cause process disturbances and accounting errors. Self-monitoring in field devices can diagnose malfunctions at an early stage and enable corrective action to be taken in good time.
Drinking water of high quality and in reliable supply is something that many people take for granted. Closely monitored and controlled processes are necessary to guarantee this quality and availability. These include precise and reliable flow measurement.
Today, electromagnetic flowmeters (EMF) are used in all stages and processes of drinking water procurement and treatment. EMFs exhibit high precision as well meters, in sea water desalination and in surface water or shore filtration. They also operate correctly with untreated water that often contains high ferromanganese content. Because EMFs have no mechanical moving parts to be slowed or obstructed by sludge deposits, they operate accurately and practically maintenance-free under these arduous conditions.
For the water supply authority flowmeter errors can result in incorrect volume measurements, billing errors and also higher costs due to overdosing of disinfectant, flocculants or other agents.
Classic methods of flowmeter testing are expensive and unsatisfactory. They often involve taking the meter out of its normal operating environment and at best only show the accuracy at the time of testing, rather than on an ongoing basis.
Self diagnosis
The most effective method of flowmeter testing involves continuous self-testing during operation.
The precision of EMFs is unaffected within wide ranges by distorted flow profiles. But very severe interference with the flow profile can cause noticeable metering errors. This can occur when the required unimpeded inlet section (5 diameters) is not provided, flaps that can cause extreme distortion of the flow profile over very long distances are installed upstream of the EMF, or when gaskets protrude into the flow on one side.
Optiflux 2300 detects flow profile distortion with its integrated flow profile monitor by periodically briefly reversing the polarity of the magnetic field in one of the two field coils (see Figure 1). This produces two opposing fields. The field in the top of the tube determines the flow rate in the top half of the tube positively. The field in the bottom of the tube determines the flow rate in the bottom half of the tube negatively because its polarity has been reversed. The electrode voltage is the sum of these two values. The voltage for this test phase is only zero when the flow rates in both halves of the tube are the same.
Figure 1. On the left: magnetic field during normal flow measurement. On the right: the half inverted magnetic field for detecting distorted flow profiles
Even deposits on the bottom of the tube or partial filling of the measuring tube are therefore detected and signalled in this way.
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