Check and take measures for output signal exceeding full scale value
Figure 7-7 The flow of checking the over-full-scale value of the output signal of the electromagnetic flowmeter (continued) 4． Check the lower (rear) gauge. Check item 4 of the flowchart. The output flow signal of the electromagnetic flow converter is transmitted to the lower-level instruments such as the flow totalizer and the flow computer. If the rear meter is electrically connected (ie active load), the power feedback on the load will damage the output circuit of the converter, and the output signal will be over-full. Figure 7-7 Electromagnetic flowmeter output signal over-full value inspection process (continued) 4. Check item 4 of the next (rear) meter check flow chart. The output flow signal of the electromagnetic flow converter is transmitted to the lower-level instruments such as the flow totalizer and the flow computer. If the rear meter is electrically connected (that is, an active load), the power feedback on the load will damage the output circuit of the converter, and the output signal will exceed the full-scale value, so electrical isolation measures must be taken. The converter output circuit has two types: allowable grounding and not allowed grounding. If the grounding is allowed, the output still exceeds the full-scale value, and the converter is faulty; if the grounding is not allowed, it can operate normally as long as the grounding is removed by mistake. 5. Check item 5 of the electric interference check flow chart introduced from the liquid. In the absence of excitation current, use a multimeter or oscilloscope to detect the interference potential at the two electrodes. This failure phenomenon often occurs in the sodium chloride electrolysis process of the soda industry and on the cathodic protection pipeline. Measures can be taken to insulate the electromagnetic flow sensor from the pipeline, so that the electrode and the liquid are at the same potential. 6. Checking the converter itself The cause of the output signal over-full-scale value caused by the converter itself is more complicated. It can be caused by a link in the circuit of each unit in the converter, and there are big differences due to the type (analog or digital). For general use units, the current electromagnetic flowmeter circuit board can be divided into interchangeable and independent units, and a test replacement circuit board (or temporarily borrowing a circuit board of another normal instrument of the same model) can be used to check and judge by substitution. First check item 6 of the flowchart, that is, check the input/output circuit. According to the respective characteristics of the two types of analog circuit converters and digital circuit converters, focus on checking several links. The analog circuit converter should be based on whether the feedback loop is open and whether the output loop is damaged; the digital circuit converter should be A /D conversion circuit and output loop analysis are the main inspection links.
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