5 methods for troubleshooting electromagnetic flowmeters

Case introduction

During the use of electromagnetic flowmeters, some faults will occur that affect the normal operation of the surface. How to find the factors behind the faults is a very important task. We need to do this Various types of inspection methods are used to identify them. Under normal circumstances, our routine inspection of electromagnetic flowmeters generally includes electrode contact resistance, electrode polarization voltage, signal cable interference, determination of the presence of ground potential and pipeline stray current flow direction. Five aspects, these five aspects can be carried out independently, and are inherently related to each other. Changes in any one factor may lead to changes in other factors. This reminds us to pay special attention when viewing:

1. Electrode contact resistance measurement. The electrode and liquid contact resistance value can directly estimate the general condition of the electrode and lining surface without removing the flow sensor from the pipeline, which is helpful to analyze the fault factors.

Measuring the resistance value of the contact between the electrode and the liquid can directly evaluate the general surface condition of the electrode and lining layer without removing the flow sensor from the pipeline, which is helpful to analyze the fault factors. It especially brings great convenience to the viewing of large-diameter electromagnetic flowmeters. This method can estimate the internal and external conditions of the flow sensor measurement tube, such as whether there is a deposition layer on the electrode and lining layer, whether the deposition layer is conductive or insulating, and the surface contamination of the electrode.

2. Polarization voltage of the electrode Measuring the polarization voltage between the electrode and the liquid will help determine whether the instability of the zero point or output fluctuation is caused by the electrode being contaminated or covered. Use the 2V DC range of a digital multimeter to measure the polarization voltage between the two electrodes and the ground (the electromagnetic flowmeter can be measured without power interruption or power failure). If the two measured values ​​are close to almost the same, it means that the electrode is not contaminated or covered, otherwise it means that the electrode is contaminated or covered. The size of the polarization voltage depends on the "electrode potential" of the electrode material and the properties of the liquid. The measured value may range from a few mV to hundreds of mV. Because in fact, the contamination conditions of the two electrodes cannot be the same and symmetrical during operation, so the voltages on the two electrodes form an asymmetric common mode voltage. The asymmetric common mode voltage becomes a differential mode signal, forming a zero offset.

3. Measurement of signal cable interference The signal cable is disturbed by external electrostatic induction and electromagnetic induction, which will cause the zero point of the electromagnetic flowmeter to change. In order to determine whether the change in zero point is due to the interference potential of the signal cable, it is necessary to determine the general scale of the interference and the degree of impact on the electromagnetic flowmeter.

4. Determine the presence or absence of ground potential electromagnetic flow juice. During normal use, if the condition of the electrical (power) machine near the sensor changes (such as leakage), the ground potential will change and the zero point will change. To check whether this is affected, you can short-circuit the converter's operating ground C terminal and maintenance ground G terminal, and use the zero point (or indicated value) change to determine whether there is a ground potential.

5. Identification of the direction of stray current flow in pipelines. Sometimes, in order to find the interference source of stray current in the pipeline, it is enough to be upstream of the flow sensor, so as to reduce the search scope and try to reduce or eliminate the influence of stray current interference.