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What are the special inspection instructions for electromagnetic flowmeters?

First, the electrode contact resistance measurement

Measuring the liquid contact resistance value of the electrode spoon can indirectly estimate the general condition of the surface of the juice electrode and the lining layer without removing the flow sensor from the pipe, and is helpful for analyzing the cause of the failure. This is especially convenient for the inspection of the population diameter electromagnetic flowmeter. It is estimated that the flow sensor measures the inner surface condition of the tube such as whether the electrode and the lining layer have a deposited layer, the conductive layer of the deposited layer is sufficiently insulating, the surface contamination state of the electrode, and the like.

The measured electrode-to-ground resistance has the following different trends compared with the original measured value:

(1) The resistance of the two electrodes increases (ie, the difference increases),
(2) The resistance value increases,
(3) The resistance value is reduced.

These three signs can determine the following possible causes of failure:

(1) There is a large drop in the insulation of the electrode at the electrode part.
(2) The surface of the electrode is covered with an insulating layer,
(3) A conductive deposition layer is attached to the electrode surface and the lining surface.

The above several failure possibilities can also be used as a precursor to predicting the failure.

Pay attention to the following points when measuring with a multimeter:

(1) The resistance value shall be read at the moment when the measuring rod contacts the terminal, and the maximum value of the pointer shall be read. The measured value shall be based on the initial one. If the values measured by polarization are re-measured, they are inconsistent;
(2) When measuring the resistance of the two electrodes, the polarity of the grounding rod must be the same, that is, the same meter is used for the same meter, the positive rod is connected to the electrode, and the negative rod is grounded.
(3) The measurement should use the same model multimeter, and use the same range, commonly used 1.5V battery working range measurement file, such as: × lkn file.

Second, the polarization voltage measurement of the electrode

The polarization between the electrode and the liquid will help determine if the zero point instability or output sloshing fault is caused by contamination or coverage of the electrode. Use the digital multimeter 2V DC file to measure the polarization voltage between the two electrodes and the ground separately (the plug-in electromagnetic flowmeter can be tested without stopping the power, or it can be stopped). If the two measurements are nearly equal, the electrode is not contaminated or covered, otherwise the electrode is contaminated or covered. The magnitude of the polarization voltage is determined by the "electrode potential" of the electrode material and the nature of the liquid, and the measured value may be between several mV and several hundred mV. Since the contamination of the two electrodes in operation is not exactly the same symmetry, the voltage across the electrodes forms an asymmetric common mode voltage. The asymmetrical common mode voltage becomes a differential mode signal, causing a zero offset.

Third, the determination of signal cable interference

The signal cable is subject to external static induction and electromagnetic induction interference, which causes the plug-in electromagnetic flowmeter to change at zero. In order to determine whether the zero point change is affected by the signal cable interference potential, it is necessary to determine the general extent of the interference and the degree of influence on the plug-in electromagnetic flowmeter.

Fourth, determine the presence or absence of ground potential

During the normal use of the electromagnetic flow juice, if the state of the electric (force) machine near the sensor changes (such as leakage), the ground potential will change and cause a zero point change. Check if there is any influence in this way. Short-circuit the converter working ground C terminal and the protective ground G terminal, and judge whether there is a ground potential by zero point (or indication value).

V. Pipe flow stray current flow direction discrimination

Sometimes the source of interference for finding pipeline spurs is still downstream upstream of the flow sensor to narrow the search range and try to reduce or eliminate the effects of stray current interference.
Case 12 (page 24) of Section VIII above is used as an example to illustrate the specific practices. As shown in Figure 11, the flow sensor connected to the pipeline is connected to the current meter by the upstream and downstream jumper wires and the grounding wires A and B. The current measured at point A is 60 mAAC, and the current at point B is zero, indicating that the source of interference is upstream of the flow sensor.

The electrode contact resistance of the electromagnetic flow sensor should be measured and recorded immediately after the newly installed instrument is debugged. After each maintenance measurement, the analysis and comparison of these data will help to determine the cause of the instrument failure in the future.

The resistance value of the electrode in contact with the liquid mainly depends on the surface area of the electrode in contact with the liquid and the conductivity of the liquid to be measured. The general structure electrode has a resistance value of 350k~Q when measuring the conductivity of 5×10-6S/cm distilled water, and the living and industrial water of conductivity 150×10-6S/cm is about 15kO, and the conductivity is 1×10-2S/ The brine of cm is about 200n.

Use a multimeter to measure the resistance between each electrode terminal and the ground when filling with liquid. Experience has shown that the difference between the contact resistance values of the two poles should be less than 10%-20%, otherwise it is faulty. Using a multimeter to measure the electrode contact resistance is not enough to correctly measure the exact value of the resistance, just to determine the general value. Accurate measurements must be made with an AC bridge such as the "Kohlraush Bridge".

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