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Measurement of contact resistance between electrode and liquid in electromagnetic flowmeter

by:Sure     2021-08-01
Measuring the contact resistance between the electrode and the liquid can indirectly evaluate the general condition of the electrode and the lining surface without removing the flow sensor from the pipeline, which is helpful to analyze the cause of the failure. In particular, it brings great convenience to the inspection of large-caliber electromagnetic flowmeters. This method can estimate the condition of the inner surface of the flow sensor measuring tube, such as whether the electrode and the lining layer have a deposited layer, whether the deposited layer is conductive or the contact resistance between the electrode and the liquid, and the electrode and the lining can be evaluated indirectly without removing the flow sensor from the pipe. The general condition of the layer surface helps to analyze the cause of the failure. In particular, it brings great convenience to the inspection of large-caliber electromagnetic flowmeters. This method can estimate the condition of the inner surface of the flow sensor measuring tube, such as whether there is a deposition layer on the electrode and the lining layer, whether the deposition layer is conductive or insulating, and the contamination of the electrode surface.  The resistance value of the electrode in contact with the liquid mainly depends on the surface area of u200bu200bthe electrode in contact with the liquid and the conductivity of the measured liquid. When measuring distilled water with a conductivity of 5×10-6S/cm, the resistance value of the electrode with a general structure is about 350k Ω; for domestic and industrial water with a conductivity of 150×10-6S/cm, it is about 15k Ω; and the conductivity is 1×10-6S. /cm of salt water is about 200 Ω   .   The contact resistance between the electrode and the liquid can be measured with an analog multimeter when it is filled with liquid. Empirically, the difference between the contact resistance values u200bu200bof the two electrodes measured separately should be less than 10% to 20%, otherwise it means there is a fault.  The electrode and liquid contact resistance of the electromagnetic flow sensor should be measured and recorded immediately after the newly installed instrument is debugged. After each maintenance, the measurement will be performed and the analysis and comparison of these data will help determine the cause of the instrument failure in the future.   If the measured electrode wetted resistance is inconsistent with the original measured value, there may be three different trends:   (1) Resistance value increases;   (2) Resistance value decreases. (3) The unbalanced value of the contact resistance of the two electrodes increases (that is, the difference increases);    These three signs can be judged to be caused by the following reasons:    (1) The electrode surface and the lining surface are attached with conductive deposits;    (2) Electrode part There is a large drop in the insulation of one electrode;    (3) The surface of the electrode is covered by an insulating layer.  Although the above signs sometimes appear, but the failure has not yet formed, it can be used as a precursor to predict the occurrence of the failure and deal with it in advance. Pay attention to the following points when measuring the electrode wetted resistance with an analog multimeter:    (1) When measuring the resistance of two electrodes, the polarity of the grounding rod must be the same, that is, use the same measuring rod of the meter, the positive rod is connected to the electrode, and the negative The rod is grounded.   (2) The resistance value should read the maximum pointer deflection at the moment the measuring rod touches the terminal, and the measured value should be based on the initial value. Such as re-measurement, the values u200bu200bmeasured due to polarization are inconsistent.  (3) To compare the measured values u200bu200bof the two periodic inspections before and after, use the same model multimeter and use the same range. The measurement file of the 1.5V battery working range is commonly used, such as: ×1k Ω    file.  Using a multimeter to measure the electrode contact resistance is not a correct way to measure the exact value of the resistance, but to determine the approximate value. Accurate measurement must use AC bridge such as 'Kohlraush bridgeFor more questions, please call customer service.
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