Specific operation method for measuring the contact resistance between the electromagnetic flowmeter electrode and the liquid

Measuring the contact resistance between the electrode and the liquid can indirectly evaluate the general surface condition of the electrode and the lining layer without removing the flow sensor from the pipeline, which is helpful for analyzing the cause of the fault. It is especially convenient for the inspection of large-caliber electromagnetic flowmeters. This method can estimate the surface condition of the flow sensor measuring tube, such as whether there is a deposit layer on the electrode and the lining layer, whether the deposit layer is conductive or insulating, and the contamination condition of the electrode surface.
 
The resistance value of the electromagnetic flowmeter electrode in contact with the liquid mainly depends on the contact surface area of ​​the electrode and the liquid and the conductivity of the measured liquid. The resistance value of the general structure electrode is about 350k Ω when measuring distilled water with a conductivity of 5×10-6S/cm; the resistance value of domestic and industrial water with a conductivity of 150×10-6S/cm is about 15k Ω; the resistance value of salt water with a conductivity of 1×10-6S/cm is about 200 Ω.
 
The contact resistance between the electrode and the liquid can be measured by an analog multimeter when the electrode is filled with liquid. Experience shows that the difference between the contact resistance values ​​of the two electrodes measured separately should be less than 10% to 20%, otherwise it means there is a fault.
 
The contact resistance between the electrode and the liquid of the electromagnetic flowmeter sensor should be measured and recorded immediately after the new instrument is debugged. It should be measured once every maintenance in the future. Analysis and comparison of these data will help to determine the cause of the instrument failure in the future.
 
If the measured electrode contact resistance is inconsistent with the original measured value, there may be three different trends:
(1) The resistance value increases;
(2) The resistance value decreases.
(3) The unbalanced value of the contact resistance between the two electrodes increases (that is, the difference increases);
 
these three signs can be judged as the following reasons:
(1) The conductive deposition layer is attached to the electrode surface and the lining surface;
(2) There is a significant decrease in the insulation of one electrode in the electrode part;
(3) The electrode surface is covered by an insulating layer.
 
Sometimes, although the above signs are present, but no fault has occurred, it can be used as a precursor to predict the fault of the electromagnetic flowmeter and take pre-processing.
 
When using an analog multimeter to measure the electrode contact resistance, the following points should be noted:
(1) When measuring the resistance of the two electrodes, the polarity of the grounding rod must be the same, that is, using the same probe of the meter, the positive rod is connected to the electrode, and the negative rod is grounded.
(2) The resistance value should be read at the maximum pointer deflection value at the moment the probe contacts the terminal, and the measured value should be based on the initial measurement. If the values ​​are re-measured, they will be inconsistent due to polarization.
(3) When comparing the values ​​measured before and after two regular inspections, use the same model of multimeter and the same range, usually using the measurement range of the 1.5V battery working range, such as: ×1k Ω.
 
Using a multimeter to measure the electrode contact resistance is not a correct way to measure the exact value of the resistance, it only determines the approximate value. Accurate measurements must use an AC bridge such as the "Kohlraush bridge" or a digital bridge. (