Thread Content
1. Check the insulation resistance of the excitation coil. When the excitation coil and its terminal connections become damp, the insulation of the excitation circuit with respect to ground decreases, which introduces common-mode interference signals that cause the converter’s zero point to drift, affecting the accuracy of measurements. In practice, due to carelessness, the junction box is not properly sealed; for example, it is common for sealing washers to be omitted at the cable entry points, allowing moisture to enter and resulting in a decrease in the insulation of the terminal connections. Dry the terminals with a heat blower; usually, the insulation can be improved or restored to its original level at the time of manufacture, and thus the fault can be resolved. 2. Check the electrode contact resistance. Measuring the contact resistance between the electrode and the liquid allows for an indirect assessment of the overall condition of the surfaces of the electrode and the lining layer, such as whether there are any deposited layers on these surfaces and whether those deposits are conductive or insulating. The electrode contact resistance of the flow sensor should be measured immediately after the newly installed instrument is calibrated, and recorded. In the future, perform a measurement after each maintenance; analyzing and comparing these data will help determine the cause of instrument failures. The contact resistance value between the electrode and the material being tested depends on the size of the contact surface (i.e., the size of the electrode tip) and the electrical conductivity of the material being tested. 3. Measure the copper resistance of the excitation coil. Measure the coil resistance using a high-precision digital multimeter or a Wheatstone bridge, and apply a temperature coefficient correction if necessary. By comparing the measured values with the instrument’s archive values, it is confirmed that the coil is in good conductive condition without any inter-turn short circuits or similar issues. In fact, the probability of inter-turn short circuits in the coil is very low. Together with the converter excitation current check, this check can indirectly evaluate whether there are any changes in the magnetic field strength of the flow sensor. When comparing the measured electrode contact resistance value with the original measurement value, if they do not match, there may be the following different trends: (1) An increase in the resistance value. It indicates that the electrode surface is covered with an insulating layer. (2) The resistance value decreases. This indicates that there is a conductive deposition layer on the electrode surface, or that the insulation properties of the electrode have decreased. Experience shows that the contact resistance values of the two electrodes are close to each other. The difference between the two resistance values should be less than 10%–20%. Otherwise, it indicates that the insulating properties of the surface coatings on the two electrodes are different, or there is a significant decrease in the insulation of the signal circuit for one of the electrodes. The electrode contact resistance can be determined using an analog multimeter by measuring the resistance between each electrode terminal and ground while the device is filled with the medium under test. When taking measurements, the following points should be noted: (1) When measuring the resistance between two electrodes, the polarities of the measuring probes must be the same; that is, one measuring probe should always be used to connect to the electrodes, while the other measuring probe should always be grounded. (2) The resistance value should be the maximum deflection of the pointer read at the very instant the measuring rod makes contact with the terminal, and the measurement value shall be based on this initial reading. Repeated measurements can yield inconsistent results due to polarization. (3) When comparing the measurement values from the two periodic inspections, it is necessary to use a multimeter of the same model and with the same range. Using a multimeter to measure the contact resistance of electrodes is not a method for obtaining accurate values; it only gives approximate values. Accurate measurement requires a digital bridge. 4. Check the insulation of the signal circuit and the insulation between the excitation circuit and the signal. The purpose of this inspection is also to assess the interference introduced due to reduced insulation. When checking the signal circuit, the signal wire must be temporarily disconnected from the electrode. There are many reasons for a decrease in insulation, and moisture ingress due to unsealed junction boxes is a common one. 5. Check the insulation resistance of the electrodes and the condition of the lining. To check the insulation resistance of electrodes, the integrity of the lining, or the condition of the adhesive layer, small-diameter instruments must be removed from the pipeline for inspection. For large-diameter instruments, the measuring medium must be drained, and then a person must enter the pipeline through a manhole to conduct inspections. Dry the inner surfaces of the electrodes and liner, ensuring no liquid residues remain, and let them dry. Use a 500V DC megohmmeter to measure the insulation resistance of each electrode relative to ground. If the lining adhesion layer must be removed, the future cleaning interval should be determined based on the layer thickness. The deterioration of electrode insulation is largely caused by sensors being exposed to external moisture or immersion in water; sometimes, blowing hot air on them can eliminate the moisture and restore their functionality. If it is indeed insulation failure, such as electrode leakage, the sensor must be replaced and sent back to the manufacturer for repair.