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Analysis of the daily inspection of the electromagnetic flowmeter sensor

1. Check the insulation resistance of the excitation coil.
After the excitation coil and its terminals are wetted, the excitation circuit is grounded to the ground, and the common mode interference signal is introduced, which causes the zero drift of the converter and affects the measurement degree. In practice, due to negligence, the junction box is not sealed, such as the installation of a sealing gasket at the end of the cable lead, the introduction of moisture, and the insulation of the terminal is a frequent failure case. Dry the terminal with a hot blower, usually insulation can be improved or restored to the factory insulation level, and the fault can be eliminated.
2. Check the electrode contact resistance.
Measuring the contact resistance of the electrode with the liquid can indirectly evaluate the general condition of the surface of the electrode and the liner layer, such as whether the electrode surface and the backing layer are attached to the deposited layer, whether the deposited layer is electrically conductive or insulative. The electrode contact resistance of the flow sensor should be measured immediately after the new instrument is commissioned and recorded. After each maintenance measurement, the analysis and comparison of these data will help to determine the cause of the instrument failure.
The contact resistance between the electrode and the measured medium depends on the size of the contact surface (ie, the size of the electrode end face) and the conductivity of the measured medium.
3. Measure the copper resistance of the excitation coil.
Use a high-precision digital multimeter or a Wheatstone bridge to measure the coil resistance and, if necessary, correct the temperature coefficient. The measured value is compared with the instrument file value, and it is confirmed that the coil is well-connected and there is no inter-turn short circuit. In fact, the probability of a short circuit between turns of the coil is very low. This check, together with the converter field current check, indirectly assesses whether the magnetic field strength of the flow sensor has changed.
The measured electrode contact resistance value is compared with the original measured value. If they are inconsistent, there may be the following different trends:
(1) The resistance value increases. It indicates that the surface of the electrode is covered with an insulating layer.
(2) The resistance value is reduced. It indicates that the electrode surface is provided with a conductive deposition layer, or the electrode insulation performance is degraded.
Experience has shown that the contact resistance values of the two electrodes are close. The difference between the two resistance values should be less than 10% to 20%. Otherwise, it indicates that the adhesion of the two electrode surface adhesion layers is different, or the insulation of one electrode signal circuit is greatly reduced.
The electrode contact resistance can be obtained by measuring the resistance between each electrode terminal and the ground when the measuring medium is filled with a pointer type multimeter. Pay attention to the following points when measuring:
(1) When measuring the resistance of two electrodes, the polarity of the measuring rod must be the same, that is, one measuring rod is fixed to the electrode, and the other measuring rod is always grounded.
(2) The resistance value shall be read from the initial deflection of the pointer when the measuring rod contacts the terminal. The measured value shall be the first one. Repeated measurements can cause different measurements due to polarization.
(3) To compare the measured values of the second periodic inspection before and after, it is necessary to use the same type of multimeter and use the same range.
Measuring the electrode contact resistance with a multimeter is not a method of measuring the exact value, but a general value. A digital bridge must be used for accurate measurements.
4. Check the insulation between the signal loop insulation and the excitation circuit/signal.
The purpose of this inspection is also to assess the interference introduced by the drop in insulation. When checking the signal loop, the signal line should be temporarily disconnected from the electrode. There are many reasons for the drop in insulation, and the junction box is not sealed into moisture is a common cause.
5. Check the insulation resistance and lining of the electrode.
To check the insulation resistance of the electrode, the integrity of the lining or the condition of the adhesive layer, the inspection can only be removed from the pipeline for the ********* instrument. For large-diameter instruments, the medium should be vented and observed from the manhole into the pipeline. Wipe the inner surface of the electrode and lining without leaving any liquid stains and dry. The insulation resistance of the two electrodes to the ground was measured with a 500 V DC megger. If the lining adhesion layer must be removed, determine the future cleaning interval period according to the thickness of the laminate. A large part of the drop in electrode insulation is caused by the external moisture or water immersion of the sensor, and sometimes it can be recovered by using a hot air blower to remove moisture. If the insulation is indeed broken, such as electrode leakage, only the sensor can be exchanged and returned to the factory for repair.

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