Electromagnetic flowmeter display small or no display failure

The electromagnetic flowmeter sensor has no flow signal output. Reasons include power failure, connecting cable failure; damage to sensors or converter components; process reasons include changes in liquid flow conditions and problems with the adhesion layer on the inner wall of the process pipeline. It can be checked and processed according to the following methods.

① Observe that the electromagnetic flowmeter has an * alarm display, and conduct corresponding inspection and processing according to the meaning of the alarm code. If there is no alarm display, check whether the power supply of the instrument is normal, whether the switch is closed, and whether the fuse is blown. Use a multimeter to measure the voltage at each level to determine the fault.

② If the power supply is normal, check the connecting cables to see if the excitation cables and signal cables are loose, and if the terminals are oxidized or corroded. Tighten the screws if necessary.

③ Check whether the resistance value of the excitation coil is normal, whether the coil is open circuit, whether there is a short circuit between turns, and whether the insulation resistance of the terminal or coil has dropped. Use a multimeter to measure the resistance value between the EX1 and EX2 terminals of the excitation coil, but disconnect the cables connected to them. The resistance value is usually around 80-150Ω, and each product has slight differences. It is more difficult to judge the inter-turn short circuit. It can only be judged by comparing it with the original recorded value. Due to the environment at which the sensor is installed, a common fault occurs when the insulation resistance of the excitation coil circuit decreases. When the insulation resistance is less than 100MQ, you should check whether the cable sealing ring of the sensor and the sealing gasket of the terminal box are damaged, and whether the sensor is immersed in water or Moisture can be dried with hot air from a hair dryer. When the detachable excitation coil gets damp, it can be put into an electric oven for drying as a whole. Paying attention to the sealing of the lead opening is an important measure to overcome moisture.

④ For newly installed or replaced sensors, check whether the sensor arrow is consistent with the direction of the fluid. It requires process coordination to determine whether the sensor is filled with liquid. Whether the sensor can be filled with liquid is related to its installation location, and it must be installed according to regulations.

⑤ Dirt adhering to the lining layer of the sensor will reduce the insulation resistance of the electromagnetic flowmeter. The sensor can be removed for observation. If there is no condition to remove the sensor, the contact resistance of the electrode and the polarization voltage of the electrode can be measured, indirectly check and judge the condition of the adhesion layer.

a. Measurement of electrode contact resistance

Measuring the contact resistance between the electromagnetic flowmeter electrode and the liquid is actually measuring the resistance value of the electrode to ground to indirectly determine the electrode and lining layer The surface condition provides a basis for fault analysis, provided that the original measurement data must be used as the basis. If the newly installed instrument is in normal operation, the contact resistance values ​​of the two electrodes should be measured and recorded. In the future, regular measurements and recordings will be made, and the recorded data will be analyzed and compared. The difference in the contact resistance values ​​of the two electrodes should be less than 10%-20%, otherwise There may be a malfunction.

If the difference in contact resistance between the two electrodes of the electromagnetic flowmeter increases, the insulation performance of one electrode may be greatly reduced. If the resistance of an electrode to ground increases, it may be that the surface of the electrode is covered by an insulating layer. If the resistance of an electrode to ground decreases, it may be that there is a conductive layer attached to the surface of the electrode or the lining surface.

When measuring the electrode contact resistance of the electromagnetic flowmeter, the sensor wiring should be disconnected and the measurement should be made with the liquid in the pipe. The pointer multimeter used must be of the same model and the same range; for example, ×1k block, the red test lead is connected to the ground, and the black test lead is connected to the electrode; when measuring, the maximum value of the pointer deflection is read immediately after the test lead touches the terminal. Do not measure repeatedly to avoid polarization.

b. Measurement of electrode polarization voltage

The 2VDC block of a digital multimeter can be used to measure the polarization voltage between the two electrodes of the electromagnetic flowmeter and the ground. If the two measurement values ​​are basically equal, it means that the electrode is not contaminated or covered by laminates; otherwise, it means that the electrode is contaminated or covered by laminates. Polarization voltages typically range from a few millivolts to a few hundred millivolts.

⑥The converter does not display a flow measurement signal. First check whether there is any problem with the wiring between the sensor and the converter; make sure the sensor is normal. If you suspect there is a problem with the converter, you can use spare parts or a normal electromagnetic flowmeter of the same model. Circuit board replacement inspection and judgment.