Analysis of Common Problems of Electromagnetic Flowmeter

Errors caused by non-axisymmetric flow When the flow velocity of the fluid in the pipe is axisymmetrically distributed, and in a uniform magnetic field, the magnitude of the electromotive force generated on the electrodes of the flowmeter has nothing to do with the flow velocity distribution of the fluid, and is proportional to the average flow velocity of the fluid, rather than When the flow velocity distribution is axisymmetric, that is, the size of the induced electromotive force generated by the electrode is also different depending on the geometric position of each flowing particle relative to the electrode. The closer to the electrode, the greater the induced electromotive force generated by the particle with a higher velocity. Ensure that the fluid flow rate is axisymmetric. If the flow velocity in the pipe is non-axisymmetric distribution, errors will be caused. Therefore, when selecting an electromagnetic flowmeter, it is necessary to ensure the requirements of the straight pipe section as much as possible to reduce the error caused by it. Influence of electrode lining attachments When measuring fluids with attached sediments, the electrode surface will be polluted, which often causes zero point changes, so attention must be paid. It is difficult to conduct quantitative analysis on the relationship between the zero point change and the degree of electrode pollution, but it can be said that the smaller the diameter of the electrode, the less affected it will be. During use, attention should be paid to cleaning the electrode to prevent adhesion. When measuring fluids with sedimentation deposits, in addition to choosing linings such as glass or polytetrachloroethylene that are difficult to adhere to sedimentation, the flow rate should also be increased. If air bubbles are contained uniformly in the fluid, the volumetric flow rate including the air bubbles is measured and makes the measured flow value unstable, introducing errors. The problem of signal transmission cable length The shorter the connecting cable between the sensor (ie electrode) and the converter, the better. However, some sites are limited by the location of the installation environment, and the distance between the converter and the sensor is relatively long. At this time, the long length of the connecting cable must be considered. The maximum length of the connecting cable between the sensor and the converter is determined by the distributed capacitance of the cable and the conductivity of the measured fluid. In actual use, when the conductivity of the measured fluid is within a certain range, the maximum length of the cable between the electrode and the converter is determined. When the cable length exceeds the maximum length, the loading effect caused by the distributed capacitance of the cable becomes a problem. In order to prevent this from happening, a double-core, two-layer shielded cable is used, and the converter provides a low-impedance voltage source so that the inner shield and the core wire get the same voltage to form a shield, even if there is distributed capacitance between the core wire and the shield. But the core wire and the shielding are at the same potential, so there is no current passing between the two, and there is no loading effect of the cable, so the signal cable can be extended to a large length. In addition, special signal transmission cables can be used to extend the large length between the converter and the sensor. The problem of fluid conductivity The reduction of fluid conductivity will increase the output impedance of the electrode, and the load effect caused by the input impedance of the converter will cause errors. Therefore, according to the following principles, the conductivity of the fluid in the electromagnetic flowmeter application is specified. lower limit of the rate. The output impedance of the electrode determines the size of the input impedance required by the converter, and the output impedance of the electrode can be considered to be dominated by the conductivity of the fluid and the size of the electrode. Technical problems of excitation Excitation technology is the key technology for the measurement performance of electromagnetic flowmeters. In practical applications, the excitation methods can be divided into AC sine wave excitation, non-sine wave AC excitation and DC excitation. AC sine wave excitation, when the AC power supply voltage (sometimes frequency) is unstable, the magnetic field strength will change, so the induced electromotive force generated between the electrodes will also change. Therefore, the signal corresponding to the calculated magnetic field strength must be taken out from the sensor as standard signal. This excitation method is easy to cause the zero point to change, and reduce its measurement accuracy. Non-sinusoidal AC excitation uses square wave or triangular wave excitation which is lower than the industrial frequency. It can be considered to generate constant DC and periodically change the polarity. Because this excitation power supply is stable, it is not necessary to remove the magnetic field strength. Changes are made.

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