Thread Content
I. Errors caused by non-axisymmetric flow When the flow velocity of the fluid within a pipe is axially symmetric and it is in a uniform magnetic field, the electromotive force generated at the electrodes of the electromagnetic flowmeter is independent of the flow velocity distribution; it is proportional to the average flow velocity of the fluid. In the case of a non-axisymmetric flow velocity distribution, that is, when each fluid particle is at a different geometric position relative to the electrodes, the induced electromotive force generated at the electrodes also varies. The closer a particle is to an electrode, the greater the induced electromotive force generated by that particle with higher velocity. Therefore, it is necessary to ensure that the flow velocity of the fluid is axially symmetric. If the flow velocity within the pipe has a non-axisymmetric distribution, it will cause errors. Therefore, when selecting an electromagnetic flowmeter, it is necessary to ensure the requirements for straight pipe sections as much as possible in order to reduce the errors caused by them. II. The issue of fluid conductivity A decrease in fluid conductivity increases the output impedance of the electrodes, and errors arise due to the loading effect caused by the input impedance of the converter. Therefore, a lower limit for the conductivity of the fluid used in electromagnetic flowmeters is specified based on the principles outlined below. The output impedance of the electrode determines the magnitude of the input impedance required by the converter, and the electrode output impedance can be considered to be determined by the conductivity of the fluid and the size of the electrode. III. Impact of electrode liner deposits When measuring fluids with deposited precipitates, the electrode surface becomes contaminated, which often leads to variations in the zero point; therefore, this aspect must be taken into consideration. It is difficult to conduct a quantitative analysis of the relationship between zero-point drift in electromagnetic flowmeters and the degree of electrode contamination. However, it can be said that the smaller the diameter of the electrodes, the less affected they are. In use, it is important to clean the electrodes to prevent deposits from forming. When measuring fluids with precipitate deposits, in addition to choosing a lining such as glass or polytetrafluoroethylene that resists deposit adhesion, the flow rate should also be increased. If the fluid contains bubbles uniformly, the volume flow rate measured includes those bubbles, which renders the measured flow value unstable and introduces errors. IV. The issue of signal transmission cable length The connection cable between the sensor (i.e., the electrode) and the converter should be as short as possible. However, in some locations on-site, due to constraints imposed by the installation environment, the distance between the converter and the sensor is relatively large; in such cases, the maximum length of the connection cable must be taken into consideration. The maximum length of the connection cable between the sensor and the converter is determined by the distributed capacitance of the cable and the conductivity of the fluid being measured. In practical use, since the conductivity of the fluid being measured lies within a certain range, this determines the maximum length of the cable between the electrodes and the converter. When the cable length exceeds the maximum length, the loading effect caused by the cable’s distributed capacitance becomes a problem. To prevent this from happening, a two-core, double-shielded cable is used. A low-impedance voltage source provided by the converter ensures that the inner shield and the core wires have the same voltage, thus creating a shield. Even if there is distributed capacitance between the core wire and the shield, since they are at the same potential, no current flows between them, and there is no load effect on the cable. As a result, the maximum length of the signal cable can be increased. Additionally, special signal transmission cables can be used to increase the maximum distance between the converter and the sensor. V. Technical issues related to excitation Excitation technology is one of the key technologies determining the measurement performance of electromagnetic flowmeters. In practical applications, excitation methods can be divided into alternating current sine wave excitation, non-sine wave alternating current excitation, and direct current excitation. With AC sine wave excitation, when the voltage of the AC power supply (and sometimes its frequency) is unstable, the magnetic field strength changes, and as a result, the induced electromotive force generated between the electrodes also varies. Therefore, it is necessary to obtain from the sensor a signal corresponding to the calculated magnetic field strength, to use as a reference signal. This excitation method tends to cause zero-point shifts, thereby reducing its measurement accuracy. Non-sinusoidal AC excitation involves using square waves or triangular waves with frequencies lower than those of industrial power, thereby generating a constant direct current whose polarity changes periodically. Since this type of excitation source is stable, there is no need to take measures to eliminate variations in magnetic field strength. For more information, please visit the company’s official website at http://www.yb1518.com/. Please retain this link when reproducing the content! http://www.yb1518.com/UploadFiles/2012717174126829.jpg