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Problems frequently encountered in the use of electromagnetic flow meters


    1.The error caused by non-axisymmetric flow, when the flow velocity of the fluid is axially symmetric, and in a uniform magnetic field, the magnitude of the electromotive force generated on the flow meter electrode is independent of the flow velocity distribution of the fluid, and is proportional to the average flow velocity of the fluid. Yet Non-axisymmetric flow velocity distribution, that is, the magnitude of the induced electromotive force generated by the counter electrode is different for each flow particle relative to the geometric position of the electrode. The closer to the electrode, the larger the induced electromotive force generated by the high velocity particle. Therefore, the fluid flow velocity must be guaranteed to be axisymmetric. When installing the electromagnetic flow meter far away, it is necessary to ensure the requirements of the straight pipe section as much as possible to reduce the error caused by it.
 


 
 
    2.The problem of fluid conductivity, the decrease in fluid conductivity will increase the output impedance of the electrode and cause an error due to the load effect caused by the input impedance of the converter. Therefore, the lower limit of the conductivity of the fluid in the application of the electromagnetic flow meter is specified according to the principles described below. The output impedance of the electrode determines the amount of 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.
 
    3.The influence of attachments in the Electric board village. When measuring a fluid with a deposit attached to it, the surface of the electrode will be contaminated, often causing changes in the points, so care must be taken. The relationship between the change of the key points and the degree of electrode contamination is difficult to perform quantitative analysis. However, it can be said that the smaller the electrode diameter, the less the impact is. In use, attention should be paid to the cleaning of the electrode to prevent adhesion. When measuring a fluid with precipitated deposits, In addition to selecting such as glass or polytetrahydroethylene, attached to the lining of the precipitate, the flow velocity should be increased. If there is a bubble in the fluid, the volume flow rate including the bubble is measured, and the measured flow value is unstable, and there is an error.

 


 
 
    Signal transmission cable length problem,--.However, some sites are limited by the location of the installation environment, and the distance between the converter and the sensor is far. In this case, the maximum length of the connection cable should 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 fluid being measured. In actual use, the conductivity of the fluid to be measured is within a certain range, thus determining the maximum length of the cable between the electrode and the converter. When the cable length exceeds the maximum length, the load effect caused by the electrical distribution capacitance becomes a problem. To prevent this from happening, a two-core, two-layer shielded cable is used, and the converter provides a low-impedance voltage source. The inner shield receives the same voltage as the core to form a shield, even if there is a distributed capacitance between the core and the shield. However, if the core wire and the shield are at the same potential, there is no current between the two, and no load effect of the cable exists. Thus, the maximum length of the signal cable can be extended, and a special signal transmission cable can be used to extend the maximum length between the converter and the sensor.

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