Three Factors Affecting Electromagnetic Flow Meter Measurement

Abstract: The information on the three major factors affecting the measurement of electromagnetic flowmeters is provided by excellent flowmeter and flowmeter manufacturers. 1. The influence of flow velocity distribution on measurement is known from fluid mechanics. When the liquid flows in the pipeline, the flow velocity of each point on the cross section of the pipeline is not equal, but whether it is laminar flow or turbulent flow, after a certain distance of straight pipe section, The velocity of the velocity can be axisymmetric. More flowmeter manufacturers choose models and price quotations. You are welcome to inquire. The following are the details of the article on the three major factors that affect the measurement of electromagnetic flowmeters. 1. The influence of flow velocity distribution on measurement is known from fluid mechanics. When the liquid flows in the pipeline, the flow velocity of each point on the cross section of the pipeline is not equal, but whether it is laminar flow or turbulent flow, after a certain distance of straight pipe section, The velocity of the velocity can be axisymmetric distribution, the velocity of each point on the cross section of the pipe is only a function of the distance from the point to the center of the pipe. The velocity is the largest at the center of the pipe axis and zero at the pipe wall. As long as the flow velocity distribution is symmetrical with respect to the central axis of the measuring tube of the electromagnetic flowmeter, the magnitude of the induced electromotive force generated on the electrode has nothing to do with the flow velocity distribution state at each point, but is only proportional to the average flow velocity of the measured liquid. Therefore, it is one of the working conditions that the uniform magnetic field type electromagnetic flowmeter must meet that the flow velocity distribution is axisymmetric. If the flow velocity distribution is asymmetric with respect to the central axis of the pipe, although the total flow rate is the same, the induced electromotive force near the electrode is large, so the measured signal is larger than the actual flow value;°In the place where the induced electromotive force is small, the resulting signal is smaller than the actual flow value, resulting in measurement error. Therefore, in order to make the flow velocity distribution symmetrical with respect to the central axis of the measuring tube, it is necessary to add a straight pipe section before the transmitter. 2. The influence of the magnetic field edge effect on the measurement Assuming that the magnetic field distribution is infinitely long, the influence of the magnetic field edge effect can be ignored. In fact, this assumption is difficult to make for actual flow meters. The impact of this edge effect on meter performance is discussed below. It is assumed that the tube wall is insulated, the length of the magnetic field coil is 2L, the radius of the measuring tube is D/2, the electrodes A and B are in the middle of the magnetic field, and the magnetic induction intensity B is parallel to the x-axis. The middle section of the magnetic field, that is, near the electrode, is roughly uniform, and the two ends gradually weaken, forming an uneven edge, and finally drops to zero. In this way, the electric field E inside the liquid is not uniform, and as a result, eddy currents will be generated in the y-z plane. The secondary magnetic flux generated by the eddy current in turn changes the working magnetic flux at the edge of the magnetic field, further destroying the uniformity of the magnetic field. At this time, the induced electromotive force measured on the electrode is not the same as the induced electromotive force under the infinite magnetic field, so an error occurs. Assuming that the axial length of the magnetic field is finite, the induced electromotive force generated between the electrodes A and B of the electromagnetic flowmeter is UAB; when the axial length of the magnetic field is infinite, the induced electromotive force between the electrodes is BvD, and S represents their ratio, that is UABS ==------ BvD Obviously, it is hoped that the closer the S value is to 1, the better. That is to say, it is hoped that the induced electromotive force generated by the on-site magnetic field is close to the induced electromotive force generated by the infinite magnetic field, and the loss of the flow signal is minimized at this time. If the tube wall is conductive, the magnetic field edge effect should be more obvious, resulting in an increase in the loss of the induced electromotive force on the electrode, so the tube wall is usually coated with an insulating layer. If the conductivity of the medium is extremely high, a large eddy current will be generated in the edge region of the magnetic field, causing a primary magnetic flux, so that the magnetic fields on both sides of the edge region of the working magnetic field are weakened and enhanced respectively. Therefore, it is not suitable to use AC excitation for the medium with high conductivity, but DC excitation. If the measured medium contains magnetically permeable substances, the magnetic field edge effect is more complicated. Due to the existence of magnetically conductive substances, the magnetic field is seriously distorted, resulting in nonlinear measurement. 3. The influence of the conductivity of the measured medium on the measurement At present, the input impedance of the electromagnetic flowmeter converter has been improved. When measuring conductive liquids, there is generally no error caused by a slight change in the conductivity of the medium, but for certain The input impedance of the converter and the conductivity of the measured medium have a lower limit, which cannot be lower than the lower limit. It is also not allowed if the conductivity of the measured medium is too large. For example, when the conductivity exceeds about 10-1 (S/cm), the flow signal will be reduced and the indicated value will be changed, that is, the indicated flow value is smaller than the actual flow value. This is because in the electromagnetic flow transmitter, the magnetic field is of finite length, and the measured conductive liquid can only generate induced electromotive force when it flows through the finite magnetic field. Therefore, the induced electromotive force representing the flow signal is the result of the conductive liquid in the magnetic field part cutting the magnetic field lines, and the conductive liquid other than the two ends of the magnetic field does not make any contribution. On the contrary, since they are also connected to the two electrodes, they also constitute a part of the external circuit.

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