Introduction to the principle and measurement method of electromagnetic flowmeter in partial pipeline

In-line electromagnetic flowmeters are usually used in closed pipes for liquid full pipe flow. For the situation where the flow rate changes greatly, the fluid sometimes fills the pipeline, and sometimes the pipeline is not full, the pipeline electromagnetic flowmeter is not applicable. If you use a weir, trough or submersible electric impact flow meter for measuring open channel flow, the head loss when the pipe is full will be too large, which will affect the outflow; the flow is too small, and the fluid cannot enter the weir. The trough pipe type electromagnetic flowmeter is usually used for sealing The pipe is full of liquid. For the situation where the flow rate changes greatly, the fluid sometimes fills the pipeline, and sometimes the pipeline is not full, the pipeline electromagnetic flowmeter is not applicable. If a weir, trough or submersible electric impact flow meter is used to measure open channel flow, the head loss when the pipe is full will be too large, which will affect the outflow; if the flow is too small, the fluid cannot enter the weir, trough or submersible electromagnetic flow sensor. Measurement. The partial-pipe electromagnetic flow sensor can be the same as the installed pipe diameter and work within a large flow range. It can be used to measure the free surface flow of closed pipes full pipe flow and unclosed pipes or open pipes, and does not produce head loss, such as For municipal drainage, wastewater treatment, and agricultural irrigation, the fluid is measured by the flow rate of natural flow.　　The electromagnetic flowmeter uses a constant cross-sectional area of u200bu200bthe sensor and measures the average flow rate to obtain the flow rate. The cross-sectional area of u200bu200bthe fluid in the partially full pipe changes. The flow measurement must not only measure the average velocity of the flow through the sensor, but also measure the cross-sectional area of u200bu200bthe fluid that flows through the sensor. That is to say, the flow measurement of the non-full pipe electromagnetic flowmeter requires at least two variables: flow rate and liquid level.　　 According to the weight function theory, the signal voltage induced on the electrode is the collection of the potentials of all the particles in the electrode section. In a non-full-tube sensor, no matter how the cross section of the water is changed, the particles in the fluid flow will always have induced potentials, and these potentials must be within the range of the electrodes.　　The electrode cannot be exposed to the air, otherwise the electrode will not get the induced flow signal. Therefore, the non-full-tube sensor structure can have a large-area electrode shown in Figure 2.30 (a) with a tube center deviated to the bottom, a multi-electrode shown in Figure 2.30 (b), and a multi-electrode shown in Figure 2.30 (c) installed at the bottom of the sensor. The liquid level detection point electrode and other structures.　　In a partially full pipeline sensor, the flow velocity distribution is determined by the hydraulic slope of the pipeline, and the measured average flow velocity needs to be corrected for different flow velocity distributions. When W×B u003d constant is satisfied, the measurement is not affected by the change of the flow velocity distribution, and W is related to the electrode area and the geometric position of the particle. In a partially full pipeline sensor, different fluid cross-sections change the geometric position of the particle potential and the electrode area, that is, change the weight function W. Some types of non-full tube sensors can change the magnetic induction intensity B through a special form of excitation, and induce different electromotive forces to obtain the water level.　　 In the non-full pipe electromagnetic flowmeter, there are many ways to measure the liquid level, most of them do not attach a liquid level sensor, and the liquid level measurement is completed by the flow measurement electrode or the liquid level detection electrode set in the cutting tube lining. Common methods for measuring liquid level are as follows.　　① Apply the principle of weight function and geometric position, use two different connection methods of excitation coils, change the ratio of the electric potential generated by the magnetic field, establish the functional relationship of the liquid level, and obtain the liquid level through the calculation of the single-chip microcomputer.　　② Liquid level measurement using capacitance method. Capacitance measurement is based on the capacitance between two metal plates arranged in the lining of the measuring tube and is proportional to the cross-sectional area of u200bu200bthe fluid to measure different liquid levels.　　 The flow measurement of the non-full-tube electromagnetic flowmeter requires complicated calculation and control of the measured electromotive force and excitation. Obviously, its development cannot be separated from the development of single-chip microcomputer and calculation technology.　　 In order to extend the measurement of low water level flow, the non-full pipe electromagnetic flow sensor can be made into a shape with a throat groove. Install the electrode on the measuring board of the throat to improve the measurement resolution.