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Intelligent electromagnetic flowmeter (1) DC excitation DC magnetic excitation EMF used to measure the liquid metal flow is, such as room temperature mercury and high temperature liquid sodium, strontium, potassium and so on. In the primary circuit of a fast breeder nuclear reactor, it is used to measure the molten sodium flow rate of more than 500 degrees Celsius. The DN300mm pipe instrument report also describes the exploration of the dry check flow value, compared with the wet method (real flow) check. Accuracy can reach 3%. In addition to the magnetic field generated by the electromagnet DC excitation, it also uses a smaller diameter sensor with a permanent magnet magnetic field. The domestic Shanghai Guanghua Instrument Factory has developed such products.
The application of this type of instrument in the general process industry in China is still an exploratory stage. It was successfully applied to the measurement of the mercury flow at room temperature in the electrolytic cell in the 20th century to the 1970s. The non-ferrous metallurgy industry has also tried to measure the flow rate of molten zinc because of the zinc liquid. In the measuring tube wall fouling and zinc oxide and other reasons, not used successfully.
(2) AC Excitation Early EMF was excited with 50Hz industrial frequency to generate a sine wave alternating magnetic field; the reason for using AC excitation was to avoid polarization on the surface of the electrode when using DC excitation, but it was susceptible to the mains power. The superposition of various inductive noises in the same phase as the flow signal, forming zero drift, etc., has now been replaced by low-frequency rectangular wave excitation. These induced noises are caused by the following reasons.
1) The eddy current generated in the liquid and the phase signal noise of the flow signal, the noise formed by the electrode contamination and the zero point drift;
2) The magnetic loss in the magnetic circuit deviates from the phase between the exciting current and the magnetic field, so that the quadrature (900) noise formed by the transformer effect originally in the signal loop is phase-shifted to generate the in-phase noise;
3) Electrostatically induced noise between the excitation coil and the signal line and between the excitation coil and the fluid.
However, the magnetic flux density of the AC Excitation Flow Sensor is usually designed to be higher, with the advantages of larger signal electromotive force (about 1 mV per 1 m/s, rectangular wave excitation only 0.2-0.3 mV) and higher signal-to-noise ratio. In addition, in the case of fluids such as solid-liquid two-phase slurries, low-frequency square-wave excitation generates a sharp slurry noise on the surface of the solid wiper electrode, which causes output signals to fluctuate, and power frequency AC-excited EMF does not have such a disadvantage. . Therefore, there are still some instrument manufacturers at home and abroad that continue to provide EMF with AC or other frequency AC excitation.
Intelligent electromagnetic flowmeter
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Intelligent electromagnetic flow meter classification by current mode
Smart electromagnetic flowmeters convert the volumetric flow of conductive liquid flowing through a pipeline into a linear electrical signal. Its conversion principle is the famous Faraday's law of electromagnetic induction, that is, the conductor passes through a magnetic field, cuts the electromagnetic wire, and generates an electromotive force. The magnetic field of the flow sensor is realized by excitation, which is divided into DC excitation, AC excitation and low frequency square wave excitation.