How the electromagnetic flowmeter DC interference potential is generated

The metal electrode of the electromagnetic flowmeter in contact with the measured electrolyte liquid will undergo an electrochemical reaction (that is, the corrosion process of the electrode material), and a polarization voltage will be generated on the electrode. While the metal material reacts electrochemically with the medium, an oxide protective film is formed on the surface of the electromagnetic flowmeter to balance the electrochemical reaction. When the slurry fluid containing solid particles or fibers flows through the electrode, the protective film on the electrode is broken, so that the balance of the electrochemical reaction is broken, and the protective film is formed on the surface of the electrode. At this time, a large electrode is generated on the electrode.化voltage.　 When measuring very low conductivity fluids, a swinging DC polarization voltage will also appear. This phenomenon is called the 'flow noise' of low conductivity measurement. When the conductivity of the measured fluid is low to a certain level, such as alcohol, pure water and other media flowing, like the charge in a capacitor moves, the displacement current cannot be ignored. Using the following formula, the flow noise can be expressed approximately.　　The flow of fluid rubs the surface of the lining to make the charge near the collecting electrode follow the movement, so that the variable polarization noise is induced on the electrode. Obviously, if the dielectric constant of the medium is high, the displacement current will be large, and the charge moving near the electrode will also increase, and the flow noise will increase as the dielectric constant E of the medium increases. According to the application conditions of the electromagnetic flowmeter, the condition for ignoring the displacement current is we/σ<<1. It can be seen that when the fluid conductivity σ decreases, the displacement current will increase. The fluid conductivity σ is inversely proportional to the voltage of the flowing noise. In a fluid with high viscosity, the charge cannot easily overcome the binding force of the fluid and dissociate to the vicinity of the electrode. Therefore, the viscosity coefficient of fluid motion is inversely proportional to the flow noise. It should be noted that the flow signal voltage induced at this time is applied to the fluid medium as an external electric field of the dielectric fluid. The higher the flow rate of the fluid, the higher the signal voltage induced on the electrode, that is, the greater the strength of the external electric field applied to the dielectric fluid, which intensifies the movement of charges, that is, increases the flow noise. In actual use, it is found that the fluid velocity and the flow noise have an exponential function relationship. In addition, it is known from physics that the frequency of the external electric field has a great influence on the polarization of the dielectric. Since polarization is a process, there is a polarization relaxation phenomenon in time. When the frequency of the electric field increases, the turning polarization (the polarization process in which polar molecules form a dielectric) is too late to 'followThe dielectric constant of will be greatly reduced. Therefore, when the excitation frequency is high, the polarization phenomenon is weakened, and the flow noise is also reduced. However, it should also be noted that the excessively high excitation frequency increases the dielectric loss of the dielectric. Under the action of the applied voltage, part of the electric energy in the dielectric is converted into heat energy. The high-frequency external electric field generates heat during the repeated polarization of the dielectric. The higher the frequency, the more prominent the heat. Therefore, the excitation frequency of flowmeters generally used for low conductivity measurement may be 100~400Hz.　　 DC interference potential tends to drift randomly. The asymmetrical DC interference potential of the two electrodes with respect to the ground will convert the DC common-mode voltage into a DC differential-mode voltage. Excessive DC differential mode voltage amplitude will block the amplifier and destroy its linearity. In the converter measurement circuit, the capacitance isolation and sampling signal removal circuit can make the DC interference voltage output very small. In the actual application of the flowmeter, measures should also be taken to prevent the inner wall of the pipeline being tested from being corroded by the medium to produce a large potential difference. At this time, it is necessary to take measures to equipotentially connect the front and rear metal pipes to reduce the occurrence of DC interference potential. 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