Briefly describe the reasons for the generation of DC interference potential of pipeline electromagnetic flowmeter

In actual industrial production, pipeline electromagnetic flowmeter is one of the most commonly used flow measurement instruments. During the measurement signal sampling process of the electromagnetic flowmeter fluid medium, there will be various factors that affect the measurement data. The components of the interference signal are relatively complex, some are very strong, and sometimes even exceed the flow signal. How to eliminate these interference signals and improve the signal-to-noise ratio has always been a difficult problem in the installation and use of electromagnetic flowmeters, and is the key to the design and installation of electromagnetic flowmeters.

The electrode of the pipeline electromagnetic flowmeter is a metal substance. Although there are matching measuring electrodes for different media, the measured electrolyte liquid will undergo an electrochemical reaction when it contacts the metal electrode (that is, the process of electrode material corrosion, which is very common when the measured medium is salt, acid, or alkali liquid), and a polarization voltage will be generated on the electrode.

While the metal material of the measuring electrode undergoes an electrochemical reaction with the liquid medium, an oxide protective film will be produced on its surface to balance the electrochemical reaction. However, whenever a slurry fluid containing solid particles or fibers flows through the electrode, the solid particles will destroy the protective film on the electrode during the friction with the electrode, breaking the balance of the electrochemical reaction. The protective film will be re-formed on the measuring electrode surface. When the electrode repeats this process, a large polarization voltage will be generated on the electrode of the pipeline electromagnetic flowmeter, thus affecting the measured signal.

In another case, when we measure a very low conductivity fluid, a swinging DC polarization voltage will also appear. We call this phenomenon "flow noise" of low conductivity measurement. When the conductivity of the measured fluid is low to a certain extent, such as when the medium such as alcohol and pure water flows, the displacement current is not negligible, just like the movement of charges in a capacitor. The following formula can be used to approximately express the size of the flow noise. The flow friction lining surface of
　　

the pipeline electromagnetic flowmeter fluid causes the charges near the electrode to follow the movement, so the changing polarization noise is induced on the electrode. Obviously, if the dielectric constant of the medium is high, the displacement current is large, the charges moving near the electrode also increase, and the flow noise increases with the increase of the dielectric constant E of the medium. According to the excitation frequency of the electromagnetic flowmeter, it may be between 100 and 400 Hz.

The DC interference potential often drifts randomly. The asymmetric DC interference potential of the two electrodes to the ground will convert the DC common mode voltage into a DC differential mode voltage. If the DC differential mode voltage amplitude is too large, it will cause the amplifier to block the pipeline electromagnetic flowmeter and destroy its linearity. In the converter measurement circuit, the capacitor isolation and sampling signal removal circuit can make the DC interference voltage output very small. In the actual application of the pipeline electromagnetic flowmeter, measures should also be taken to prevent the inner wall of the measured pipeline from being corroded by the pipeline electromagnetic flowmeter medium to produce a large potential difference. When this happens, we need to take timely measures to connect the front and rear metal pipes with equal potential to reduce the occurrence of DC interference potential and ensure the stability of the electromagnetic flowmeter measurement signal.