Electromagnetic flowmeter electrode maintenance and cleaning method

In the measurement of sewage, slurry and other media, the inner wall of the pipe and the surface of the electrode are susceptible to scaling and adhesion. When the conductivity of the fouling material is different from the conductivity of the measured medium, measurement errors will result. The attachment of sludge and oil to the electrode will also cause the meter output to swing and drift. Therefore, in some cases, the electrode needs to be maintained. For example, clean the electrodes and replace the electrodes.

Electrode cleaning commonly used methods are as follows:

(l) Electrochemical methods

The metal electrode has an electrochemical phenomenon in the electrolyte fluid. According to the principle of electrochemistry, there is an interface electric field between the electrode and the fluid, and the interface electric field between the electrode and the fluid is caused by the electric double layer existing in the electrode/fluid phase. The study of the electric field at the interface between the electrode and the fluid reveals that the molecules, atoms, or ions of the material have an enriched or poorly adsorbed phenomenon at the interface, and that most of the inorganic anions are surface-active substances with a typical ion adsorption law and the surface of inorganic cations. The activity is very small, so the electrochemical cleaning electrode only considers the anion adsorption. Adsorption of anions is closely related to electrode potential. Adsorption occurs mainly at a potential range that is more positive than the zero charge potential, ie, the telegraph surface with an opposite charge. On the surface of the electrode with the same charge, when the residual charge density is slightly larger, the electrostatic repulsive force is greater than the adsorption force, and the anion desorbs quickly, which is the principle of electrochemical cleaning. Some companies apply a voltage drop of about 1.2. 1.4V to both electrodes by applying the voltage drop of the two forward diodes to the signal loop. Because the voltage applied across the two electrodes is a negative dc common-mode voltage, there is no amplifier saturation. The dc common-mode voltage is superimposed on the tiny alternating flow signal. The capacitor isolates the dc and the pre-amplifier rejects the common-mode voltage. The dc common-mode voltage does not affect the flow measurement. The negative DC voltage applied to the electrodes forms a negative electric field that can repel substances that adhere to the electrodes and reach the date of cleaning the electrodes. This method effectively, automatically and continuously performs electrode washing in AC excitation. However, for the low-frequency rectangular wave excitation, the effect is not necessarily good due to the high amplitude of the polarization voltage, so it has rarely been seen recently.

(2) Mechanical removal method

The mechanical removal method is achieved by installing a special mechanical structure on the electrode to achieve electrode removal. There are currently two forms:

One is to use a mechanical scraper. A gua knife with a thin shaft made of stainless steel was used to draw out the gua knife through the hollow electrode, and a mechanical seal was used between the thin shaft and the hollow electrode to prevent the medium from being ejected, so that the gua knife became a mechanical scraper. When turning the thin shaft from the outside, the gua knife rotates close to the electrode end plane to scrape dirt. The scraper can be manually or automatically scraped by a motor-driven fine shaft. The other is in the tubular electrode, equipped with a wire brush to remove dirt, and the shaft is wrapped in a sealed “O” ring to prevent fluid leakage. This type of cleaning device requires someone to constantly pull the wire to clean the electrode.

(3) Ultrasonic cleaning method

The 45 to 65 kHz ultrasonic wave generated by the ultrasonic generator is applied to the electrode to increase the energy of the ultrasonic wave. The applied voltage is almost concentrated on the attached matter. The high voltage breaks down the attached matter and is then washed away by the fluid. From the viewpoint of safety, using the electric breakdown method must be a direct connection of the AC (50Hz or 60Hz) high voltage signal to the sensor signal output terminal in the event of a flowmeter interrupt measurement, disconnection of the signal line between the sensor and the converter and the converter, and power outage. Clean it.

(5) Improve the average within the measuring tube

Flow rate and the use of small electrodes with pointed tip When measuring easily fouled, adhered media, it is usually possible to select a sensor smaller than the diameter of the process to increase the flow rate. Experience has shown that the average flow velocity in the tube is higher than 2m/s, and the deposition adhesion is generally less likely. There is also an instantaneous increase in flow rate of 3-5m/s (depending on the attachment condition) to flush the adhesion layer. The tip of the electrode protrudes into a pointed shape, and the force of flushing with the fluid is large (because the flow velocity of the tube wall is equal to zero, and the tip of the tube exits from the boundary layer of the tube into the flow velocity layer), so the possibility of adhesion contamination is small. In addition, due to the large signal internal resistance of the small-area electrode itself, the influence of changes in the internal resistance of the signal caused by the adhesion of the electrode is small, so the influence on the instrument measurement is also small.

I hope that the above analysis will help everyone. Because it can only be used if it is used correctly. And we will provide you with this information in a timely manner. Guarantee the normal use of your products.

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