When measuring sewage, slurry and other media, scaling and attachments are likely to occur on the inner wall of the pipe and the electrode surface. When the conductivity of the fouling substance is different from the conductivity of the measured medium, it will bring measurement errors. The attachment of sludge and oil to the electrode will also cause the output of the instrument to swing and drift. Therefore, in some cases, the electrode needs to be maintained. For example, clean the electrode and replace the electrode.
The common methods of electrode cleaning are as follows:
(l) Electrochemical method
The metal electrode has an electrochemical phenomenon in the electrolyte fluid. According to the principle of electrochemistry, there is an interfacial electric field between the electrode and the fluid. The interfacial electric field between the electrode and the fluid is caused by the electric double layer that exists between the electrode and the fluid. The study of the electric field between the electrode and the fluid found that the molecules, atoms or ions of the substance have rich or poor adsorption at the interface, and it is found that most inorganic anions are surface-active substances, which have typical ion adsorption rules, and the surface of inorganic cations The activity is very small, so the electrochemical cleaning electrode only considers the case of anion adsorption. The adsorption of anions is closely related to the electrode potential. The adsorption mainly occurs in the range of potentials that is more positive than the zero charge potential, that is, the surface of the telegraph with different charges. On the surface of the electrode with the same charge, when the residual charge density is slightly larger, the electrostatic repulsion is greater than the adsorption force, and the anion is quickly desorbed. This is the principle of electrochemical cleaning. Some companies apply a negative voltage of about 18.104.22.168V to the two electrodes in the form of common mode by adding the voltage drop of the two forward diodes to the signal loop. Because the voltage applied to the two electrodes is a negative DC common mode voltage, it will not cause the amplifier to saturate. The DC common mode voltage is superimposed on the tiny alternating flow signal, the DC is isolated by the capacitor, and the common mode voltage is suppressed by the preamplifier. The DC common mode voltage does not affect the flow measurement. The negative DC voltage applied to the electrode forms a negative electric field that can repel the material attached to the electrode and reach the day of cleaning the electrode. This method can effectively, automatically and continuously decontaminate electrodes in AC excitation. However, for low-frequency rectangular wave excitation, because of the high polarization voltage amplitude, the effect is not necessarily very good, so it has rarely been seen recently.
(2) Mechanical removal method
The mechanical removal method is to remove the electrode by installing a special mechanical structure on the electrode. There are currently two forms:
One is to use a mechanical scraper. A stainless steel scraper with a thin shaft is used to draw the scraper out through the hollow electrode. A mechanical seal is used between the thin shaft and the hollow electrode to prevent the medium from becoming outside, so it becomes a mechanical scraper. When the thin shaft is rotated from the outside, the scraper rotates against the plane of the electrode end to scrape off the dirt. This type of scraper can be manually scraped, or it can be scraped automatically by a motor-driven thin shaft. The other is to install a wire brush for removing dirt in the tubular electrode, and the shaft is wrapped in a sealed 'O' ring to prevent fluid leakage. This cleaning device requires someone to pull the wire frequently to clean the electrode.
(3) Method of ultrasonic cleaning
The ultrasonic voltage of 45~65kHz generated by the ultrasonic generator is applied to the electrode to increase the energy concentration of the ultrasonic wave. The applied voltage is almost concentrated on the attachment. The high voltage will break the attachment and then be washed away by the fluid. For safety reasons, the electrical breakdown method must be to interrupt the measurement with a flowmeter, disconnect the signal line between the sensor and the converter and the converter, and connect the AC (50Hz or 60Hz) high voltage electricity directly to the sensor signal output terminal in the event of power failure For cleaning.
(4) Improve the average in the measuring tube
Flow rate and the use of a pointed small area electrode When measuring media that is prone to fouling and adhesion, you can usually choose a sensor with a smaller diameter than the process tube to increase the flow rate. Experience shows that the average flow velocity in the pipe is higher than 2m/s, and the possibility of sedimentation and adhesion is generally small. There are also measures to increase the flow rate by 3-5m/s instantaneously (depending on the attachment situation) to wash the adhesion layer. The electrode head protrudes into a sharp shape, and is subject to large fluid erosion force (because the flow velocity of the pipe wall is equal to zero, the tip is separated from the boundary layer of the pipe wall and enters the flow velocity layer), so the possibility of adhesion and contamination is small. In addition, due to the large internal signal resistance of the electrode with a small area, the influence of the change in the internal resistance of the signal caused by the adhesion of the electrode is small, so the impact on the instrument measurement is also small.