Electrode polarization analysis of electromagnetic flowmeters

The advantages of electromagnetic flowmeters are: first, high measurement accuracy; second, there are no obstructions in the sensor pipe, wide measurement range and low maintenance cost. The only parts of the electromagnetic flowmeter that are in contact with the measured medium are the lining and the electrode. Generally, there will be no problems after the lining is selected during model selection. What really affects the measurement accuracy is the electrode. In addition to selecting the electrode during model selection, Yes, in the later stage, regular cleaning and other maintenance work must be done according to the complexity of the working environment. The electrode will undergo electrode surface effects over time and the properties of the medium, affecting the measurement accuracy or even failing to work.

The electrode surface effect generally refers to the chemical reaction effect on the electrode surface, electrochemical phenomena, polarization potential and the catalytic effect of the electrode. The editor has made a summary for everyone’s reference.

1. Catalyst effect: refers to the chemical reaction of the measured medium under the action of the electrode, which affects the measurement. For example, when measuring hydrogen peroxide with a platinum electrode electromagnetic flowmeter, there will be no problem under normal pressure. However, when measuring hydrogen peroxide under low pressure (less than 0.3Mpa), aerosol will be generated on the electrode surface, blocking the electrical path. causing the output signal to fluctuate.

2. Polarization potential: refers to the electrode-induced electromotive force having different polarities on the two electrodes, causing the electrolyte to become polarized on the electrode surface. Early electromagnetic flowmeters used DC excitation electrodes, which caused serious polarization phenomena. Nowadays, low-frequency square wave excitation is often used. Low-frequency square wave excitation has the advantages of both DC excitation and AC excitation, although alternating excitation weakens the polarization potential by several orders of magnitude. , but it does not completely eliminate the influence of polarization potential interference.

3. Electrochemical phenomenon: refers to the contact between the electrode and the measured medium, and the potential change occurs on the electrode surface. For example, when measuring the flow of a fluid containing solid particles such as sediment or fiber slurry, the solid particles rub against the electrode. Although the electrode is not corroded, the contact potential of the impacted surface of the electrode changes suddenly, causing spikes in the instrument output signal to cause interference.

4. Chemical reaction effect: refers to the formation of a passivation film or oxide layer on the surface of the electrode after the electrode is in contact with the measured medium. At this time, the electrode is not corroded, but the formed passivation film or oxide layer is. Increased contact resistance on the electrode surface. For example, the electrode material of the electromagnetic flowmeter is tantalum, and the measured medium is sewage. Tantalum electrodes cannot be used to measure water because when measuring water flow, an insulating layer will form on the surface of the tantalum electrode to block the transmission of induced voltage.

In summary, we can see that the flow meter is a precision instrument, and all factors must be considered comprehensively when selecting the model. Otherwise, the subsequent use of the flow meter will be affected or even impossible to use. The application range of electromagnetic flowmeters is becoming more and more extensive, and more and more problems are exposed during use. Most of the problems are caused by improper selection. Therefore, when choosing a flowmeter, you must contact the flowmeter manufacturer. Check the technical parameters.