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Electromagnetic flowmeter electrode selection precautions

Electromagnetic flowmeter electrode selection considerations
Electrochemical potential changes and polarization phenomena create interference potentials that create noise. Slurry noise and flow noise are the manifestations of electrode surface noise. The polarization potential is that the electrode induced electromotive force is different in polarity of the two electrodes, resulting in polarization of the electrolyte on the electrode surface. The low-frequency rectangular excitation combines the advantages of DC excitation and AC excitation. Although alternating excitation weakens the polarization potential by several orders of magnitude, it does not completely eliminate the influence of polarization potential interference. The polarization potential is related to the nature of the liquid medium and the properties of the material of the drain.
Slurry noise is when measuring the flow rate of liquid-solid two-phase conductive liquid such as mud fiber slurry, solid particles (or bubbles in liquid) rubbing over the surface of the electrode, the electrochemical potential of the electrode surface is suddenly deuterated, and the pulse of the output signal is spiked. .
The flow noise is the liquid flow elbow with a lower amount of 电I, and the liquid potential of the electrode changes periodically. The electrochemical potential of the electrode changes periodically, resulting in random noise with increasing frequency as the flow rate increases, causing the instrument output to appear. Fluctuation.
For electrode surface noise, materials with low electrochemical and polarization potentials of the liquid to be tested and low noise electrodes can be selected.
The measured medium generates a chemical reaction under the action of the catalyst of the electrode to affect the measurement. For example, the platinum electrode EhrW generates an aerosol on the surface of the electrode when measuring hydrogen peroxide, and the output fluctuates when the flow rate is zero.
For the medium to avoid or reduce the surface effect of the electrode - electrode material matching, there is not enough information to check as corrosive, only limited experience, yet to be accumulated in practice. Several examples are listed below.
钽 is resistant to water. However, if the water flow rate is measured by the erbium electrode EMF, an insulating layer is formed on the surface of the ruthenium electrode, causing the meter to malfunction or a large noise after a short period of operation. Strontium electrodes cannot be used for lye such as sodium hydroxide. In the process flow, even if the electrode is in contact with water or "non-acid" liquid for a very short time (such as cleaning the pipe system), it will affect the normal use of the instrument.
The platinum-rhodium alloy electrode or the platinum electrode has good corrosion resistance to hydrochloric acid, and the platinum electrode EMF is used for measuring hydrochloric acid to obtain satisfactory results. However, the measurement of higher concentrations of hydrochloric acid (above lOqo) produces severe noise. When the platinum electrode is used to measure low-pressure hydrogen peroxide (pressure below 0.3 MPa), an aerosol is generated on the surface of the electrode due to the action of the catalyst, which blocks the electrical path and affects the operation.
Hastelloy is corrosion-resistant to hydrochloric acid, which is not high in temperature and concentration. There are several good examples of application, but when the concentration exceeds a certain value, noise is generated. The test of changing the concentration of hydrochloric acid in the field showed that the concentration gradually increased over 15qo.
At 20%, the meter output will sway and the output will be up to 20% at 25%. Acids such as nitric acid and sulfuric acid also have similar practical experience.
The electromagnetic flowmeter used in the water plant is used to measure the aluminum sulfate solution mixed with the raw water to agglomerate the suspension. The mixing ratio is commonly used to measure the aluminum sulfate solution by EMF, and the acid-resistant steel electrode can be used to obtain satisfactory results. The planters have encountered {Half-Alloy B-electrode EMF with 15% aluminum sulfate. The output sloshing phenomenon also occurs during use. After changing to the acid-resistant steel electrode, it works normally.
Platinum and rhodium electrodes have good measurement effects on various concentrations of sulfuric acid, nitric acid, hydrochloric acid and hydrofluoric acid, but the platinum electrode produces noise with hydrochloric acid concentration greater than 10%. The concentration of tantalum electrode is greater than lOgo. Hydrofluoric acid is not resistant to corrosion.
Although platinum and rhodium are better in corrosion resistance, they are expensive, and a pair of tantalum electrodes are as high as several hundred to nearly one thousand yuan. The platinum-iridium electrode is more than twice as expensive as the tantalum electrode.
The grounding ring is connected to the flow sensor of the plastic pipe or the metal pipe lined with the insulating lining. Their corrosion resistance requirements are lower than the electrode, allowing some corrosion and regular replacement. Acid-resistant steel or Hastelloy is usually used. Due to the large size, it is less economical to use precious metals such as ruthenium platinum. A grounding ring is not required if the metal process pipe is in direct contact with the fluid.

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