Electromagnetic flow meters have many advantages,but if the selection, installation and use are improper,they will cause an increase in error,unstable indication, and even damage to the body.
(1) The liquid in the pipe is not fully filled. Due to insufficient back pressure or poor installation position of the flow sensor, the measured liquid in the pipe is not fully filled, and the fault phenomenon is different due to the degree of not fully filled and flow condition.If a small amount of gas shows stratified flow or wave flow in the pipeline, the fault phenomenon shows that the error increases, that is, the flow measurement value is not consistent the actual value.If the flow is bubble flow or plug flow,the fault phenomenon will not only show that the measured value is not consistent with the actual value, but also appear output sloshing because of the gas phase covering the electrode surface instantaneously. If the gas phase part of the flow cross-sectional area of the stratified flow in the horizontal pipeline increases, that is, the degree of liquid not fully filled increases,and the output sloshing will also occur.If the degree of liquid not fully filled is more serious,so that the liquid level below the electrode, the output over-fullness phenomenon will occur.
(2) The liquid contains solid phase, and the liquid contains solids such as powder, particles or fibers,which may cause faults, such as: ① slurry noise; ②electrode surface contamination; ③conductive deposition layer or insulating deposition layer covers the electrode or lining;④the lining is worn or covered by sediment,and the flow cross-sectional area is reduced.
(3) Liquids that may crystallize, electromagnetic flow meters should be carefully used with some easily crystallized chemical materials. They can be measured normally under normal temperature conditions,because the pipe for conveying the fluid has good heat insulation,it will not crystallize during the heat preservation work,but the measuring tube of the electromagnetic flow sensor is difficult to carry out the heat insulation,therefore, when the fluid flows through the measuring tube,it is easy to cause a layer of solid on the inner wall due to the temperature drop.Since the flow meter measurement using other principles also has crystallization problems, therefore, in the absence of other better methods, a "ring" electromagnetic flow sensor with a very short measuring tube length can be selected, and the upstream pipeline of the flow meter can be strengthened with heat insulation.In the connection method of pipeline,it should be considered that the flow sensor is easy to be disassembled and installed,and it can be easily disassembled and maintained once it is crystallized.
(4) Problems caused by improper selection of electrodes and grounding ring materials.The parts of electromagnetic flow meter contacting with the medium are electrode and grounding ring,and the electromagnetic flow meter causing fault due to mismatch between material and measured medium,in addition to the problem of corrosion resistance,there are surface effects of electrodes. Surface effects should include:①chemical reactions (passive film formed on the surface etc.);②electrochemical and polarization phenomena (generating potential); ③catalyst action(aerial fog formed on the electrode surface, etc.).Grounding rings also have these effects, but the impact is less.
(5) The problem that the liquid conductivity exceeds the allowable range,and if the liquid conductivity is close to the lower limit, there may be a sloshing phenomenon.Because the lower limit specified by the manufacturer's instrument specification is the lowest value that can be measured under various conditions of better use,and the actual conditions are not all ideal, so low-degree distilled water or deionized water is encountered many times,and its conductivity is close to the lower limit value 5 specified by the electromagnetic flow meter specification, and output sloshing occurs when using.The lower limit of the conductivity that can be measured stably is generally considered 1~2 orders of magnitude higher.
The conductivity of the liquid can be found in the relevant manual.If the data is not available, the sample can be measured with a conductivity meter.However, there are cases where sampling from the pipeline to laboratory determination is considered to be available and the actual electromagnetic flow meter does not work.This is because the liquid at the time of measuring conductivity is different from the liquid in the pipeline. For example, the liquid has absorbed CO2 or NO in the atmosphere to form carbonic acid or nitric acid, and the electrical conductivity is increased.