Discussion on the Cause of Measurement Error of Electromagnetic Flowmeter and its Improvement
Time:2022-11-08 Read:719
Reading guide: On the basis of expounding the working principle and characteristics of electromagnetic flowmeter, this article discusses the causes of its measurement error and the improvement measures for reference.

1. Operating principle of electromagnetic flowmeter

Electromagnetic flowmeter is a special instrument for flow measurement based on the principle of electromagnetic induction, which can accurately measure the volume flow of conductive liquid in industrial production. The calculation formula is E=kBDv, where E represents the induced electromotive force, in V; K is the coefficient; B is the magnetic induction intensity, T; D is the inner diameter of the measuring pipe, in m; V is the liquid flow velocity, in m/s.

The application process of the electromagnetic flowmeter, can be considered the practical application of electromagnetic induction law in the industrial production field. Based on the calculation formula of the electromagnetic flowmeter, when the magnetic induction intensity and pipe diameter are relatively stable, there is a positive change relationship in the induced potential and pipe volume flow, and there is also a positive change trend in the induced potential and measured dielectric volume flow.

2. Characteristics of electromagnetic flowmeter

The electromagnetic flowmeter with its own uniqueness shows obvious characteristics in specific applications, which is basically composed of the following three aspects: first, in the structure of the electromagnetic flowmeter, because the internal parts of the measuring pipe and the barrier fluid are not movable, it will not produce mechanical inertia and pressure loss, and its response is relatively sensitive. Second, the measuring range is wide, which can reach 10:1 or even 100:1 in general. At the same time, the velocity range is mainly (1~6) m/s, even (0.5~10) m/s. The range of measured pipe diameter shall be 2mm at least and 3000mm at most. Third, it can support the measurement of liquid volume flow containing solid particles or with a certain conductivity (acid, alkali, salt solution), and can also measure the pulsating flow.

Although the electromagnetic flowmeter has many advantages and is quite outstanding, there are still shortcomings, mainly in the following two aspects: First, it is not allowed to use too high temperature and pressure. In specific use, the temperature of the electromagnetic flowmeter shall be controlled within 120 ℃, and the operating pressure shall not exceed 1.6MPa. Such requirements exist because the temperature will affect the material deterioration, deformation, and expansion of the pipe lining to a certain extent. The specific value of service pressure is usually determined by such factors as the sealing condition of the electrode part, pipeline strength, and flange specification. Second, the measuring range of an electromagnetic flowmeter is greatly limited, and it can not be used for the measurement of non-conductive fluid flow, such as steam, gas, and petroleum products.

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3. Measurement error caused by the electromagnetic flowmeter and improvement countermeasures

3.1 Significant change and improvement of the conductivity of the liquid to be measured

Once the conductivity of the liquid to be measured changes greatly, the displayed value will also fluctuate greatly, and even have a severe impact on the normal operation of the corresponding control system. When the conductivity of the liquid to be measured is low, it will form an impedance to the output of the electrode. When the actual conductivity is lower than the lower limit, the normal operation of the instrument will be interrupted. In view of the causes of such errors, improvements can be made in the following aspects: First, when selecting an electromagnetic flowmeter, try to select the type with low conductivity on the premise of meeting the requirements. The other is to assemble a straight pipe section or reactor that can fully mix materials or achieve a chemical reaction. Third, other flowmeters with different principles are introduced.

3.2 Bubble phenomenon or non-full tube state of liquid to be measured and improvement

Bubbles are usually formed after the dissolved gas in the liquid gradually develops into a free state or is inhaled into the outside world. If the volume of bubbles contained in the flow is large, the measurement accuracy will be affected. However, when the bubble has an excessive diameter or even a larger value than the motor diameter, the state in the measurement and display process will be extremely unstable, and it is impossible to avoid fluctuations. In view of the causes of such errors, improvements can be made in the following aspects: (1) Install a gas collector on the electromagnetic flowmeter and perform the exhaust operation according to the cycle. (2) Reasonably replace and adjust the installation position. (3) Install vertical pipes on the electromagnetic flowmeter to ensure its direction is from bottom to top. (4) During the installation of the sensor, it is necessary to keep a proper distance from the drain. (5) A sensor is installed at the position of the control valve to control its position upstream or downstream of the pump.

3.3 Space electromagnetic wave interference and improvement

When the electromagnetic flowmeter is used in the measurement practice, if there is a long cable between the converter and the sensor, and there is strong electromagnetic interference around, the interference signal will be introduced due to the existence of the cable, and eventually, there will be a common mode interference phenomenon, which will lead to nonlinear, significant distortion or large amplitude shaking of the flowmeter, and the accuracy of the measurement will be greatly reduced. In view of the causes of such errors, the following measures can be taken to solve them: (1) During the installation of an electromagnetic flowmeter, it is necessary to deeply analyze the surrounding environment to ensure that the electromagnetic flowmeter has a strong magnetic field. (2) The cable length shall be controlled within the appropriate range as far as possible, and relevant shielding measures shall be implemented, such as passing the cable into the grounding steel pipe to prevent the power line and cable from passing into the same pipe. (3) The selection of shielded cables that meet the requirements can also effectively reduce the interference caused by electromagnetic waves.

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3.4 Connecting cable problems and improvement

The electromagnetic flowmeter is a system composed of specific cables, converters, and sensors. Therefore, the cable length, the number of shielding layers, the cross-sectional area of the conductor, the insulation, and the distributed capacitance will affect its measurement results, and even interfere with the normal operation of the electromagnetic flowmeter. Therefore, when installing the electromagnetic flowmeter, it is not only necessary to determine the cable length with reference to the conductor cross-sectional area, the number of shielding layers, the conductivity of the liquid to be measured, and the distributed capacitance, but also to avoid the situation of the cable intermediate joints, and properly handle the ends to ensure a good connection. In addition, it is also necessary to ensure that the cables used to meet the standard requirements.

3.5 Layer and improvement in measuring pipe

Based on the application object of the electromagnetic flowmeter, it mainly measures non-clean fluid. If a certain amount of sediment and other substances exist in the non-clean fluid during the actual measurement, the normal use and measurement of the electromagnetic flowmeter will inevitably be affected, such as polluting the pipeline and electrode surface of the electromagnetic flowmeter, resulting in measurement errors. Faced with the causes of such errors, relevant personnel should do a good job of regularly cleaning the electromagnetic flowmeter in their daily work, and appropriately increase the flow rate. In addition, polytetrachloroethylene can be selected as the lining material.

3.6 Electrode selection, liquid flow rate, and improvement

In the practical application of an electromagnetic flowmeter, its electrode and internal materials will directly contact the liquid to be measured, so the selection of electrode and lining materials should be based on the liquid to be measured. Complete the determination of lining material properties in combination with the properties of the liquid to be measured, and strictly control the measured temperature in the actual measurement to avoid wearing or deformation of lining materials due to unreasonable selection of lining materials or insufficient temperature control, which will lead to faster adhesion and increase the occurrence rate of measurement errors. In view of this situation, while highlighting the pertinence of lining material selection, the application of an electromagnetic flowmeter also needs to reasonably select electrodes, properly control the liquid flow rate, and ensure that it is within a reasonable range.

3.7 Asymmetric state of measuring liquid and improvement

When an electromagnetic flowmeter is used to measure the flow of relevant liquid, if the liquid to be measured has an asymmetric state, it will inevitably cause measurement error. Liquid asymmetry is usually manifested in the combination of single vortex flow or straight flow along the pipeline axis. In this case, the integral of the pipe section is the liquid volume flow. If there are deficiencies in the upstream straight pipe section, generally, the flow can be adjusted by combining the flow regulator to control the same value between the inner diameter of the flowmeter and the inner diameter of the pipe within a certain range of upstream and downstream to ensure that the upstream straight pipe section is sufficient.

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3.8 Symmetry of electrode and excitation coil and improvement

There is a strict requirement for symmetry when machining the magnetic coil and electrode of the electromagnetic flowmeter. If there is asymmetry, it will inevitably cause asymmetry deviation, which will affect the measurement results, and eventually, there will be measurement errors. At the same time, the installation of the electromagnetic flowmeter also strictly requires the vibration of the installation site. For example, the installation of the integrated electromagnetic flowmeter needs to be in a place with low vibration. If the vibration exceeds the standard, there will be errors in the measurement, and even affect the normal operation of the instrument. Therefore, before actual installation, relevant personnel need to carry out strict measurements of vibration at the installation location to ensure compliance with installation standards.

4. Summary

In the specific use of an electromagnetic flowmeter, due to a large number of influencing factors, the measurement results will inevitably produce errors. In order to reduce or avoid measurement error and ensure accuracy, it is necessary to conduct in-depth research on the basic principle of electromagnetic flowmeter, carefully analyze each measurement link, find out the cause of the error, and introduce targeted treatment measures to control measurement error, so as to ensure that its due role can be fully played on the premise of eliminating measurement error.