Analysis of Measurement Error of Electromagnetic Flowmeter

Nowadays, technology is developing rapidly. In this article, we will explain in depth the application scenarios of electromagnetic flowmeter measurement error analysis and the advantages of electromagnetic flowmeter measurement error analysis compared with other domestic products. We hope it will be helpful to everyone.
 

Electromagnetic flowmeter is a kind of velocity flowmeter. It uses the induced electromotive force generated by the flow of conductive fluid in the magnetic field to calculate the fluid flow rate. It can be said that the accuracy of the electromagnetic flowmeter is closely related to the rational and effective use of energy and the settlement of trade. Therefore, its measurement error has become a very important issue.

1. The principle of electromagnetic flowmeter..Electromagnetic

flowmeter is an instrument for measuring the volume flow of conductive liquid made by Faraday's law of electromagnetic induction. The basic principle is Faraday's law of electromagnetic induction, that is, when a conductor cuts the magnetic lines of force in a magnetic field, an induced electromotive force is generated at both ends of the conductor. Of course, the above formula is established under certain strict conditions. The magnetic field is uniformly distributed under constant magnetic field.. The flow velocity of the measured fluid is axially symmetrically distributed. That is to say, if the flow velocity near the electrode is non-axially symmetrical and large, the measured flow

signal is larger than the actual flow value. The flow signal measured at the place where it is symmetrical to the electrode is small, as shown in the figure.. When a non-magnetic liquid flows in a non-magnetic measuring tube perpendicular to the magnetic field, an induced electromotive force proportional to the flow rate is generated in the direction perpendicular to the flow direction. The direction of the electromotive force follows the "right-hand rule.. .. .... .. -...... .. .... .. ", and

its value is as follows..

.... The measured liquid is non-magnetic.. According to the principle of electromagnetic flowmeter, it is obvious that if the medium of the measured liquid is magnetic, it will seriously interfere with the measurement results.

.... The conductivity of the measured liquid is uniform and isotropic.. The conductivity of the measured liquid is required to be uniform and isotropic, because, from the formula.. it can be seen that the induced electromotive force is generated by the conductive medium cutting the magnetic lines of force. When the conductivity of the measured medium is uneven, isotropic and non-isotropic, a certain additional electromotive force will inevitably be generated, which will also cause inaccurate measurement results. At the same time, if the conductivity of the measured medium is lower than a certain lower limit, the internal resistance generated is large, which bears a large amount of induced electromotive force, which is too large relative to the converter input impedance and affects the measurement results. In addition, due to the limitation of the result principle of the electromagnetic flowmeter itself,

the conductivity of the measured medium is not allowed to be too large.

2. Types and analysis of errors.

From the above, we can see that the essence of the measurement of A is mainly the measurement of induced electromotive force. In practical applications, for electromagnetic flowmeters that have passed factory calibration, the accuracy in application is often affected by many factors, but it is nothing more than the induced electromotive force, magnetic field strength, measuring tube, and the degree of compliance with the above theoretical working conditions. Therefore, the errors of electromagnetic flowmeters in actual work can be roughly summarized into the following categories.

1. Design and selection factors.

In the actual application of electromagnetic flowmeters, the selection of flowmeters is very important. Some design projects ignore the characteristics of electromagnetic flowmeters themselves, and certain problems occur in the engineering design and selection links, causing unnecessary errors. ... Errors caused by improper matching of the inner diameter between the flow sensor and the connecting pipe. The inner diameter difference between the flow sensor and the connecting pipe is

too large. It is stipulated that the inner diameter of the connecting pipe shall not be less than the inner diameter of the flow sensor and shall not be greater than the inner diameter. In the absence of manufacturer recommendation, the error caused by improper selection of the instrument range. The measured normal working flow should be less than the full scale of the flowmeter. Considering the accuracy factor, it should be greater than the full scale of the instrument. It is best to be around. This problem should be taken seriously, especially because the adjustment of the equipment operation capacity, but the problem of the flowmeter is ignored, which often causes unnecessary errors. Errors caused by ignoring the physical and chemical properties of the measured medium. For some measured media, especially many chemical companies, the physical and chemical properties of the measured medium are ignored. Under high temperature, high pressure and strong corrosive conditions, the selection of inappropriate electromagnetic flowmeters will also cause large errors. Because the lining of the electromagnetic flowmeter is made of insulating non-magnetic material, many chemical raw materials will corrode the lining and electrodes and cause measurement errors. Therefore, in this case, pressure-resistant and corrosion-resistant electromagnetic flowmeters or other suitable flowmeters are generally selected. Errors caused by flow field conditions Errors caused by the measured medium not being full in the pipe. In the application of electromagnetic flowmeters, many application units do not install the flowmeter in the appropriate position due to the limitations of the construction site and installation location,

resulting in the failure to fill the liquid in the measuring pipe. If there are bubbles in the fluid, it will cause output shaking, unstable flow, increased error and even over-full phenomenon. Therefore, during the installation of the flowmeter, it should be installed vertically as much as possible. If the site conditions only allow horizontal installation, it is necessary to ensure that the two electrodes are on the same horizontal plane, and the flowmeter should be placed at the lowest point of the pipeline as much as possible to ensure a certain back pressure at the outlet. It is best to reserve an exhaust valve at an appropriate position in the pipeline. The correct installation method... Error caused by velocity distribution.

When the measured medium flows in the pipe, it is often greatly affected by the pipeline conditions. When there are valves, elbows, tees and other pipe fittings upstream of the electromagnetic flowmeter, or when foreign objects enter the pipeline due to poor maintenance and interference, it often has a greater impact on the velocity distribution of the flowmeter, thereby affecting the measurement accuracy of the flowmeter. Therefore, the flow regulating valve should be placed downstream of the flowmeter as much as possible. For the horizontally installed flowmeter, the upstream... times the diameter... and the downstream... times the diameter... straight pipe section should be ensured to achieve the purpose of rectification. .., Errors caused by working medium........ Errors caused by bubbles in the fluid.. When the measured medium flows in the pipe, the gas dissolved in the liquid will often be released when the temperature or pressure changes greatly. In addition, when the flow meter is installed in an inappropriate position, such as after a throttling pipe, a sudden contraction pipe, or when the opening of the upstream regulating valve of the fluid is too small, it can be seen from the Bernoulli equation.. .. .. .. .. ......' .. .. .. .. .. ...... .. .. that as the flow rate increases and the pressure decreases, a certain amount of bubbles will be generated, and these bubbles will also interfere with the electrode and cause measurement errors. .... Errors caused by working medium contamination of the flow meter.. Electromagnetic flow meters are used to detect certain special media, such as certain liquid chemical products, specially treated water and other fluids.

A large number of crystals or electrodes are often generated during work, which seriously affects the measurement of the electrode on the one hand, and on the other hand, siltation is formed in the pipe to change the inner diameter of the flow meter. In addition, when measuring fluids with attached sediments, the electrode surface will be contaminated, which often causes zero point changes. Therefore, when using electromagnetic flowmeters in these special occasions, on the one hand, cleaning holes should be added or pipelines should be cleaned, or the meter should be cleaned regularly. On the other hand, when designing the flow rate, the flow rate of the medium flowing through the flowmeter should be large enough to prevent the deposition of pollution, or new electromagnetic flowmeters with "electrode self-cleaning" function and "capacitive" should be selected to overcome the above shortcomings. , Errors caused by installation and site conditions.
.... Errors caused by the lack of grounding of the sensor housing. People generally believe that the sensor is installed on a metal pipe, and the pipe is buried underground, and the pipeline is very long, so the instrument does not need to be grounded. However, this often ignores two problems. One is that the pipeline is generally treated with anti-corrosion, and the other is that the connecting flange and the pipeline are separated by punching or rubber pads, which greatly increases the grounding resistance of the sensor, affecting the normal operation of the flowmeter and causing errors. At the same time, since the detection electromotive force is very small, generally a few millivolts, it is also very easy to be affected by "stray current". .... ..Error caused by interference of output signal..For general electromagnetic flowmeters, the connecting cable between the sensor, that is, the electrode, and
the converter should be as short as possible. If the signal transmission cable is too long, the load effect caused by the distributed capacitance of the cable will increase the measurement error and increase the possibility of signal interference.

And be sure not to route the signal line and the power line together. Route the signal line smoothly to prevent "parasitic capacitance" and unnecessary interference. If conditions permit, try to use a digital output instrument to obtain the best measurement results. .... ..Errors caused by strong electric and magnetic interference..In the working environment of the flow meter, try to avoid being too close to strong electric and magnetic equipment. Because, after the flow meter is grounded, if there are other grounded equipment nearby, if the grounding voltage drop is generated on the equipment, the grounding potential of the electromagnetic flow meter may change, affecting the measurement and causing errors.. Similarly, if the flow meter works near a strong magnetic field, such as a transformer and other equipment, when the magnetic field strength exceeds its electromagnetic compatibility requirements, it will also have a greater impact on the measurement results.

3. Conclusion

The above are some superficial views put forward by the author on the principle and manifestation of errors in the current electromagnetic flowmeter application. In actual situations, there should be more experience and understanding worth summarizing, but I still hope that this article can provide some help to the majority of electromagnetic flowmeter users. At the same time, I also hope that flowmeter users can strengthen their work in the selection, maintenance and application of equipment, regularly perform maintenance inspections, data comparisons, and metrological verifications to avoid unnecessary errors and optimize the use of electromagnetic flowmeters.

In summary, this article has explained the measurement error analysis of electromagnetic flowmeters. I believe that everyone has a deeper understanding of the measurement error analysis of electromagnetic flowmeters. I hope that this article can be of great reference value to all readers.