Abstract: Electromagnetic flowmeters are widely used in the measurement of various conductive liquid flows in industrial processes. This paper introduces the structural principle and characteristics of electromagnetic flowmeter, analyzes the causes of electromagnetic flowmeter error from three aspects, and analyzes the causes and solutions.
Electromagnetic flow sensors are commonly used in the measurement of various liquid flows and are widely used in the current industry. For example, corrosive substances such as acid and alkali can measure the flow in the pipe with a specific cross-sectional area through the electromagnetic flow sensor. It is not only convenient to measure, but also avoids contact with corrosive liquids. It has formed a unique application. field.
1 Electromagnetic flowmeter works
Electromagnetic flowmeter is a new type of measuring instrument. It mainly uses electromagnetic induction to measure the flow of liquid. The electromagnetic flowmeter not only measures a variety of fluids such as steam, purified water, sediment and slurry, but also has high measurement accuracy. In the specific application, the electromagnetic flowmeter is based on the principle that the conductor cuts the magnetic wire to generate the induced potential. When the fluid passes through the specific pipeline, the potential difference between the two ends of the pipeline is formed, and the potential difference has a direct relationship with the velocity of the flow. The conversion can be calculated from the potential difference across the pipe to calculate the flow rate of the fluid . As shown in Fig. 1, it is a schematic diagram of the structure of the electromagnetic flowmeter, and finally the flow is measured by collecting electrons through the electrodes.
Figure 1 shows the structure of the electromagnetic flowmeter
2 Main characteristics of electromagnetic flowmeter
The characteristics of the electromagnetic flowmeter mainly include the following items:
1) A wide range of applications. Electromagnetic flowmeters measure the flow rate of a fluid without being affected by factors such as temperature and viscosity, and because of the unobstructed flow in the measuring tube, the requirements for straight pipe sections are low. Moreover, even a mixture with sediment can be measured because it is not in direct contact with the fluid. Therefore, the electromagnetic flowmeter has a wide application range and can be applied to flow rate measurement of various fluids.
2) The measurement accuracy is high. The electromagnetic flowmeter has a high measurement accuracy. If the measurement is performed strictly according to the corresponding standard and the measurement environment is ensured, the precise fluid flow rate can be obtained. In the specific use, the parameters can be set according to the specific situation, and the programming can also be personalized. In particular, the novel excitation method can further improve the accuracy of measurement. In the specific application, the electromagnetic flowmeter will show the potential difference of the millivolt level, and the calculation method is also systematic, so the calculated fluid flow is very accurate.
3) Long service life. The electromagnetic flowmeter has a self-diagnostic function, and its internal movable parts are few, and there is no additional pressure loss in the pipe, so the service life is extremely long. Generally, after installing the electromagnetic flowmeter, the corresponding lining protection electrode is added to reduce the damage of the corrosive substance in the fluid to the electromagnetic flowmeter, so the service life is longer .
4) Remote control is possible. The electromagnetic flowmeter is more sensitive, and the electrical signal can be output as a standard signal through the converter output to achieve long-distance transmission, which facilitates the control of the flow rate measurement. This feature of the electromagnetic flowmeter is of great help to the management of the network. In many areas with harsh environments, it is not necessary to set up special statisticians, which reduces the cost of traffic statistics, and the feedback is more timely and accurate.
3 Error analysis
Electromagnetic flowmeters have high measurement accuracy, but because of the harsh environment they use, they are easily affected by many factors, resulting in low measurement accuracy. In practical applications, due to improper operation, unreasonable equipment selection, unscientific installation and other factors, measurement errors are generated when using electromagnetic flowmeters for flow measurement . The following three aspects are used to analyze the causes of errors in the flow measurement of the electromagnetic flowmeter.
3.1 Improper selection
Improper selection is one of the main causes of electromagnetic flowmeter error, in many situations.
In the case where the flow rate measurement range of the electromagnetic flowmeter does not match the actual flow rate, and the inner diameter of the pipe is matched, there is a corresponding problem, which causes the statistical data to be inaccurate. In addition, in the electromagnetic flowmeter, in order to avoid the contact of the electrode with the corrosive liquid, lining protection is adopted, and in many applications, the reason is not taken into consideration, the selected electrode and the lining are not appropriate, and the wear is caused by wear. Deformation, corrosion, scaling and other causes of internal lining deformation, which will also produce corresponding errors.
There are many types of excitation methods for electromagnetic flowmeters. Different excitation methods have different application ranges, and some are prone to interference currents, and some may cause zero fluctuations. When selecting, it needs to be determined according to the specific measuring fluid. Under normal circumstances, the voltage and frequency should be kept stable, so that the magnetic field can be as constant as possible and the measurement error is lower. However, in some cases, the excitation mode of the electromagnetic flowmeter is improperly selected, resulting in a disorder of the magnetic field, and thus the measured data has a certain deviation. In some applications, the fluid is a solid-liquid mixture. The measurement of such a fluid is complicated, and a one-way electromagnetic flowmeter cannot be used. Instead, a solid-liquid separation section should be used to mount the sensor. In many applications, this problem is neglected, which in turn causes measurement errors.
3.2 Influence of the liquid to be tested
Because the main function of the electromagnetic flowmeter is to calculate the flow of the fluid, and the fluid
The situation may be very complicated, so the liquid to be tested will also cause errors in the measurement of the electromagnetic flowmeter. The main reasons and countermeasures are as follows:
1) The conductivity of the liquid to be tested is too large or too low. If the conductivity of the liquid to be tested is too large, the measured value will fluctuate significantly, causing disturbance of the control system, which in turn affects the accuracy of the measurement. If the conductivity of the liquid to be tested is low, the normal output of the electrode cannot be guaranteed. In many cases, the electromagnetic flowmeter does not count the corresponding data, which directly affects the accuracy of the measurement. In this case, it is generally necessary to classify the electromagnetic flowmeter and install a straight pipe section so that the flow rate of the fluid can be more accurately counted.
2) The fluid contains a lot of bubbles. Air bubbles can affect the measurement accuracy of the electromagnetic flowmeter, because the gas is dissolved in the liquid, but it is likely to be freed during the flow to form bubbles. The density of these bubbles is greatly different from the fluid, so it will affect The accuracy of the measurement. Moreover, bubbles that are too large in diameter can cause fluctuations in the measurement display, and the data obtained are not representative. In view of this situation, a gas collecting device can be installed near the electromagnetic flowmeter, and at the same time, the electromagnetic flowmeter is installed downstream of the pump and upstream of the control valve to reduce the influence of air bubbles on the measurement accuracy of the electromagnetic flowmeter.
3) The change in conductivity of the liquid to be measured. The measured liquid is likely to change in conductivity during transportation, which directly affects the accuracy of the measurement. Because the conductivity directly determines the potential difference across the electromagnetic flowmeter, if the conductivity is not constant during the flow, there will be a significant deviation in the measured value. Of course, this kind of situation usually occurs less. If this happens, you can use other principles of wandering to make measurements, to avoid the accuracy of the measurement affecting the accuracy of the results.
4) The effect of the precipitate. The general fluid is not a pure liquid, so it contains a variety of impurities inside, it is easy to precipitate in the flow process, pollute the electrode of the electromagnetic flowmeter, and thus the measurement accuracy of the electromagnetic flowmeter is affected. In order to solve this problem, it is first possible to increase the flow rate of the liquid within a controllable range and reduce the precipitation of impurities. In addition, materials such as polytetrafluoroethylene can also be used as the electrode for protecting the electromagnetic flowmeter from the lining to improve the accuracy. Of course, the best way is to clean the electromagnetic flowmeter regularly so that the measurement accuracy can be guaranteed to the greatest extent.
3.3 External interference
Interference from the outside may also affect the accuracy of the electromagnetic flowmeter.
The reasons and solutions are as follows:
1) The cable between the converter and the sensor is long. In an electromagnetic environment, if the cable length between the sensor and the converter is large, then corresponding interference will occur.
The interference is directly reflected in the measuring device, and its value shows a nonlinear change. In response to this problem, the shielding effect can be used to reduce the influence of interference. In the grounding pipe, the cable is introduced alone, and the length of the cable is reasonably shortened, so that the magnetic field interference can be minimized.
2) The grounding resistance of the electromagnetic flowmeter is too large. The output signal of the electromagnetic flowmeter is small because it is only the potential difference caused by the electrons in the fluid, generally only a few millivolts, so the anti-interference ability is weak. In order to solve this problem, usually the zero potential of the electromagnetic flowmeter needs reliable grounding to ensure that its internal resistance is small, and at the same time, the corresponding insulating material is applied on the inner wall of the electromagnetic flowmeter, so as to ensure the independence of the electromagnetic flowmeter. Reduce the impact of various disturbances on measurement accuracy.
3) The excitation coil of the electromagnetic flowmeter is asymmetrical. The accuracy of the electromagnetic flowmeter is high, which depends on the symmetry of the excitation coil. If the excitation coil inside the electromagnetic flowmeter is asymmetrical, the potential deviation will occur during the measurement, which will directly lead to inaccurate measurement results. In order to solve this problem, it is necessary to ensure the quality of the electromagnetic flowmeter, and it is preferable to perform the test before use. Of course, it is also possible to increase the protection against vibration in the installation place of the electromagnetic flowmeter, so that the working environment of the instrument can be effectively guaranteed, thereby effectively improving the measurement accuracy of the electromagnetic flowmeter.
Electromagnetic flowmeters are affected by external factors during use, but can be used to reduce the influence of external factors on the accuracy of electromagnetic flowmeters through targeted analysis and measures ring. The above points are the main influencing factors of the electromagnetic flowmeter in use, of course, there are other factors. The user can make judgments according to his actual use situation, take preventive measures in advance, and perform debugging and evaluation in advance, and then put them into use after the requirements are met, so that the interference can be reduced and the accuracy of the electromagnetic flow secondary statistical flow can be improved.
In current industrial production, there are many places where fluid flow rate measurement is required, and some fluids are highly corrosive and cannot be directly contacted, which increases the difficulty of the work. The appearance of the electromagnetic flowmeter solves the above problems well. It calculates the flow velocity by forming a potential difference between the electrons in the fluid. It not only has a wide range of application, but also has high measurement accuracy, which is of great help to industrial production. However, in the specific use of the electromagnetic flowmeter, a certain error is also generated. Based on this, Huaheng Instrument starts from the working principle of electromagnetic flowmeter, first analyzes the characteristics of electromagnetic flowmeter, then discusses the causes of the error of electromagnetic flowmeter, and discusses the corresponding solutions. It is hoped that electromagnetic flow can be given. Reasonable use of the meter provides certain reference opinions to promote the long-term development of the technology.