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Explanation of the causes of electromagnetic flowmeter error

The electromagnetic flowmeter has high measurement accuracy, but because of its harsh environment, it is 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 [4]. 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 cases, the flowmeter measurement range of the electromagnetic flowmeter does not match the actual flow velocity, and the inner diameter of the pipeline also has corresponding problems, which will result in statistical data. Not precise. In addition, in the electromagnetic flowmeter, in order to avoid contact between the electrode and the corrosive liquid, lining protection is adopted, and many uses do not take this into consideration, and the selected electrode and the lining are not appropriate, and may be worn in use. 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 modes 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 [5].

3.2 Influence of the liquid to be tested
Since the most important function of the electromagnetic flowmeter is to calculate the flow rate of the fluid, and the condition of the fluid may be very complicated, the liquid to be tested may 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, the measured value will have a significant deviation. 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 controlled range and reduce the precipitation of impurities. In addition, materials such as polytetrafluoroethylene can also be used as an 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 [6].

3.3 External interference
Interference from the outside may also affect the accuracy of the electromagnetic flowmeter. The specific 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, corresponding interference will occur, and these disturbances will be directly reflected on the measuring device, and the values show nonlinear changes. In order to solve this problem, the shielding effect can be used to reduce the influence of interference. In the grounding pipe, the cable is separately introduced, 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 anti-vibration protection 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.

The electromagnetic flowmeter is affected by external factors during use, but the impact of external factors on the accuracy of the electromagnetic flowmeter can be reduced through targeted analysis and measures. 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. After the requirements are met, they can be put into use, which can reduce the interference and improve the accuracy of the secondary flow of electromagnetic flow.

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