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The reason for the large measurement error of the electromagnetic flowmeter may be explained here.

The electromagnetic flowmeter has the advantages of no pressure loss, high precision and moderate price. It is widely favored by petrochemical, chemical and other industries and plays an important role in flow metering. However, in practical applications, due to improper operation, unreasonable equipment selection, and unscientific installation, measurement errors are difficult to avoid, causing trouble to the user. Therefore, the majority of instrument personnel should pay attention to the various factors that cause the error of the electromagnetic flowmeter.
In general, the main effects of electromagnetic flowmeter errors can be divided into three categories: improper selection, liquid impact and interference.
I. Liquid flow rate to be measured The flow rate of the electromagnetic flowmeter can be measured generally in the range of 0.5 to 10 m/s, and the economical flow rate is in the range of 1.5 to 3 m/s. In actual use, the inner diameter of the measuring tube should be determined according to the flow rate to be measured and the flow rate range of the electromagnetic flowmeter.
2. Electrode and lining material selection electrode and lining material are directly in contact with the liquid to be tested. The electrode and lining material should be selected according to the characteristics of the liquid to be tested (such as corrosiveness, abrasiveness, etc.) and working temperature. If improperly selected, it will result in Problems such as fast attachment speed, corrosion, scale, wear, and lining deformation, which in turn cause measurement errors.
Third, the excitation stability electromagnetic flowmeter excitation methods are DC excitation, AC sine wave excitation and dual-frequency rectangular wave excitation, DC excitation is easy to produce electrode polarization and DC interference problems, AC sinusoidal excitation is easy to cause zero point change, and dual frequency Rectangular wave excitation has excellent zero-point stability of low-frequency rectangular wave excitation, and high-frequency rectangular wave excitation has strong ability to suppress fluid noise. It is an ideal excitation mode. In practical applications, the stability of the power supply voltage and frequency should be ensured as much as possible to ensure that the magnetic field strength is constant and the measurement error caused by the change of the magnetic field strength is reduced.
4. Mixing phase fluid measurement When measuring the flow rate of liquid-solid mixed phase fluid (such as sediment-containing water) with an electromagnetic flowmeter, if an electromagnetic flowmeter calibrated by single-phase liquid is selected, measurement error will occur. Sensors are installed at straight sections that do not cause liquid-solid phase separation.

Liquid to be tested
1. When the conductivity of the liquid to be tested changes drastically, the conductivity of the liquid to be tested is large, which will cause large fluctuations in the displayed value. If the problem is very serious, the control system is difficult to achieve normal operation; and the conductivity of the liquid to be tested is too low. When the electrode is difficult to achieve normal output, if the conductivity of the liquid to be tested is below the lower limit value in the operation, the electromagnetic flowmeter is difficult to function normally. In view of these situations, first of all, based on actual needs, combined with relevant standards and requirements, the choice of electromagnetic flowmeter type; secondly, install reactor or straight pipe section to ensure the full mixing of materials, promote the smooth realization of chemical reactions; Re-select the flowmeter type.
2. The liquid bubble or non-full tube to be tested is mainly derived from the bubble dissolved in the liquid and the bubble in the free state. Flows containing a large volume of bubbles can affect the accuracy of the measurement. If the diameter of the bubble is too large, even exceeding the value of the electrode diameter, an unstable state will occur during the measurement display, and the fluctuation cannot be avoided. In view of this situation, first, the gas collector can be installed on the electromagnetic flowmeter, and the exhaust operation is performed according to the cycle; secondly, the installation position is reasonably replaced; again, the vertical pipe is installed on the electromagnetic flowmeter, and the protection is guaranteed from the bottom. The direction of the upper direction; fourth, when installing the sensor, avoid too close to the discharge port; fifth, install the sensor at the control valve position, in its upstream position, or downstream of the pump.
3. The conductivity of the liquid to be tested is too low. The conductivity of the liquid to be measured is lowered, which will increase the output impedance of the electrode, and the load error caused by the input impedance of the converter will cause measurement error. If the actual conductivity is lower than the lower limit, the instrument cannot In normal operation, the indication will fluctuate. Countermeasures: Select other low-conductivity electromagnetic flowmeters that meet the requirements, such as capacitive electromagnetic flowmeters; use other principle flowmeters, such as orifice plates.
4. Measurement of the liquid in an asymmetrical state measurement, the liquid to be tested is asymmetric, there are two kinds of flow combinations: one is a single vortex flow; the other is a linear flow along the pipeline axis, the volume flow of the liquid is The integral of the pipe section. For the shortage of the upstream straight pipe section, the flow regulator can be used for adjustment; secondly, the inner diameter of the pipe within the reasonable range of the upstream and downstream is guaranteed to have the same value as the inner diameter of the flowmeter; again, sufficient straight pipe section is reserved for the upstream.
5. The electromagnetic flowmeter in the measuring tube is often used to measure non-clean fluid. The non-cleaning fluid contains some substances such as sediment inside, which causes the surface of the electrode of the electromagnetic flowmeter or the pipeline to be contaminated, resulting in measurement error. In response to this situation, firstly, the electromagnetic flowmeter is cleaned regularly; secondly, the flow rate is reasonably raised, and it is controlled to a state of 4 m/s; again, a lining of a material such as polytetrachloroethylene is applied.

First, the space electromagnetic interference converter and the sensor ask for a long cable line. In a strong electromagnetic environment, it is very susceptible to interference, which causes the instrument to have a non-linear measurement value, which is difficult to display normally. In view of this situation, firstly, the introduction of shielding measures can be carried out in the grounded steel pipe to separate the cable and use the shielded cable up to standard; secondly, the cable length is reasonably shortened; again, it is kept at a long distance from the strong magnetic field.
Second, the connection cable problem The essence of electromagnetic flow application is to achieve the connection between the converter and the sensor by means of a specific cable, forming a complete system, so the conductor cross-sectional area, capacitance, cable site, etc. will have adverse effects. In view of this situation, first of all, to ensure that the cable model meets the requirements, to achieve an effective connection of the ends, to prevent the occurrence of intermediate joints; secondly, the control length range is usually as short as possible
Third, the grounding problem Because the output signal of the sensor is very small, usually only a few millivolts, in order to improve the anti-interference ability, the zero potential of the sensor must be reliably grounded separately, and the ground point of the sensor output signal should be electrically connected with the fluid to be measured. The grounding resistance of the sensor should be less than 10Ω. When the insulation is applied to the pipe connecting the sensor or the non-metallic pipe is used, the grounding ring should be installed on both sides of the sensor and grounded reliably to ground the fluid. The fluid potential is the same as the ground potential.
Fourth, the electrode and the excitation coil symmetrical point installation point vibration
The excitation coil and the electrode of the electromagnetic flowmeter should be symmetrical. Once asymmetrical, the deviation during the production process will be triggered, and the measurement result is difficult to ensure accuracy. In addition, the installation site needs to achieve a high anti-vibration standard, otherwise the accuracy of the measurement value cannot be guaranteed, and even the abnormal operation of the instrument is induced.
Grounded and the sensor output signal ground point should be electrically connected to the fluid being measured. The grounding resistance of the sensor should be less than 10Ω. When the insulation is applied to the pipe connecting the sensor or the non-metallic pipe is used, the grounding ring should be installed on both sides of the sensor and grounded reliably to ground the fluid. The fluid potential is the same as the ground potential.

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