Electromagnetic flowmeter measurement error, the problem may be here

Electromagnetic flowmeters have advantages such as no pressure loss, high precision, and moderate price, and are widely favored by petrochemical and chemical industries. They play an important role in flow measurement. However, in practical applications, due to improper operation, unreasonable device selection, and unscientific installation, measurement errors are difficult to avoid and cause troubles for users. Therefore, the majority of instrumentation personnel should pay attention to various factors that cause electromagnetic flowmeter errors.
In general, the main influencing factors of the electromagnetic flowmeter error can be divided into three categories: improper selection, influence and interference of the tested liquid.
Improper selection
1 The flow rate of the electromagnetic flowmeter to be measured The measurable flow rate range of the electromagnetic flowmeter is generally 0.5 to 10 m/s, and the economic flow rate range is 1.5 to 3 m/s. In actual use, the inner diameter of the measuring tube is determined according to the size of the flow to be measured and the measurable flow rate range of the electromagnetic flowmeter.
2 Electrode and lining material selection The electrode and lining material are directly in contact with the liquid to be measured. The electrode and lining material should be selected according to the characteristics of the liquid to be measured (such as corrosiveness, abrasiveness, etc.) and the operating temperature. If the electrode is selected improperly, it will cause adhesion. Speed, corrosion, scaling, wear, lining deformation, and other issues, and then produce measurement errors.
3 Electromagnetically stabilized electromagnetic flowmeters have DC excitation, AC sinusoidal excitation, and dual-frequency square-wave excitation. DC excitation is prone to electrode polarization and DC interference problems. AC sinusoidal excitation can easily cause zero-point changes, while dual-frequency square waves Excitation has excellent zero 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 method. In practical applications, the stability of the power supply voltage and frequency should be ensured 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 Mixed phase fluid measurement When using an electromagnetic flowmeter to measure the flow of a liquid-solid mixed phase fluid (eg, sediment-laden water), if an electromagnetic flowmeter calibrated with a single-phase liquid is used, measurement error will occur. At this time, select Sensors will be installed at the straight section where liquid-solid phase separation will occur.
The impact of the liquid to be measured
1 drastic changes in the conductivity of the liquid to be measured When the conductivity of the liquid to be measured is large, large fluctuations in the displayed value will be triggered. If the problem is very serious, the control system will be difficult to achieve normal operation; and when the liquid conductivity of the liquid to be measured is too low. It is difficult for the electrode to achieve normal output. If the conductivity of the liquid to be measured is below the lower limit during operation, the electromagnetic flowmeter will be difficult to function normally. To address these situations, first, based on actual needs, in conjunction with relevant standards and requirements, select the type of electromagnetic flowmeter; secondly, install reactors or straight pipe sections to ensure adequate mixing of materials and promote the smooth implementation of chemical reactions; again, Reselect the flow meter type.
2 Liquid bubbles to be measured or non-full tubes For bubbles, they mainly originate from the dissolved gas in the liquid and develop into bubbles in the free state and bubbles in the outside. Flows containing a large volume of bubbles can affect the accuracy of the measurement. If the bubble diameter is too large, even exceeding the value of the electrode diameter, instability will occur during the measurement display, and fluctuations cannot be avoided. In view of this situation, first, the gas collector can be installed on the electromagnetic flowmeter, and the exhaust operation can be performed according to the cycle; secondly, the installation position can be reasonably changed; once again, the vertical pipe is installed on the electromagnetic flowmeter and the protection is guaranteed. On the direction of; Fourth, when installing the sensor, avoid too close to the discharge port; Fifth, the sensor is installed in the control valve position, in its upstream position, or downstream of the pump.
3 The conductivity of the liquid to be measured is too low. The decrease in the conductivity of the measured liquid will increase the output impedance of the electrode and cause the measurement error due to the load effect caused by the input impedance of the converter. If the actual conductivity is lower than the lower limit, the instrument will not function properly. Work, showing value 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 Liquid Asymmetry In the measurement, there is an asymmetry in the liquid to be measured. There are mainly two kinds of flow combinations: one is a single vortex flow; the other is a linear flow along the axis of the pipeline, and the volume flow of the liquid is the pipeline. Cross section points. For the lack of upstream straight section, the flow regulator can be used to adjust; secondly, to ensure that the inner diameter of the pipe in the reasonable range of upstream and downstream has the same value as the inner diameter of the flowmeter; again, enough straight pipe sections are reserved for the upstream.
5 Measuring tubes Electromagnetic flow meters are often used to measure non-clean fluids. The non-clean fluid contains some sediment and other substances, which cause contamination on the surface of the electromagnetic flowmeter electrode or inside the pipeline, causing measurement error. In view of this situation, first, the electromagnetic flowmeter is periodically cleaned; secondly, the flow rate is reasonably raised to control it at a state of 4m/s; again, the lining of polytetrafluoroethylene and other materials is applied.
Interference effect
1 Space electromagnetic interference converters and sensors are required to have long cable lengths. In a strong electromagnetic environment, they are susceptible to interference, causing non-linear measurement of the measured values ​​of the instrument. It is difficult to display normally. In view of this situation, first, the introduction of shielding measures, the cable can be introduced in the grounding tube alone, and use the shielded cable up to standard; Second, reasonably shorten the length of the cable; Again, keep a long distance with a strong magnetic field.
2 Connection cable problems The essence of the electromagnetic flow application is to use a specific cable to connect the converter and the sensor to form a complete system. Therefore, the cross-sectional area of ​​the conductor, capacitance, and cable site will all have adverse effects. First of all, to ensure that the cable type meets the requirements, to achieve an effective connection at the end, to prevent the occurrence of intermediate joints; Second, control the length range, usually the shorter the better
3 Grounding problem Because the output signal of the sensor is very small, usually only a few millivolts, in order to improve the anti-jamming capability, 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 measured fluid. The grounding resistance of the sensor should be less than 10Ω. When the insulating layer is applied in the pipeline connecting the sensor or the non-metallic pipeline is used, the grounding ring should be installed on both sides of the sensor and grounded reliably so that the fluid is grounded. The fluid potential is the same as the ground potential.
4 Electrode and Electromagnetic Coil Symmetry Point Installation Point The excitation coil and electrode of the vibrating electromagnetic flowmeter must be symmetrical. Once asymmetrical, deviations will occur in the production process, and the measurement results will be difficult to ensure accuracy. In addition, the installation site must achieve high anti-vibration standards, otherwise it cannot guarantee the accuracy of the measured value, or even induce the instrument to work abnormally.

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