In-depth study of electromagnetic flowmeter technology
First, the error caused by non-axisymmetric flow
When the flow velocity of the fluid is axisymmetrically distributed in the tube, and in a uniform magnetic field, the magnitude of the electromotive force generated on the flowmeter electrode is independent of the flow velocity distribution of the fluid and is proportional to the average flow velocity of the fluid, rather than the axisymmetric flow velocity distribution. The magnitude of the induced electromotive force generated by the counter electrode is different for each fluid particle with respect to the geometrical position of the electrode. The closer to the electrode, the greater the induced electromotive force generated by the particle with higher velocity. Therefore, it must be ensured that the fluid velocity is axisymmetric. If the flow velocity in the tube is non-axisymmetric, it will cause errors. Therefore, in the optional electromagnetic flowmeter, it is necessary to ensure the straight pipe section as much as possible to reduce the error caused by it.
Second, the fluid conductivity problem Fluid conductivity decreases, will increase the output impedance of the electrode, and due to the load effect caused by the converter input impedance error, therefore, according to the following principles, the provisions of the electromagnetic flowmeter application fluid The lower limit of the conductivity.
The output impedance of the electrode determines the size of the input impedance required by the converter, and the output impedance of the electrode can be considered to be dominated by the conductivity of the fluid and the size of the electrode.
In the theoretical analysis, the electrode is used as a point electrode and the size is negligible. In fact, the electrode has a certain size. When the diameter of the circular plate electrode is in contact with a semi-infinitely broadened fluid with a conductivity of K, the broadening resistance is 1. /2Kd, therefore, if the pipe diameter D>>d, then the output impedance of the electrode is the sum of the two broadening resistances, which is equal to 1/Kd.
The lower limit of the general measured fluid conductivity is 5?S/cm to 10?S/cm. Therefore, if the electrode diameter is 1cm, the output impedance of the electrode is 1/Kd=100kΩ to 200kΩ, which is the output impedance. The effect is limited to 0.1% or less, and the input impedance of the converter should be about 200 MΩ.
Third, the technical problems of electrode lining attachments
When measuring fluids with attached deposits, the surface of the electrodes will be contaminated and often cause zero point changes, so care must be taken.
The relationship between the zero point change and the electrode contamination degree is difficult to quantitatively analyze, but it can be said that the smaller the electrode diameter is, the less the influence is affected. In use, attention should be paid to the cleaning of the electrode to prevent adhesion.
The error, Δε, that occurs when a deposit adheres to the lining, and if the thickness of the attachment is the same, the following equation can be used:
Δε=1-2/[1+(Kω/Kf)+(1-Kω/Kf)×(1-2t/D)2], where Kω and Kf are the conductivity of the attached matter and the measuring fluid, respectively. Attachment thickness t, diameter D.
If Kω and Kf are equal in the formula, there is no error, and if the conductivity of the attached matter is low, the above equation holds. However, since the output impedance of the electrode is increased, it is limited, for example, if the insulating deposit is immersed in the fluid. This situation. On the other hand, if a metal powder or the like adheres, the induced electric potential is short-circuited due to the high-conductivity adhesive layer, so that the output of the electrode is low and negative deviation occurs.
When measuring a fluid with deposits, besides selecting a liner such as glass or polytetrafluoroethylene that is difficult to deposit, the flow rate should be increased. If the air bubble is uniformly contained in the fluid, the volumetric flow rate including the air bubble is measured, and the measured flow rate value is destabilized, introducing an error.
In summary, when selecting a flowmeter, especially a large-diameter electromagnetic flowmeter, the maintenance of the electrodes and lining of the sensor should be considered in the future. For example, scraper electrodes or replaceable electrodes of Shanghai Guanghua Ai Meite Instrument Co., Ltd. may be used, or a cleaning inlet hole may be preset at an appropriate position upstream or downstream of the sensor for cleaning the sensor in the future.
Fourth, the technical problem of signal transmission cable length
The shorter the connection cable between the sensor (ie the electrode) and the converter, the better. However, some sites are limited by the location of the installation environment, and the distance between the converter and the sensor is relatively long. In this case, the maximum length of the connecting cable must be considered. The maximum length of the connecting cable between the transducer and the transducer is determined by the distributed capacitance of the cable and the conductivity of the fluid being measured.
In actual use, when the conductivity of the fluid to be measured is within a certain range, the maximum length of the cable between the electrode and the converter is determined. When the cable length exceeds the maximum length, the loading effect caused by the cable's distributed capacitance becomes a problem. To prevent this from happening, use a two-core, two-layer shielded cable. A low-impedance voltage source is provided by the converter so that the inner shield and the core get the same voltage to form a shield, even if there is a distributed capacitance between the core and the shield. However, if the core wire and the shield are at the same potential, no current flows between them, and no load effect of the cable exists, so the maximum length of the signal cable can be extended. In addition, a special signal transmission cable can be used to extend the maximum length between the transducer and the sensor.
Fifth, the technical problems of excitation
Excitation technology is one of the key technologies for the measurement performance of electromagnetic flowmeters. The excitation method can be divided into AC sine wave excitation, non-sine wave AC excitation and DC excitation in practical applications.
AC sine wave excitation, when the AC power supply voltage (sometimes the frequency) is unstable, the magnetic field strength will change, so the induced electromotive force generated between the electrodes also changes, so the signal corresponding to the calculated magnetic field strength must be taken out from the sensor as Standard signal. This excitation method can easily cause a zero point change and reduce its measurement accuracy.
Non-sinusoidal AC excitation is a method of using a square wave or triangular wave excitation below the industrial frequency. It can be considered as a way to generate a constant DC and periodically change the polarity. Because this excitation power supply is stable, it is not necessary to remove the magnetic field strength. Change and operate.
The main problem of AC excitation is that the induction noise is serious.
The DC excitation method is an important obstacle to the polarization potential at the electrode. Therefore, a certain value of DC excitation method is only applicable to the measurement of non-electrolyte (such as liquid metal) liquids.
When measuring aqueous solutions such as tap water and source water, periodic intermittent DC excitation is generally used. The rest period should be chosen to be an integral multiple of the AC power cycle, eliminating the noise of the AC mains frequency, eliminating the eddy currents of the AC magnetic field and polarization disturbances of the DC magnetic field.
Excitation frequency is reduced, zero stability can be improved, but the instrument's anti-low frequency interference ability is weakened, the response speed is slow, if the excitation frequency is high, the ability to resist low frequency interference is enhanced, but the zero stability of the instrument is reduced. This problem was solved by the low-frequency rectangular wave (1/2 Hz to 1/32 of 50 Hz) in the 1970s, which solved the long-disturbed power frequency interference of the electromagnetic flowmeter, improved the stability of the zero point and the measurement accuracy. In the 1980s, three-valued low-frequency rectangular wave excitation technology (with 1/8 cycle of 50Hz and excitation current with sinusoidal law) emerged, which has better zero stability and resolves the influence of interference potential. However, the response speed is reduced, and electrical noise is generated when measuring fluids containing solid particles and fiber fluids with low conductivity, such as mud, pulp, etc. (This is caused by the fluid rubbing the electrode and causing the oxide film on the electrode surface to peel off again). The output signal wobbles unsteady; in the late 1980s, a dual-frequency square-wave excitation method was introduced for these problems. The excitation waveform consists of a low-frequency (6.25 Hz) rectangular wave and a high-frequency (75 Hz) rectangular wave superposition. Sampling the flow signal corresponding to it, obtaining the two signals of the low frequency and high frequency characteristics can be processed to reproduce the actual flow signal value. Therefore, this technology not only has the excellent zero stability of the low-frequency rectangular wave excitation technology, but also has the strong ability to suppress the fluid noise with the high-frequency rectangular wave excitation technology.
Six, sensor grounding technical problems
The flow signal detected by the electromagnetic flowmeter sensor electrode is a millivolt level, and based on the potential of the fluid in the sensor, so the external interference has a great influence on it. Therefore, a good grounding largely determines the measurement of the flowmeter. Accuracy. The fluid being measured itself acts as an electrical conductor and other irrelevant electromagnetic interference must be excluded. The potential signal detected by the electrode is not disturbed by the external parasitic potential. The sensor should have a good separate ground wire, grounding resistance is less than 10Ω. If there is an insulation layer or a non-metallic pipe in the pipe connecting the sensor, a grounding ring should be installed on both sides of the sensor.
Seventh, concluding remarks
In the future, with the development and application of electronic and computer technologies, the electromagnetic flowmeter has expanded its application range and can measure some fluids that could not be measured in the past; it can perform various error compensations and improve the measurement accuracy; it has abnormalities in the conversion circuit and abnormalities in the detection part. , mis-set, empty control, over-limit alarm and other self-diagnostic functions; remote communication can be achieved through a communicator or a computer to adjust the zero point, range change, damping change, etc. of the electromagnetic flowmeter. In recent years, manufacturers have introduced various forms of electromagnetic flowmeters to accommodate the measurement of fluids of different properties. Such as: ceramic lining electromagnetic flowmeter, electromagnetic flowmeter without electrodes and the use of multi-electromagnetic flow. This is an internal level gauge related data from Yihua monitoring and control.
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