Split electromagnetic flowmeter application issues to discuss and analyze

Project Overview

1. Errors caused by non-axisymmetric flow

The velocity distribution of the split electromagnetic flowmeter is symmetrically distributed about the tube axis, generating a unified electrodynamic force for the total fluid flow of the electrode, and the liquid flow of the flowmeter The average fluid flow velocity is measured by the magnitude of the velocity distribution, but is not proportional to the magnetic field rate, but is independent. The axis of symmetry, that is, the particle is a moving object. The induced electromotive force at each different position of the electrode depends on the shape of the electrode. When the particle speed is greater and closer, the induced electromotive force is larger, and the generated electromotive force is different. Therefore, You need to ensure that the fluid flow has an axis of symmetry. will result in an error such as non-axisymmetric pipe flow distribution. Split electromagnetic flowmeter option to ensure straight pipe requirements to minimize errors.

Second, the fluid conductivity problem

The fluid conductivity below increases the error in the output impedance and the load effect on the input impedance of the electrode's converter. In principle, The application conductivity of the fluid split electromagnetic flowmeter is limited as described below. You can determine the conversion you need primarily by the size of the electrode and the conductive fluid. The size of the input impedance of the device. In theoretical analysis, the size of the point electrode can actually be ignored. The conductivity of the electrode, e", the electrode diameter and the circular plate if it is a semi-infinite expansion in contact with the fluid, have a certain Scale Therefore, D>>D pipe diameter propagation resistance is at 2 to 1/2KD spread resistance equal to 1/Kd electrode output impedance. /cm Roh Moo-hyun ~ 10 microseconds/cm Conductive fluid Roh Moo-hyun is 5μs, the output impedance is the common measurement lower limit of 200kg electrode - 0.1 diameter electrode, 1/Kd = 100KΩ 1cm% The input impedance of the converter is limited and insufficient The degree must be 200MΩ.

Three, lined electrode fixture

In sediment fluid measurement, the surface electrode is often caused by changes in the middle of the night and is contaminated, which must be noted. But it is more difficult to conduct a quantitative analysis on the relationship between the zero electrode and the changing pollution level. It can be said that the smaller the diameter of the electrode, the following effect of cleaning the electrode is used to prevent deposition and special attention should be paid. -×(1-2T/DΔε=1-2/[1+(KΩ/KF)+(KΩ/KF1): If you are connecting the same thickness of precipitated lining the error Δε can be by type kΩ By connecting the KF·clamp, the thickness of the T clamp measures the conductivity of the fluid, and d,2) diameter can be calculated. Is equal to the KF type and KΩ, if there is no error in the conductivity of the lamp, but the low expression of the glass rotameter is set, it is used as a thermal insulation, such as hip material stains immersed in this One way to limit this situation is to increase the output impedance of the electrode in the liquid. Negative bias, therefore, makes the electrode output low due to short circuits in the highly conductive bonding layer, which may induce attachments, such as metal powder, on the contrary. As well as other options further, it is difficult to attach to the lining of the sedimentation and measurement equipment of the sedimentation, glass and polyvinyl chloride in liquids, increasing the flow rate. Foam, including fluids containing foam with a uniform flow volume, makes the measured flow value unstable by introducing error in the measurement. When summarizing sensor electrodes and linings, you will need to take into consideration future maintenance issues and flowmeter selection, especially large-diameter split electromagnetic flowmeters. You, if you choose a preset electrode cleaning hole, in the appropriate position upstream or downstream of the sensor, or scraper or replacement electrode Special Equipment Co., Ltd., Shanghai Hua Ai USA in order to clean the sensor in the future.

IV. The length of cable TV signal transmission

The connection between the sensor (i.e. electrode) and the conversion cable is as short as possible. On some sites, the distance between the position of the converter and the sensor should be considered, and the maximum length of the cable connection is limited by the installation environment. The maximum length of the cable connection between sensor and transmitter depends on the conductivity of the fluid and the amount of distribution of the measuring cable. , as it is practical to use between ranges when measuring the conductivity of a fluid when determining the maximum length of the cable between the electrode and the transmitter. If the cable length exceeds the maximum length, the volume of the distribution cable becomes an issue due to the load. To prevent the occurrence of distributed capacitance, use a double-shielded cable with dual cores, a converter to form a shield between the cores, and a shield inside the core wire to obtain the same low voltage but provide the impedance of the voltage source, shielded wire, shield "For the same potential of the core, it is now that the influence of the load cable does not pass through and does not exist, it will extend the length of the signal cable. In addition, you can also use special signal transmission cables to extend the transmitter and The maximum length between sensors.

5. Excitation technology

Excitation technology is one of the key technologies for the performance of split electromagnetic flowmeters. The excitation mode can be divided into AC excitation and The non-sinusoidal excitation AC sine wave is commercialized in the DC excitation mode. The excitation AC sine wave is unstable and therefore changes the magnetic field frequency (sometimes) and the strength of the supply voltage AC, thus causing changes in the induced electromotive force between the electrodes. Calculating the signal coming from the sensor, the corresponding standard signal and the magnetic field strength must be removed. In order to reduce the accuracy of this excitation method, it is easy to cause changes in the AC excitation non-sinusoidal waveform, but below the excitation frequency. The wave or triangular wave, which excites the stable industry, produces a constant DC power that changes polarity in a manner that requires removing the strength of the magnetic field that would otherwise be produced by AC excitation. The excitation mode has become an important obstacle to the electrode polarization potential. Therefore, the DC excitation mode of a specific value for non-electrolytes (eg, liquid metals) will be applied to the measurement of water resources. For measurements of water and other solutions, in DC excitation mode, the intermittent cycle should generally be considered an integer multiple of the intermittent cycle of the AC power supply.The metal tube float flowmeter eliminates the interference of the AC magnetic field by selecting the DC magnetic field and eddy current polarization, and you can remove the noise of high-power AC frequencies. Excitation frequency, but you can reduce the excitation frequency interference capability, prevent high and low frequencies in order to reduce the stability of the instrument to zero and improve it to zero.