How to deal with the influence of interference on the flow meter

Abstract: The information on how to deal with the influence of interference on the flow meter is provided by the excellent flow meter and flow meter manufacturers. Among the many flow detection solutions, the vortex flowmeter has the characteristics of high measurement accuracy, small pressure loss, easy installation, unaffected by the physical properties of the measured medium, and easy remote transmission of signals. The application technology is becoming more and more perfect, especially in large Pipe diameter and water,. More flowmeter manufacturers choose models and price quotations. You are welcome to inquire. The following is the article details on how to deal with the impact of interference on flowmeters. Among the many flow detection solutions, the vortex flowmeter has the characteristics of high measurement accuracy, small pressure loss, easy installation, unaffected by the physical properties of the measured medium, and easy remote transmission of signals. The application technology is becoming more and more perfect, especially in large It is more widely used in flow measurement of pipe diameter and liquid media such as water and oil. The vortex flowmeter is based on the Karman vortex principle in fluid dynamics. Within a certain range of Reynolds number, the flow velocity or volume flow of the fluid is proportional to the vortex frequency and has nothing to do with the physical properties of the fluid (pressure, temperature, density, etc.), namely: Q=k where: Q is the volume flow; k is the instrument constant; f is the vortex frequency. According to the above measurement principle, one of the characteristics of the vortex flowmeter is that it is susceptible to electromagnetic and mechanical vibration interference, which limits the normal use of the vortex flowmeter in some occasions, which is also the working condition of the vortex flowmeter. Solving the problem of anti-interference is an effective way to expand the lower limit of the range and improve the vortex flowmeter. 2 Working conditions The bluff body of the vortex flowmeter uses piezoelectric crystals to detect the vortex frequency, and the piezoelectric signal is amplified and triggered by AC to turn the vortex frequency into a pulse signal. The pulse signal is sent to the secondary instrument to display the measured flow after conversion. Among them, the amplification factor K of the AC amplifier and the threshold voltage of the trigger can be adjusted, as shown in Figure 1. In Figure 1, the signal voltage is E, the interference signal is converted to the input end as V, the threshold voltage U is converted to the input end as u, and the AC amplification factor is K. Since u=UK, the effect of adjusting K or U is the same. In order to make the threshold voltage prevent the interference signal to ensure that the trigger can output a valid signal, the interference signal V must be less than the threshold voltage u, and the effective signal voltage E is greater than the threshold voltage u, that is, the working conditions of the vortex flowmeter are: E> u> The magnitude of the V interference signal V determines the lower limit of the range of the vortex flowmeter. Therefore, to expand the lower limit of the range of the vortex flowmeter must start with reducing the interference signal. Adjusting the AC magnification K can only strengthen the output signal, and the lower limit of the range cannot be expanded. 3 Anti-interference measures The interference signals of the vortex flowmeter mainly include electromagnetic interference and mechanical vibration interference. How to solve these two anti-interference problems becomes the key to improving the vortex flowmeter. Vortex flowmeters usually use metal casings, and the shielding effect of the casing can prevent electric field and radio frequency interference; for magnetic field interference, it can be solved by optimizing non-magnetic components and rational wiring of printed circuit boards in the internal circuit design. The development and improvement of the manufacturing process are also not a problem. Therefore, the anti-electromagnetic interference is mainly anti-ground current interference. The piezoelectric crystal of the vortex flowmeter is mounted on the bluff body structure, and one end of the piezoelectric crystal is connected to the casing, so the signal preamplifier must be grounded. The output signal of the vortex flowmeter is sent to the secondary instrument, and the DC power required for signal amplification is provided by the secondary instrument. There may be a step voltage between the ground wire of the piezoelectric crystal and the ground wire of the secondary instrument to form a current. When this current flows in the ground wire of the signal amplifier, there will be a voltage drop. This voltage drop is superimposed with the effective signal and cannot be separated, which is the ground wire current interference. The solution to the ground wire current interference of the vortex flowmeter is to reduce or eliminate the ground wire current. The most thorough solution is to isolate the DC power supply from the secondary instrument. That is, the DC power supply is isolated by the transformer and then rectified into DC to supply the vortex flowmeter, so that there is no electrical connection between the ground wire of the secondary instrument and the ground wire of the piezoelectric crystal. At the same time, the effective measurement signal is converted into a pulse signal after pre-amplification, and is output to the secondary instrument through the pulse transformer, which fundamentally eliminates the influence of the ground current and is an extremely effective anti-interference measure. However, the method of transformer isolation is relatively expensive, bulky, and difficult to implement in the manufacturing process, which greatly reduces the practicability. Optical isolation current limiting and anti-interference measures can effectively reduce the interference of ground current. The principle is shown in Figure 2. In the figure, a is the grounding point of the piezoelectric crystal, and b is the grounding point of the secondary instrument. A resistor r is connected to the ground loop, so the ground current between points a and b is limited by the resistor r, and the voltage drop between the two points a and b is across the resistor r. The voltage drop across the resistor r reflected on the positive line of the power supply is blocked by the three-terminal voltage regulator R. The resistance of the preamplifier ground loop is much smaller than that of the resistor r. There is only a small ground current in the preamp ground. After the effective signal of the piezoelectric crystal is amplified, it is isolated and output by the optical isolation device. In this way, the interference of the ground current can be reduced by at least an order of magnitude. It can be seen that in order to make the power supply voltage have enough margin to block the step voltage when using the optical isolation current limiting anti-interference,

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