Doppler Flow Meter Basics

Abstract: Doppler flow meter basic knowledge information is provided by excellent flow meter and flow meter production and quotation manufacturers. Doppler flowmeter is a flow detection technology that uses Doppler frequency shift to obtain fluid flow velocity data, and then calculates the average flow velocity of the section with a mathematical model according to the original flow velocity and liquid level data, and then multiplies it by the fluid cross-sectional area to obtain the flow rate. Formula: flow. For more flowmeter manufacturers to select models and price quotations, you are welcome to inquire. The following is the details of the article on the basic knowledge of Doppler flowmeters. Doppler flowmeters use Doppler frequency shift to obtain fluid flow rate data, and then According to the original flow rate and liquid level data, the average flow rate of the section is calculated by a mathematical model and then multiplied by the fluid cross-sectional area to obtain the flow rate detection technology (the basic formula of flow rate: flow rate Q = average flow rate V * cross-sectional area A). Therefore, Dopp The flow measurement process of the Le flowmeter can be specifically: measuring flow velocity, average flow velocity calculation, cross-sectional area calculation, and flow calculation. The latter two are simple geometric/mathematical calculations. For flow velocity measurement, the first thing to explain is Doppler Frequency shift. When there is relative motion between the wave source and the observer, the frequency received by the observer is different from the real frequency of the wave source, and the difference between the frequency of the received signal and the frequency of the sound source is called Doppler frequency shift. Its size is determined by the relative velocity between the two, a physical phenomenon called the Doppler effect. When measuring the flow velocity, the wave emitted by the probe encounters the suspended matter, solid particles, air bubbles, and even unstable particles in the medium. The vortex, disturbance, etc. of the state will produce reflected waves deviating from the transmitting frequency, resulting in Doppler frequency variation, and this effect is proportional to the moving velocity of the reflector. The receiving probe receives the reflected waves at each point and undergoes complex signal processing. , dynamic spectrum analysis and precision calculation, the flow velocity of the fluid in the pipe section can be obtained. At present, the most used waves in Doppler flowmeters are ultrasonic and radar waves, and Doppler flowmeters that use any wave as the measurement wave , the flow rate quantification principle used is the Doppler frequency shift principle. Basically, it can be considered that under this principle, the probes of various waves measure the flow velocity at a certain point or within the measurement range (referring to the coverage of sound waves or radar waves. Flow velocity measurement, not There is no essential difference in the accuracy of the flow velocity of the entire section), which is basically about 1-2%. The difference between ultrasonic and radar waves in current measurement applications is that ultrasonic waves are usually used for measurement under fluids, and the latter is generally used for The measurement above the fluid. Therefore, the ultrasonic wave can obtain the flow velocity data at different depths in the fluid according to the Doppler frequency shift of the echoes collected and analyzed at different times. However, the radar wave can only obtain the flow velocity data on the surface of the fluid because it is installed above the liquid. In terms of installation, the radar wave does not need to be installed below the liquid, which is not only convenient for installation and maintenance, but also lowers the failure rate of the instrument. Whether it is an ultrasonic Doppler flowmeter or a radar Doppler flowmeter, the sound waves emitted by the probe itself and The coverage area of ​​radar waves is extremely limited. In fact, it is impossible to directly measure the average flow velocity of the entire water-passing section. Therefore, how to obtain the average flow velocity of the section through the limited raw flow velocity data obtained by the probe is very important. At present, the acquisition of the average flow velocity is very important. Generally, the average flow velocity of the section is calculated according to the mathematical model according to the flow velocity data of a certain depth or several depths. The radar wave flowmeter generally only has the ability to measure the surface flow velocity , therefore, the average velocity can only be calculated from the surface velocity. The ultrasonic flowmeter, because it is installed below the liquid level, can obtain multiple depth flow rates, and can obtain the average flow rate calculated from multiple flow rates. It is not difficult to see that there must be a theoretical model in the built-in software of the flowmeter to find out the relationship between the average flow velocity of the section and the flow velocity measured by our probe according to the section shape and water flow characteristics, so as to calculate the average flow velocity of the current section. However, in fact, the motion of water flow is complex and changeable, and the hydraulic model is usually only suitable for uniform flow. Therefore, no matter what flowmeter is used, it is impossible to accurately calculate the flow when encountering turbulent flow. The reason is that the average flow velocity of the turbulent cross-section cannot be Calculation (unless the flow velocity at each point on the section can be measured and weighted averaged, which is theoretically impossible). Even if it is also a laminar flow, there will still be slight disturbances and eddies in the water flow (due to the existence of particles and siltation, it is impossible to form an ideal water flow). In this case, relatively speaking, underwater ultrasonic waves ( upward firing) can indeed calculate flow more accurately than radar waves (surface velocity). The reason is that underwater ultrasonic waves can measure the flow velocity at different depths in sections, so as to obtain a relatively accurate flow velocity distribution approximately, while the radar can only measure the surface flow velocity, and cannot measure the actual flow velocity distribution at all. This underwater ultrasonic wave can correct the relationship between the measured flow rate and the average flow rate according to the actual measurement results.Thus, a more accurate average flow velocity can be obtained, while the radar can only assume the current flow state and use a theoretical model to calculate the average flow velocity. Then the difference between the two when calculating the flow can be seen at a glance. In addition, it should be particularly pointed out that since ultrasonic Doppler can obtain flow velocity data at multiple depths, in addition to calculating the average flow velocity from these flow velocity, these raw flow velocity data at different depths have additional processing methods.

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