Working principle of ultrasonic flowmeter

Working principle of ultrasonic flowmeter When ultrasonic waves are transmitted in a moving fluid, they carry information on the flow rate of the fluid. Therefore, the flow velocity of the fluid can be detected through the received ultrasonic waves, and then converted into flow. Ultrasonic pulses travel through the pipe from sensor to sensor, like a ferryman crossing a river. When the gas is inactive, the sound pulse travels in both directions with the same velocity (sonic speed, C). If the gas in the pipeline has a certain flow velocity V (the flow velocity is not equal to zero), the sound pulse along the flow direction will be transmitted faster, while the sound pulse against the flow direction will be transmitted slower. In this way, the downstream transmission time tD will be shorter, while the upstream transmission time tU will be longer. The longer or shorter ones mentioned here are compared with the transmission time when the gas is inactive; according to the detection method, it can be divided into transmission speed difference method, Doppler method, beam shift method, noise method and related methods. Different types of ultrasonic flowmeters. Acoustic wave flowmeter is a kind that has only begun to be used with the rapid development of integrated circuit technology in the past ten years. According to the principle of signal detection, the current ultrasonic flowmeter can be roughly divided into transmission speed difference method (including: direct time difference method, time difference method, phase difference method, frequency difference method) beam offset method, Doppler method, related method, space Filtering method and noise method and other types. Among them, the noise method is the simplest in principle and structure, easy to measure and carry, cheap in price but low in accuracy, and is suitable for use in occasions where the accuracy of flow measurement is not high. Because the basic principles of the direct time difference method, time difference method, frequency difference method and phase difference method reflect the flow velocity of the fluid by measuring the difference between the upstream and downstream reporting speeds of ultrasonic pulses, they are collectively referred to as the transmission speed difference method. Among them, the frequency difference method and the time difference method overcome the error caused by the change of sound velocity with the fluid temperature, and have high accuracy, so they are widely used. According to the different configuration methods of the transducer, the transmission speed difference is divided into: Z method (transmission method), V method (reflection method), X method (crossover method) and so on. The beam offset method reflects the fluid flow velocity by using the transmission direction of the ultrasonic beam in the fluid to shift with the change of the fluid flow velocity. When the flow velocity is low, the sensitivity is very low and the applicability is not great. The Doppler method uses the principle of acoustic Doppler to determine the fluid flow rate by measuring the ultrasonic Doppler frequency shift scattered by the scattering body in the inhomogeneous fluid. It is suitable for fluid flow measurement containing suspended particles and bubbles. The relevant method is to use relevant technology to measure the flow rate. In principle, the measurement accuracy of this method has nothing to do with the sound velocity in the fluid, so it has nothing to do with the fluid temperature, concentration, etc., so the measurement accuracy is high and the application range is wide. But the price of related devices is expensive, and the circuit is more complicated. After the widespread use of microprocessors, this shortcoming can be overcome. The noise method (listening method) uses the principle that the noise generated when the fluid in the pipeline is active is related to the flow velocity of the fluid. After detecting the noise, the flow velocity or flow value is indicated. The method is simple and the equipment price is cheap, but the accuracy is low. The above methods have their own characteristics and should be based on the properties of the measured fluid. Factors such as flow rate distribution, pipeline installation location, and requirements for measurement accuracy are selected. Generally speaking, because the temperature of the working medium in industrial production cannot be kept stable, the frequency difference method and time difference method are often used. The direct transit time method is only used when the pipe diameter is large. The selection criteria for the transducer installation method are usually: when the fluid flows parallel to the tube axis, the Z method is used; when the flow direction is not parallel to the tube or the installation location of the pipeline restricts the transducer installation distance, the V method is used. Law or Law X. When the flow field distribution is uneven and the straight pipe section in front of the meter is short, multi-channel (such as two-channel or four-channel) can also be used to overcome flow measurement errors caused by flow velocity disturbances. The Doppler method is suitable for measuring two-phase flow, and it can avoid the defects of clogging, abrasion, and adhesion caused by suspended particles or air bubbles on the surface of the routine, so it can be developed rapidly. With the development of industry and the development of energy-saving work, the development of energy-saving measures such as the transportation and application of kerosene-oil mixing (COM) and coal cement (CWM) fuels, as well as the development of energy-saving methods such as adding water to fuel oil, all open up a wide range of applications for Doppler ultrasonic flowmeters. vista.

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