Basic principle and types of the hottest ultrasoni

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Basic principle and type of ultrasonic flowmeter

when ultrasonic wave propagates in flowing fluid, it carries the information of fluid velocity. Therefore, the flow rate of the fluid can be detected through the received ultrasonic wave, which can be converted into flow. According to the detection methods, ultrasonic flow meters can be divided into different types, such as propagation velocity difference method, Doppler method, beam offset method, noise method and correlation method. Acoustic flowmeter is a kind of non-contact instrument that has been applied only with the rapid development of integrated circuit technology in recent decades. It is suitable for measuring fluids that are not easy to contact and observe and large pipe runoff. It can be linked with the water level gauge to measure the flow of open water flow. The ultrasonic flow ratio does not need to maintain the instrument in the fluid to install the measuring element, so it will not change the flow state of the fluid and will not produce additional resistance. The installation and maintenance of the instrument will not affect the operation of the production pipeline, so it is an ideal energy-saving flowmeter

as we all know, the current industrial flow measurement generally has the problems of large pipe diameter and large flow measurement. This is because the general flowmeter will bring difficulties in manufacturing and transportation with the increase of the measured pipe diameter. The cost will increase, the energy loss will increase, and the installation will not only avoid these shortcomings, but also the ultrasonic flowmeter can be avoided. Because all kinds of ultrasonic flow meters can be installed outside the pipe for non-contact flow measurement, the instrument cost basically has nothing to do with the diameter of the measured pipe, while the cost of other types of flow meters increases significantly with the increase of the diameter, so the larger the diameter, the better the function price ratio of ultrasonic flow meters than other types of flow meters with the same function. It is considered to be a good instrument for measuring large pipe runoff. The Doppler ultrasonic flowmeter can measure the flow of two-phase medium, so it can be used for the measurement of sewage and sewage. In power plants, it is much more convenient to use portable ultrasonic flowmeter to measure large pipe runoff such as water inflow of hydraulic turbine and circulating water volume of steam turbine than pitot tube flowmeter in the past. Ultrasonic flow measurement can also be used for gas measurement. The applicable range of pipe diameter ranges from 2cm to 5m, from open and closed channels several meters wide to rivers 500m wide

in addition, the flow measurement accuracy of ultrasonic measuring instruments is hardly affected by the temperature, pressure, viscosity, density and other parameters of the measured fluid. In addition, the ball screw can be used to make non-contact and portable measuring instruments with less driving power, so it can solve the flow measurement problems of highly corrosive, non-conductive, radioactive, flammable and explosive media that are difficult to measure by other types of instruments. In addition, in view of the characteristics of non-contact measurement, coupled with a reasonable electronic circuit, one instrument can adapt to a variety of pipe diameter measurement and a variety of flow range measurement. The adaptability of ultrasonic flowmeter is incomparable with other instruments. Ultrasonic flowmeter has some of the above advantages, so it has been paid more and more attention and developed towards product serialization and generalization. Now it has been made into standard, high-temperature, explosion-proof and wet instruments with different sound channels to adapt to the flow measurement of different media, different occasions and different pipeline conditions

at present, the disadvantages of the ultrasonic flowmeter are mainly that the temperature range of the measurable fluid is limited by the temperature resistance of the ultrasonic energy exchange aluminum and the coupling material between the transducer and the pipeline, and the original data of the measured fluid's sound transmission velocity at high temperature are incomplete. At present, it can only be used to measure fluids below 200 ℃ in China. In addition, the measuring circuit of ultrasonic flowmeter is more complex than that of general flowmeter. This is because, in general industrial measurement, the flow rate of liquid is often several meters per second, while the propagation speed of sound wave in liquid is about 1500m/s. The maximum change of sound speed caused by the change of measured fluid flow rate (flow) is also 10-3 orders of magnitude. If the accuracy of measuring flow rate is required to be 1%, the accuracy of measuring sound speed needs to be ~ orders of magnitude. Therefore, it must have a perfect measuring circuit to achieve it, This is the reason why ultrasonic flowmeter can be applied in practice only under the premise of the rapid development of integrated circuit technology

ultrasonic flowmeter consists of ultrasonic transducer, electronic circuit and flow display and accumulation system. The ultrasonic transmitting transducer converts electric energy into ultrasonic energy and transmits it into the measured fluid. The ultrasonic signal received by the receiver is amplified by the electronic circuit and converted into an electrical signal representing the flow to the display and integration instrument for display and integration. In this way, the flow detection and display are realized

piezoelectric transducers are commonly used in ultrasonic flow meters. It uses the piezoelectric effect of piezoelectric materials, and uses a suitable transmitting circuit to add electric energy to the piezoelectric element of the transmitting transducer to generate ultrasonic vibration. The ultrasonic wave is transmitted into the fluid at a certain angle, then received by the receiving transducer, and converted into electric energy by the piezoelectric element for detection. The transmitting transducer uses the inverse piezoelectric effect of piezoelectric elements, while the receiving transducer uses the piezoelectric effect

the piezoelectric element of the transducer of ultrasonic flowmeter is often made into a circular sheet and vibrates along the thickness. The diameter of the sheet exceeds 10 times of the thickness to ensure the directivity of the vibration. Lead zirconate titanate is widely used as piezoelectric element materials. In order to fix the piezoelectric element and make the ultrasonic wave enter the fluid at a proper angle, it is necessary to put the element into the acoustic wedge to form the whole transducer (also known as the probe). The material of sound wedge requires not only high strength and aging resistance, but also small energy loss after ultrasonic wedge, i.e. transmission coefficient close to 1. The commonly used sound wedge material is plexiglass. Because it is transparent, the assembly of piezoelectric components in the sound wedge can be observed. In addition, some rubber, plastic and bakelite can also be used as sound wedge materials

the electronic circuit of ultrasonic flowmeter includes transmitting, receiving, signal processing and display circuits. The measured instantaneous flow and cumulative flow values are displayed by digital or analog quantities

according to the principle of signal detection, the current ultrasonic flowmeter can be roughly divided into propagation velocity difference method (including direct time difference method, time difference method, phase difference method and frequency difference method), beam offset method, Doppler method, correlation method, spatial filtering method and noise method, as shown in the figure. The noise method has the simplest principle and structure, is easy to measure and carry, is cheap but has low accuracy, and is suitable for use in occasions where the accuracy of flow measurement is not high. Since the basic principles of direct time difference method, time difference method, frequency difference method and phase difference method all reflect the flow velocity of fluid by measuring the difference between the velocity of ultrasonic pulse downstream and that we are willing to share our expertise in the field of non-metallic materials, they are also collectively referred to as propagation velocity difference method. Frequency difference method and time difference method are widely used because they overcome the error caused by the change of sound velocity with fluid temperature and have high accuracy. According to the different configuration methods of transducers, the propagation velocity difference can be divided into: Z method (penetration 5, thermal expansion rate pass method), V method (reflection method), X method (cross method), etc. The beam shift method reflects the fluid velocity by using the shift of the propagation direction of the ultrasonic beam in the fluid with the change of the fluid velocity. At low velocity, the sensitivity is very low and the applicability is not great. The Doppler method uses the acoustic Doppler principle to determine the fluid flow by measuring the ultrasonic Doppler frequency shift scattered by the scatterers in the uneven fluid. It is suitable for the measurement of fluid flow including suspended particles, bubbles and so on. The correlation method is to use the correlation technology to measure the flow. 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 correlator is expensive and the circuit is complicated. This shortcoming can be overcome after the popularization and application of microprocessors. The noise method (listening method) is based on the principle that the noise generated by the fluid flow in the pipeline is related to the flow velocity of the fluid, and the flow velocity or flow value is indicated by detecting the noise. The method is simple, the equipment is cheap, but the accuracy is low

each of the above methods has its own characteristics, which should be selected according to the properties of the measured fluid, flow velocity distribution, pipeline installation location, requirements for measurement accuracy and other factors. Generally speaking, because the temperature of the working medium can not be kept constant in industrial production, the frequency difference method and time difference method are often used. The direct time difference method is used only when the pipe diameter is large. The general selection principle for the installation method of the energy exchanger is: when the fluid flows parallel along the pipe axis, the Z method is selected; When the flow direction is not parallel to the tube or the installation location of the tube restricts the installation interval of the transducer, the V method or X method shall be used. When the flow field distribution is uneven and the straight pipe section in front of the surface is short, multi-channel (such as dual channel or four channel) can also be used to overcome the flow measurement error caused by velocity disturbance. Doppler method is suitable for measuring two-phase flow, which can avoid the disadvantages of conventional instruments that can not run due to blockage, wear and adhesion caused by suspended particles or bubbles, so it can be developed rapidly. With the development of industry and energy-saving work, the development of energy-saving methods such as kerosene mixing (COM), coal cement mixture (CWM) fuel transportation and application, and fuel oil adding water for combustion support, all open up broad prospects for the application of Doppler ultrasonic flowmeter. (end)

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