The flow sensor of the ultrasonic flow meter emits ultrasonic waves that carry information about the fluid flow rate as it propagates through the pipe into the flowing fluid. Therefore, the flow rate of the fluid can be detected by the ultrasonic wave received by another flow sensor, and converted into a flow rate. According to the detection method, it can be divided into different types of ultrasonic flowmeters such as propagation velocity difference method (time difference method), Doppler method, beam offset method, noise method and correlation method.
Ultrasonic flowmeters are non-contact meters that are suitable for measuring fluids that are not easily accessible or inaccessible and difficult to observe, as well as large pipe runoff. It works with an ultrasonic position meter to measure the flow of open water. The use of ultrasonic flowmeters does not require the installation of measuring elements in the fluid, so it does not change the flow state of the fluid, and does not create additional resistance. The installation and maintenance of the instrument can not affect the operation of the production pipeline and is therefore an ideal energy-saving flowmeter. 1 w/ y3 K; I5 l1 X) G
    At present, industrial flow measurement generally has problems of large diameter, large flow, small diameter, small flow, high temperature liquid, and gas measurement. This is because the general flowmeter will bring manufacturing and transportation as the diameter of the measuring pipe increases. The difficulty of the increase, the increase in cost, the increase in energy loss, and the installation of not only these shortcomings, ultrasonic flowmeters can be avoided. Because all kinds of ultrasonic flowmeters can be installed outside the pipe, non-contact flow measurement, the cost of the instrument is basically independent of the size of the pipe to be tested, while other types of flowmeters increase with the increase in caliber, so the cost is increased. The flowmeter is superior to the other functions of the same type of flowmeter. It is considered to be a good large-diameter flow measuring instrument. The Doppler ultrasonic flowmeter can measure the flow of two-phase medium, so it can be used for the measurement of dirty sewage such as sewers and sewage. In power plants, the use of portable ultrasonic flowmeters to measure large pipe diameters such as turbine water inflow and turbine circulating water is much more convenient than in the past. Ultrasonic flow juice can also be used for gas measurement. Pipe diameters range from 6mm to 6m , from a few meters wide open channels, culverts to 500m wide rivers.
  In addition, the accuracy of the flow measurement of the ultrasonic measuring instrument is almost independent of the temperature, pressure, viscosity, density and other parameters of the measured fluid, and can be made into non-contact and portable measuring instruments, so it can solve the problem that other types of instruments are difficult to measure. Flow measurement problems for corrosive, non-conductive, radioactive, and flammable and explosive media. In addition, in view of the non-contact measurement characteristics, coupled with reasonable electronic circuits, one instrument can adapt to a variety of pipe diameter measurements and a variety of flow range measurements. The adaptability of ultrasonic flowmeters is also unmatched by other instruments. Ultrasonic flowmeters have some of the above advantages, so it has received more and more attention and has been developed into a series of products and generalization. It has been made into standard, high-temperature, explosion-proof and wet instruments of different channels to adapt to different media. Flow measurement for occasions and different pipeline conditions.
  The shortcomings of ultrasonic flowmeters are mainly that the temperature range of the measurable fluid is limited by the temperature resistance of the ultrasonically exchanged aluminum and the coupling material between the transducer and the pipeline, and the raw data of the sound velocity of the fluid under test at high temperature. Not complete. At present, it can only be used to measure fluids below 200 °C. The temperature exceeds 200 °C. Only the high-temperature ultrasonic flowmeters produced by German FLEXIM manufacturers can measure, but the price is high. In addition, the measurement line of the ultrasonic flowmeter is more complicated than the general flowmeter. This is because, in general industrial metering, the flow rate of liquid is often several meters per second, and the propagation speed of sound waves in liquid is about 1500 m / s . The change of flow velocity ( flow rate ) of the measured fluid brings the maximum change of sound velocity to 10 - 3 orders of magnitude. If the accuracy of measuring the flow rate is required to be 1 %, the accuracy of measuring the speed of sound needs to be of the order of 10-5 to 10-6 , so a perfect measuring circuit must be implemented, which is the ultrasonic flowmeter only in the integrated circuit. The reason for the practical application can be obtained under the premise of rapid development of technology.
   Ultrasonic flowmeter consists of three parts: ultrasonic transducer, electronic circuit and flow display and accumulation system. The ultrasonic transmitting transducer converts electrical energy into ultrasonic energy and emits it into the fluid to be measured. The ultrasonic signal received by the receiver is amplified by the electronic circuit and converted into an electrical signal representing the flow to be supplied to the display and integrated meter for display. And the total calculation. This enables the detection and display of traffic.
  Ultrasonic flowmeters are commonly used in piezoelectric transducers. It utilizes the piezoelectric effect of the piezoelectric material and applies an appropriate transmitting circuit to the piezoelectric element of the transmitting transducer to generate ultrasonic vibration. Ultrasonic waves are incident into the fluid at an angle and then received by the receiving transducer and converted to electrical energy by the piezoelectric element for detection. The transmitting transducer utilizes the inverse piezoelectric effect of the piezoelectric element, while the receiving transducer utilizes the piezoelectric effect.
The piezoelectric elements of the ultrasonic flowmeter transducer are often made into a circular sheet that vibrates along the thickness. The diameter of the sheet exceeds 10 times the thickness to ensure the directionality of the vibration. The piezoelectric element material is mostly lead zirconate titanate. In order to fix the piezoelectric element, the ultrasonic wave is injected into the fluid at a suitable angle, and the component is required to be in the human voice wedge to form the entire transducer ( also called the probe ) . The material of the acoustic wedge not only requires high strength and aging resistance, but also requires that the ultrasonic energy after the wedge is small, that is, the transmission coefficient is close to 1 . The commonly used acoustic wedge material is plexiglass, because it is transparent, and the assembly of piezoelectric elements in the wedge can be observed. In addition, some rubber, plastic and bakelite can also be used as the acoustic wedge material.
The electronics of the ultrasonic flow meter include transmit, receive, signal processing, and display circuitry. The measured instantaneous flow and cumulative flow values ​​are displayed in digital or analog quantities.
  According to the principle of signal detection, the current ultrasonic flowmeter can roughly divide the propagation velocity difference method ( including: direct time difference method, time difference method, phase difference method, frequency difference method ) beam offset method, Doppler method, correlation method, space Filtering and noise methods are shown in the figure. Among them, the principle and structure of the noise method are the simplest, which is convenient for measurement and carrying. The price is cheap but the accuracy is low, and it is suitable for occasions where the accuracy of flow measurement is not high. Since the basic principles of the direct time difference method, the time difference method, the frequency difference method, and the phase difference method all reflect the flow velocity of the fluid by measuring the difference between the speed of the ultrasonic pulse forward flow and the reverse flow report, they are collectively referred to as the propagation speed type flow method. Among them, the frequency difference method and the time difference method overcome the error caused by the change of the speed of sound with the temperature of the fluid, and the accuracy is high, so it is widely used. According to the configuration method of the transducer, the propagation speed difference is further divided into: Z method ( transmission method ) , V method ( reflection method ) , X method ( cross method ), and the like. The beam offset method uses the displacement direction of the ultrasonic beam in the fluid to change with the fluid flow rate to reflect the fluid flow rate. At low flow rate, the sensitivity is very low and the applicability is not great. The Doppler method uses the acoustic Doppler principle to determine the fluid flow rate by measuring the ultrasonic Doppler shift of the scatterer scattering in a non-uniform fluid, and is suitable for fluid flow measurement including suspended particles and bubbles. The correlation method uses the relevant technology to measure the flow rate. In principle, the measurement accuracy of this method is independent of the sound velocity in the fluid, and thus has nothing to do with the fluid temperature and concentration, so the measurement accuracy is high and the application range is wide. However, the price of the correlator is expensive and the line is complicated. This shortcoming can be overcome after the microprocessor is popularized. The noise method ( listening method ) is a principle that uses noise generated when a fluid flows in a pipe is related to a flow velocity of a fluid, and detects a flow rate or a flow rate value by detecting noise. The method is simple, the equipment is cheap, but the accuracy is low.
 Several methods have their own characteristics and should be based on the nature of the fluid being tested. The flow rate distribution, the location of the piping installation, and the requirements for measurement accuracy are selected. Generally speaking, since the temperature of the working medium in industrial production often cannot be kept constant, the frequency difference method and the time difference method are often used. The direct time difference method is used only when the pipe diameter is large. The selection principle of the transducer installation method is generally: when the fluid flows in parallel along the tube axis, the Z method is selected; when the flow direction is not parallel with the tube uranium or the pipeline installation location limits the installation interval of the transducer, V is used. Law or X method. Multi-channel ( such as two-channel or four-channel ) can also be used to overcome the flow measurement error caused by the flow velocity disturbance when the flow field is unevenly distributed and the straight pipe section is short. The Doppler method is suitable for measuring two-phase flow, and can avoid the drawbacks of conventional instruments caused by clogging, abrasion, adhesion and inoperability caused by suspended particles or bubbles, and thus can be rapidly developed. With the development of industry and the development of energy-saving work, the development of energy-saving methods such as kerosene mixing (COM) , coal-cement composite (CWM) fuel transportation and fuel oil-assisted combustion have opened up a broad application for Doppler ultrasonic flowmeters. prospect
   With the deepening of the application, feedback from users, technological innovation, and re-application. Now some manufacturers of ultrasonic flowmeters have put together a variety of measurement methods into a set of mainframes, which is convenient for the users to use and measure. At the same time, it also reflects the idea that technology is the service of production and life.
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