As we all know, the current industrial flow measurement generally has problems of large pipe diameter and large flow measurement. This is because the general flow meter will cause difficulties in manufacturing and transportation with the increase of the measurement pipe diameter, increase the cost, and increase the energy loss. Large, installation not only these disadvantages, ultrasonic flowmeters can be avoided. 比相同功能其它类型流量计的功能价格比越优越。 Because all types of ultrasonic flowmeters can be installed outside the pipe and non-contact flow measurement, the cost of the instrument is basically independent of the size of the pipe being measured, while other types of flowmeters have a large increase in cost as the diameter increases, so the larger the ultrasonic wave The flowmeter has a better price / performance ratio than other types of flowmeters with the same function. It is considered to be a good instrument for measuring large-diameter flow. The Doppler ultrasonic flowmeter can measure the flow of dual-phase medium, so it can be used for the measurement of sewage and sewage. In power plants, portable ultrasonic flowmeters are used to measure large pipe diameters, such as water intake of turbines and circulating water of turbines, which is much more convenient than the traditional tube peeling flowmeter. Ultrasonic flow-through juice can also be used for gas measurement. The applicable range of pipe diameter is from 2cm to 5m, and it can be used from a few meters wide open channel and underdrain to 500m wide river.
In addition, the accuracy of flow measurement of ultrasonic measuring instruments is hardly affected by parameters such as temperature, pressure, viscosity, and density of the fluid being measured. It can also be made into non-contact and portable measuring instruments, so it can solve the problems that other types of instruments are difficult to measure. Flow measurement of corrosive, non-conductive, radioactive and flammable and explosive media. In addition, in view of the characteristics of non-contact measurement, and equipped with reasonable electronic circuits, one instrument can be adapted to a variety of pipe diameter measurement and a variety of flow range measurement. The adaptability of ultrasonic flow meters is also unmatched by other instruments. The ultrasonic flowmeter has some of the advantages mentioned above, so it has been increasingly valued and developed towards product series and generalization. Now it has been made into standard, high temperature, explosion-proof, and wet-type meters with different channels to adapt to different media. Measurements of occasions and different pipeline conditions.
The current shortcomings of the ultrasonic flowmeter are mainly that the temperature range of the measurable fluid is limited by the temperature resistance of the ultrasonic transducer aluminum and the coupling material between the transducer and the pipe, and the raw data of the sound velocity of the fluid under test at high temperature Incomplete. At present China can only be used to measure fluids below 200 ° C. 的测量线路比一般流量计复杂。 In addition, the measuring circuit of the ultrasonic flowmeter is more complicated than the general flowmeter. This is because the velocity of liquid in general industrial measurement is often a few meters per second, and the propagation speed of sound waves in the liquid is about 1500 m / s. The change in the velocity (flow rate) of the measured fluid brings the largest change in sound velocity, which is also 10- 3 orders of magnitude. If the accuracy of measuring the flow velocity is 1%, the accuracy of measuring the speed of sound needs to be on the order of 10-5 to 10-6. Therefore, it must have a perfect measurement circuit to achieve this. Reasons for practical application can only be obtained under the premise of rapid technological development.
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 signals received by the receiver are amplified by the electronic circuit and converted into electrical signals representing the flow. And accumulation. This realizes the detection and display of the flow.
Ultrasonic flow meters often use piezoelectric transducers. 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, so that it generates ultrasonic vibration. Ultrasonic waves are injected into the fluid at an angle to propagate, and then they are received by the receiving transducer and converted into electrical energy by the piezoelectric element for detection. The transmitting transducer uses the inverse piezoelectric effect of the piezoelectric element, while the receiving transducer uses the piezoelectric effect.
The piezoelectric element of the ultrasonic flowmeter transducer is often made into a circular sheet and vibrates along its thickness. The diameter of the sheet exceeds 10 times the thickness to ensure the directivity of the vibration. The piezoelectric element material mostly uses lead zirconate titanate. In order to fix the piezoelectric element, so that the ultrasonic wave is injected into the fluid at an appropriate angle, it is necessary to put the element into the vocal 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 a small energy loss after the ultrasonic wave passes through the acoustic wedge, 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 acoustic wedge can be observed. In addition, some rubber, plastic and bakelite can also be used as acoustic wedge materials.
The electronic circuit of the ultrasonic flowmeter includes transmitting, receiving, signal processing and display circuits. The measured instantaneous flow and accumulated flow values are displayed digitally or analogously.
大致可分传播速度差法(包括：直接时差法、时差法、相位差法、频差法)波束偏移法、多普勒法、相关法、空间滤波法及噪声法等类型，如图所示。 According to the principle of signal detection, the current ultrasonic flowmeter can be roughly divided into the propagation velocity difference method (including: direct time difference method, time difference method, phase difference method, frequency difference method) beam shift method, Doppler method, correlation method, space Filter and noise methods are shown in the figure. Among them, the principle and structure of the noise method are the simplest, easy to measure and carry, and the price is cheap but the accuracy is low, which is suitable for use in places where the accuracy of the 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 all reflect the flow velocity of the fluid by measuring the difference between the forward and backward flow speeds of the ultrasonic pulses, they are collectively referred to as the propagation speed difference method. Among them, the frequency difference method and the time difference method overcome the error caused by the change of the sound velocity with the fluid temperature, and have higher accuracy, so they are widely used. According to the different configuration methods of the transducer, the propagation speed difference is divided into: Z method (transmission method), V method (reflection method), X method (cross method) and so on. The beam migration method uses the deviation of the propagation direction of the ultrasonic beam in the fluid with the fluid velocity to reflect 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 rate by measuring the ultrasonic Doppler frequency shift of the scatterer in a heterogeneous fluid. It is suitable for fluid flow measurement including suspended particles and bubbles. The correlation method uses related technology to measure the flow rate. In principle, the measurement accuracy of this method has nothing to do with the speed of sound in the fluid, and therefore has nothing to do with the temperature and concentration of the fluid. Therefore, the measurement accuracy is high and the scope of application is wide. But the correlator is expensive and the line is complicated. After the popularity of microprocessors, this shortcoming can be overcome. The noise method (listening method) is based on the principle that the noise generated by the fluid flowing in the pipe is related to the flow velocity of the fluid, and the flow rate or flow value is expressed 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 and should be based on the properties of the fluid being measured. Selection of flow velocity distribution, pipeline installation location, and requirements for measurement accuracy. 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 mostly used. The direct time difference method is used only when the pipe diameter is large. The principles for selecting the installation method of the transducer are generally: when the fluid flows along the tube axis in parallel, the Z method is used; when the flow direction is not parallel to the tube uranium or the installation location of the pipeline restricts the transducer installation interval, V is used Method or X method. When the flow field is unevenly distributed and the straight pipe section at the front of the table is short, 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. The Doppler method is suitable for measuring two-phase flow, and can avoid the disadvantages of blockage, abrasion, adhesion and inability of conventional instruments caused by suspended particles or air bubbles, so it has developed rapidly. With the development of industry and the development of energy-saving work, the development of energy-saving methods such as the transportation and application of kerosene mixture (COM), coal-cement cement (CWM) fuel, and the addition of fuel oil and water to support combustion have opened up a wide range of Doppler ultrasonic flowmeter applications prospect.