Introduction ultrasonic flowmeter
Published on：2014-12-2 10:34:56 Hits：
Basic principles and types of ultrasonic flowmeter
The carrier fluid flow in the flow of information when the ultrasonic wave propagation in the fluid. Therefore, by receiving the ultrasonic wave can be detected by the flow rate of the fluid, and thus converted to a flow rate. According detection methods can be divided into propagation velocity difference, Doppler method, the beam offset method, the noise law and related laws, such as different types of ultrasonic flowmeter. Since the acoustic flowmeter is a past ten years with the rapid development of integrated circuit technology began to applications
Non-contact instrument, suitable for measuring difficult to reach and observe the fluid and a large diameter flow. It is linked with the level gauge can be open flow measurement of water flow. Do not use an ultrasonic flow ratio measuring element is installed in the fluid flow and therefore does not change the state of the fluid, does not produce additional resistance, meter installation and maintenance can run and therefore does not affect the production pipeline is an ideal energy-saving flow meter.
As we all know, the current industrial flow measurement prevalence of large diameter, high flow measurement difficult problem, because the general flow meter to measure the diameter with the increase will bring difficulties on manufacturing and transportation, the cost increase, energy loss plus large, not only the installation of these shortcomings, the ultrasonic flowmeter can be avoided. Because various types of ultrasonic flowmeter can be installed outside the tube, non-contact flow measurement, instrumentation cost is substantially independent of the measured pipe diameter size, while other types of flow meters with diameter increases, a substantial increase in cost, so the larger diameter ultrasonic the more superior than the price of the meter functions the same function than other types of flowmeters. Is considered to be a good large diameter flow measurement instruments, ultrasonic Doppler flow meter can measure two-phase medium, it can be used to measure the water, sewer and sewage dirty stream. In power plants, using portable ultrasonic flowmeter into the water turbine, steam turbine and other large diameter circulating water flow off the skin than in the past is much more convenient tube flow meter. Ultrasound can be used for gas flow measurement juice. Scope diameter from 2cm to 5m, from a few meters wide and open channels, culverts to 500m wide river can apply.
In addition, the ultrasonic flow measurement accuracy of the measuring instrument is almost unaffected by the measured fluid temperature, pressure, viscosity, density and other parameters, can be made of non-contact measuring instruments and portable, it can solve other types of instruments are difficult to measure the intensity flow-corrosive, non-conductive, radioactive and explosive media measurement problems. In addition, given the characteristics of the non-contact measurement, together with a reasonable electronic circuits, an instrument can be adapted to a variety of diameters and a variety of flow measurement range measurements. Ability to adapt to other instruments ultrasonic flowmeter is incomparable. Ultrasonic flowmeters have said some of the advantages it more and more attention and the product series, universal development, is now made of different channels standard, high temperature, explosion-proof, wet-type instrument to adapt to different media, different traffic situations and different piping conditions
The disadvantage of the ultrasonic flowmeter are primarily of the current can be measured by the fluid temperature range coupling material between the transducer and the pipe to limit the degree of temperature of aluminum and an ultrasonic transducer, and the raw data at a high temperature fluid to be measured the acoustic velocity insufficiency. At present, China can only fluid for measuring below 200 ℃. In addition, the measurement line ultrasonic flowmeter complex than the general flow meter. This is because, in general industrial liquid flow rate measurement is often a few meters per second, while the speed of sound in the liquid is approximately 1500m / s or so, the measured fluid flow rate (flow rate) to bring the amount of change in the maximum change in velocity is 10 - three orders of magnitude. If the accuracy required to measure the flow rate is 1%, then the speed of sound measurement accuracy required for 10-5 ~ 10-6 order of magnitude, so there must be a sound measuring circuit can be achieved, which is the only integrated ultrasonic flowmeter under the premise of rapid technological development in order to get practical reasons.
Ultrasonic flowmeter by the ultrasonic transducers, electronic circuits and flow display and accumulation system consists of three parts. Ultrasonic transmitting transducer converts electrical energy into ultrasonic energy, and emitted into the fluid to be measured, the receiver receives the ultrasonic signal, the electronic circuit is amplified and converted to electrical signals representative of the flow rate supplied to the display and the meter display totalizer and totalizer. This realization of the traffic detection and display.
Ultrasonic flowmeter common piezoelectric transducer. It uses a piezoelectric effect of the piezoelectric material, the use of an appropriate transmission power is applied to the transmitter circuit to the transducer of the piezoelectric element, to produce an ultrasonic vibration advised. An ultrasonic wave propagation in the fluid incident at an angle, and then received by the receiving transducer, and by the piezoelectric element into electrical energy in order to detect. Transmitting transducer using a piezoelectric element is the inverse piezoelectric effect, and the receiver transducer is the use of the piezoelectric effect.
Ultrasonic flow meter of the piezoelectric transducer element normally made of a circular sheet, the thickness vibration. Diameter of more than 10 times the sheet thickness to ensure that the directional vibration. More use of the piezoelectric element material lead zirconate titanate. The piezoelectric element is fixed, at right angles to the ultrasonic wave incident on the fluid, to be the sound of the enemy wedge element, the whole constituting the transducer (also known as probe). Sound wedge material requires not only high strength, anti-aging, and requires energy loss by the sound of the wedge after a small ultrasonic transmission coefficient that is close to 1. Commonly used materials are glass wedge sound, because it is transparent, and the sound can be observed that the assembly of the piezoelectric element is a wedge. In addition, some rubber, plastic and bakelite also can make sound wedge material.
The electronic circuit includes a transmitter ultrasonic flowmeter, receiving, signal processing and display circuits. The measured values of the instantaneous flow rate and cumulative flow display digital or analog.
According to the principles of signal detection, the current propagation velocity of ultrasonic flowmeter roughly divided difference method (including: direct difference method, the time difference method, the phase difference method, the frequency difference method) beam offset method, the Doppler method, correlation, space type of filtering and noise method, as shown. Among the principles and structure of the noise method is simple, easy to measure and carry, inexpensive but less accurate, suitable for the occasion in flow measurement accuracy less demanding use. As a direct difference method, the time difference method, the frequency difference between the law and the basic principles of the phase difference method is by measuring the ultrasonic pulse downstream and upstream transmission speed to reflect the difference between the chime of the velocity of the fluid, it is also referred to as the propagation speed difference method. Where the frequency difference method and the time difference method to overcome the error with the fluid speed of sound caused by temperature changes, high degree of accuracy, it is widely used. According to different transducer configuration, the propagation speed 差拨 divided into: Z method (through law), V method (reflection method), X method (cross method). Beam offset method is the fluid flow rate variation with the use of an offset in the direction of propagation of the ultrasonic beam to reflect the fluid flow rate of fluid at low flow rates, the sensitivity is not very low applicability. Doppler method is the use of acoustic Doppler principle, uneven scattering body fluid by measuring the scattering of ultrasound Bo Duopu
Doppler shift to determine the fluid flow, suitable for containing suspended particles, bubbles and other fluid flow measurement. Correlation method is the use of relevant technology to measure flow, on principle, the measurement accuracy of this method is independent of the speed of sound in the fluid, which has nothing to do with the fluid temperature, concentration, and thus the measurement accuracy, a wide range. But correlator expensive, more complicated route. After the microprocessor universal application, this shortcoming can be overcome. Noise Method (listening method) is related to the principle of the flow rate of the fluid noise generated when using the fluid flow within the pipe, said flow rate or flow rate value by detecting noise. Its method is simple, inexpensive equipment prices, but low accuracy.
Several methods have their own characteristics, should be based on the measured fluid properties. Velocity distribution, pipeline installation site as well as on the measurement accuracy requirements and other factors to select. Generally the temperature of the working fluid in industrial production often can not remain constant, it is more timely use of the frequency difference method difference method. Only in diameter big difference when using the direct method. Transducer installation method selection principles generally: when the fluid flow along the tube axis parallel to, the choice of Z method; when the flow direction is not parallel with the tube or pipe uranium installation site the transducer mounting space is limited, the use of V method or X method. When uneven flow distribution when the table before another short straight sections, may also be multi-channel (for example, two-channel or four-channel) to overcome the disturbance caused by the flow velocity measurement error. Doppler method suitable for measuring flow, can avoid the shortcomings of conventional instruments by aerosols or air bubbles caused by the blockage, abrasion, adhesion and can not run, and thus to develop rapidly. With the development of energy-saving work carried out development and industry, kerosene mixed (COM), coal cement together (CWM) delivery and application of fuel and combustion of fuel oil plus water and other energy methods are applied to open up broad Doppler ultrasonic flowmeter prospects.
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