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Ultrasonic flowmeter merit and demerit

The disadvantages of ultrasonic flowmeter is mainly to the current body temperature range by ultrasonic can in aluminum and coupling between transducer and pipeline material heat resistance level of restrictions, be sound transmission speed of flow and the heat raw data is not complete.At present our country can only be used to measure the fluid under 200 ℃.In addition, ultrasonic flowmeter measurement line is more complicated than the general flow meter.This is because, in general industrial measurement of liquid flow velocity is often a few meters per second, and propagation velocity of sound waves in a fluid is about 1500 m/s or so, the current body of velocity (flow) changes to sound velocity variation of the maximum 10-3 orders of magnitude. If you want to measure the velocity of the accuracy is 1%, the measuring accuracy of the sound velocity for 10 to 5 ~ 10-6 orders of magnitude, so must have perfect measuring circuit to realize, that is ultrasonic flowmeter is only under the former title of integrated circuit technology rapid development can be the cause of the practical application.
Ultrasonic flowmeter by the ultrasonic transducer, electronic circuit and flow display and cumulative system of three parts.Ultrasonic transmitting transducer converts electrical energy into ultrasonic energy, and launch it to the flow in the body, the receiver, the received ultrasonic signal amplified by electronic circuit and converted to represent the flow of electrical supply display and integrating instrument for display and integrating.Thus to achieve the traffic detection and display.
Ultrasonic flowmeter is used piezoelectric transducer.It USES the piezoelectric effect of piezoelectric materials, using the optimal transmission circuit of add electricity to the transmitting transducer on the piezoelectric element, make its produce ultrasonic is advised.Ultrasonic to spread into the fluid in a certain Angle, and the receiving transducer, and the piezoelectric element into electrical energy, in order to detect.Transmitting transducer using the converse piezoelectric effect of piezoelectric element, and the receiving transducer is using the piezoelectric effect.
Ultrasonic flow meter transducer of the piezoelectric element often make circular wafer, along the thickness of the vibration.Wafer diameter thickness of more than 10 times, to ensure the direction of vibration.With lead zirconate titanate piezoelectric element material.For fixed piezoelectric element, make the ultrasonic wave into the fluid at the right Angle, need to memorize the components in wedge, a transducer (also called detector) as a whole.The wedge of material not only require high strength, resistance to ageing, and required ultrasonic acoustic energy loss after small wedge is the transmission coefficient is close to 1.Commonly used wedge material is organic glass, because it is transparent, can be observed in wedge piezoelectric element assembly.In addition, some rubber, plastic, bakelite material can also be wedge.
Ultrasonic flowmeter electronic circuit including transmitting and receiving, signal processing and display circuit.Measured the instantaneous flow and cumulative flow value with digital or analog display.
According to the principle of signal detection, the ultrasonic flowmeter can be divided into roughly propagation velocity difference method (including direct time difference method, the time difference, phase difference, frequency difference method) beam deflection method, doppler method, correlation method, spatial filtering and noise method, such as type, as shown.With noise method, principle and structure of the most simple, easy to measure and carry, cheap but low accuracy, suitable for use in the occasion of the flow measurement accuracy requirements is not high.Due to time difference method, the direct method of time difference, frequency difference method and the basic principle of phase difference method by measuring ultrasonic pulse transmit time downstream and upstream speed to reflect the difference between the flow velocity of the fluid, so it is also referred to as propagation velocity difference method.The frequency offset and time difference method overcomes the sound velocity vary with fluid temperature, the error of accuracy is higher, so widely used.According to the configuration method of transducer, speed difference set is divided into: Z method (through), V (reflection method), X (crossover), etc.Beam deflection method is the use of ultrasonic beam propagation direction vary with the fluid velocity in the fluid shifts to reflect the fluid velocity, when low velocity of flow, the sensitivity is very low applicability. Doppler method is using the acoustic doppler principle, by measuring the uneven fluid in the body of scattering of ultrasonic doppler
The frequency shift to determine the fluid flow, is suitable for containing suspended particles, such as bubble fluid flow measurement.Measure flow correlation method is to use the related technology, in principle, the method of measurement accuracy has nothing to do with the sound velocity in the fluid, and therefore with the fluid temperature, the concentration, and high measurement accuracy, wide application scope.But the correlator price expensive, the line is more complex.After the microprocessor to popularize and apply this shortcoming can be overcome.Noise method (listening method) is the noise emitted by use of multiphase flow in pipeline, and on the principle of fluid flow through testing the noise said velocity or flow value.The method is simple, the equipment is cheap, but low accuracy.
The above several kinds of methods have different characteristics, should according to the current liquid. Velocity distribution, piping installation location and the requirement of measurement accuracy.Generally due to industrial production jobs often cannot maintain a constant temperature, so the more the frequency difference method is adopted in time difference method.Only in pipe diameter is large by direct time difference method.The choice of transducer installation method principle is: when the flowing fluid in the tube axis parallel with Z method;When the flow direction and tube uranium not parallel or piping installation place is restricted, the transducer installation interval by the methods of V or X.When the uneven distribution of flow field and straight pipe before the table is relatively short, also can use multichannel (such as double track or four channels) to overcome disturbance traffic flow velocity measurement error.Doppler method suitable for measuring two phase flow, can avoid the conventional caused by suspended solids or bubble blockage, abrasion, adhesion and cannot run ills, thus developing rapidly.With the development of the industrial development and energy conservation, the kerosene mixed (COM), coal, cement (CWM) transmission and application of the fuel and the fuel oil and water development of energy saving methods, such as fuel, have multiple doppler ultrasonic flowmeter application open up broad prospects.

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| Updated:2015.03.12    Source:    Clicks:

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Yvonne Yu