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Industry Information
Ultrasonic flow meters do not need to install measuring elements and do not change the flow state of the fluid
2012-3-19 13:53:51

由超声波换能器、电子线路及流量显示和累积系统三部分组成。 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.
的电子线路包括发射、接收、信号处理和显示电路。 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.
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.

的注意事项 Precautions when using ultrasonic flowmeter

1. Does the instantaneous flowmeter fluctuate?
A, the signal intensity fluctuates greatly; B, the measurement fluid itself fluctuates greatly;
Solution: Adjust the position of the probe and increase the signal intensity (maintained above 3%) to ensure the signal intensity is stable. If the fluid fluctuates greatly, the position is not good. Select a new point to ensure the requirements of the first 10D and 5D.
2. The signal of the external clamp flowmeter is low?
The pipe diameter is too large, the pipe scale is serious, or the installation method is wrong.
Solution: For large pipe diameter and serious fouling, it is recommended to use a plug-in probe or choose a "Z" type installation method.
3. The signal of the plug-in probe decreases after a period of use?
The probe may be offset or the surface of the probe may be thick.
Solution: Re-adjust the position of the probe and clear the transmitting surface of the probe.
4. There is no display at startup to check whether the power supply property corresponds to the rated value of the instrument and whether the fuse is blown. If the above problems are not recommended, please send the instrument back to our company for inspection by a professional technician.
5. After the instrument is turned on, the meter has only backlight and no characters are displayed. This situation is generally the loss of the program chip. It is recommended that the instrument be sent back to our company for processing.
6. The meter cannot be used under the strong interference on site?
The power supply has a large fluctuation range, there are inverters or strong magnetic field interference around it, and the ground wire is incorrect.
Solution: Provide a stable power supply for the instrument. Install the instrument away from the inverter and strong magnetic field interference, and have a good grounding wire.