首先由信号超声波发生器来产生一个特定频率的信号，这个信号可以是正弦信号，也可以是脉冲信号，这个特定频率就是超声波流量计换能器的频率，一般在超声波流量计中使用到的超声波频率为25KHz、28KHz、35KHz、40KHz；1OOKHz或以上现在尚未大量使用。 The ultrasonic flowmeter first uses a signal ultrasonic generator to generate a signal with a specific frequency. This signal can be a sinusoidal signal or a pulse signal. This specific frequency is the frequency of the ultrasonic flowmeter transducer. It is generally used in ultrasonic flowmeters. The obtained ultrasonic frequency is 25KHz, 28KHz, 35KHz, 40KHz; 1OOKHz or above has not been widely used yet.
A relatively complete ultrasonic generator should also have a feedback link, which mainly provides feedback signals in two aspects: The first is to provide an output power signal. We know that when the power supply (voltage) of the ultrasonic generator changes. The output power of the ultrasonic generator will also change. At this time, the mechanical vibration is reflected on the ultrasonic flowmeter transducer, and the cleaning effect is unstable. Therefore, the output power needs to be stabilized, and the power amplifier is adjusted correspondingly through the power feedback signal, so that the power amplification is stable. The second is to provide a frequency tracking signal. When the ultrasonic flowmeter transducer works at the resonance frequency point, it has the highest efficiency and the most stable operation, and the resonance frequency point of the ultrasonic transducer will change due to assembly reasons and work aging. Of course, the frequency of this change is only drift, change It is not very large, the frequency tracking signal can control the signal ultrasonic generator, so that the frequency of the signal ultrasonic generator can track the resonance frequency point of the ultrasonic transducer within a certain range. Let the ultrasonic generator work at its best. Of course, with the development of modern electronic technology, especially the microprocessor (uP) and signal processor (DSP), the ultrasonic generator is becoming more and more powerful, but no matter how it changes, its core function should be as described above The content is just that the technology of each part is different. However, analog power amplifiers have several disadvantages: (1) It is not easy to use modern microprocessors for processing, because this circuit presents a typical analog circuit characteristic, and digital processing is more complicated, involving A / D (analog to digital) And D / A (digital to analog), the cost is relatively high, the reliability is low. (2) The analog control circuit has the disadvantages of low control accuracy, slow dynamic response, inconvenient parameter setting, serious temperature drift, and easy aging. The advent of special analog integrated control chips has greatly simplified the control circuits of power electronic circuits. The switching frequency of the control signal is increased, and an analog regulator with a correction link can be directly formed by simply connecting a few resistance-capacitance components, thereby improving the reliability of the circuit. However, due to the existence of resistance-capacitance components, the inherent defects of analog control circuits, such as the accuracy and consistency of component parameters and component aging, still exist. (3) In addition, the analog integrated control chip also has problems such as large power consumption, low integration, insufficient control flexibility, and poor generality. Using digital control instead of analog control can eliminate the shortcomings of conventional analog regulators such as temperature drift, which is conducive to parameter tuning and variable parameter adjustment. It is easy to adjust the control scheme and implement a variety of new control strategies through program software changes. Can reduce the number of components, simplify the hardware structure, thereby improving the reliability of the system. In addition, it can also realize automatic storage of operating data and automatic fault diagnosis, which helps to realize the intelligent operation of power electronic devices.
应注意的事项：超声波流量计的超声波流量计原理一般用户只需大致了解即可，用户最注重仪表性能和实用。 Matters needing attention when selecting and using ultrasonic flowmeters : The principle of ultrasonic flowmeters for ultrasonic flowmeters is generally only understood by users, and users pay most attention to instrument performance and practicality. This article just wants to focus on the selection and installation of ultrasonic flowmeters, and combined with our company's experience to provide users with some methods and experience that can be used for reasonable selection and use.
At present, most of the ultrasonic flowmeters used in the water supply industry on the market are time-lag type. Such flowmeters have entered a relatively mature stage, and the flowmeters of medium and large pipelines in the water supply industry can fully meet the requirements. According to different applications, it can be divided into fixed super ultrasonic flowmeter and portable ultrasonic flowmeter.
1.Portable ultrasonic flowmeter
It is mainly used for temporary comparison measurement. The mobility is very large, and the price is more expensive than the fixed type. If it is used for fixed measurement, it is a bit wasteful. The portable ultrasonic flowmeter is temporarily measured in use, and the pipe diameter and pipe are not fixed. In addition to the correct input of various data, it is necessary to ensure that the upstream and downstream straight pipe sections. The installation of transducers (also called probes) is also very important. For small pipes (below 300mm), use the "V" method as much as possible. This can extend the sound path and improve the measurement accuracy. For pipes above 300mm, only the "Z" method can be used to measure. According to our actual experience, it is difficult to find signals using the "V" method for pipes above 300mm. Of course, if the signal can be found and the signal is strong and stable, it is better to use the "V" method. "V" installation requires much shorter straight sections than "Z" installation.
2. Fixed ultrasonic flowmeter
The fixed type is divided into three types: outer clamp type, plug-in type and pipe section type.
(1) Outside clip type
The early flowmeters were all clip-on, which everyone is familiar with and is also the most widely used ultrasonic flowmeter. But the biggest disadvantage of this flow meter is that it cannot be used on cement pipes. So in recent years, there have been plug-in ultrasonic flowmeters.
(2) Plug-in ultrasonic flowmeter
This type of flowmeter uses a special pipe opening tool to open a small hole in the pipe wall, and uses a pipe clamp (or plywood) to insert the transducer into the pipe and directly contact the water. Its installation and measurement are not limited, and its signal is Transmit and receive only through the water medium, the signal is strong and stable.
Disadvantages: Large pipe probe installation (especially pipes with rough outer walls) is not easy to accurately locate, and old pipes that have been buried underground for many years (due to many factors such as production processes, manufacturers, etc.) cannot accurately determine the pipe wall thickness and inner diameter, which affects the measurement accuracy (accurate (Hereinafter referred to as accuracy).
(3) Tube type ultrasonic flowmeter
Some pipes have severe attenuation of ultrasonic signals due to sparse materials, poor sound guidance or severe rust, gaps between the lining and the pipe wall, etc. When the external clamp type ultrasonic flowmeter cannot measure normally, the pipe section type (also called full pipe type ) Ultrasonic flowmeter. It integrates the transducer with a standard short tube. The flowmeter is calibrated one by one when it leaves the factory. The accuracy can reach level 0.5, which is mainly used in places with high measurement data. Note: The accuracy must be ensured when the upstream and downstream straight pipe sections are guaranteed.
Disadvantages: one-time investment is large and must be installed without current
(4) In recent years, two-channel, four-channel and other multi-channel ultrasonic flowmeters have appeared. Multi-channel flowmeters have a great effect on improving measurement accuracy, but the price is high. If funding is sufficient, measurement accuracy requirements High selection of such ultrasonic flow meters.
We believe that multiple fixed ultrasonic flowmeters and a portable ultrasonic flowmeter are reasonable for the calibration of fixed flowmeters, because ultrasonic flowmeters can fully meet the requirements of water supply companies. The advantages and disadvantages of various ultrasonic flowmeters are briefly introduced, and users can choose according to the actual situation of the enterprise. Pay attention to several points when buying:
1. Learn more about the information so as not to be fooled by exaggerated publicity. 2. Choose a manufacturer with research and development capabilities to ensure quality and after-sales service. 3. The selected ultrasonic flowmeter should be cheap and good, that is, it can meet the needs, and there is no need to be greedy for more, which causes many functions to be idle and not used, which increases the purchase cost.
After the purchase of fixed ultrasonic flowmeters (pipe segment type, plug-in type, and outer clip type), the factory usually sends someone to guide the installation on site. But pay attention to the following points: 1. The connection cable between the host and the transducer (most manufacturers specify it within 300M) should be as short as possible. One is to reduce signal attenuation, and the other is to prevent external interference, so that the host can work stably for a long time. And avoid burying in parallel with the power cable at a short distance. If it is unavoidable, you should wear steel pipe to strengthen the shield, or shorten the signal cable and then add the remote display device. 2. Zero calibration, when the fluid is static, the display value of the instrument is called "zero" (the display value at this time is not necessarily zero). When the indication value of the flow meter is not zero, the value of the "zero point" will be superimposed on the true value of the flow meter at any time, so that the measured value will be biased. The purpose of zero adjustment is to eliminate the installation zero point and ensure measurement accuracy. Stop water calibration as much as possible. If you can't stop water calibration, you can take other measures. 3. After the fixed flowmeter (except with standard pipe) is installed, it should be checked one by one. The first is to use a portable ultrasonic flowmeter for calibration, which is relatively simple. The second is to use a clear water tank for pipelines that cannot be calibrated with a portable flowmeter (such as the removal of a turbine flowmeter that we have installed in the original pipeline). After that, the installation port is left, so that it can be calibrated with the turbine flow meter at any time, so that you can know it). 4. Ensure that the straight pipe sections upstream and downstream during installation are a prerequisite to ensure accurate measurement.
敏感于流过换能器的流速分布剖面，管道中流体的稳定与否很重要，为了保证计量的准确各厂家都规定上游要保证不小于10D，下游不小于5D的直管段，直管段长度对精度影响程度如何，中山水司所做的测试实验，足以说明这个问题。 The ultrasonic flowmeter is sensitive to the profile of the flow velocity flowing through the transducer. The stability of the fluid in the pipeline is very important. In order to ensure the accuracy of the measurement, various manufacturers have stipulated that the upstream must be not less than 10D, and the downstream is not less than 5D. How much the length of the pipe section affects the accuracy, the test experiments made by Zhongshan Water Company are enough to illustrate this problem.