Introduction to the principle and application of ultrasonic measurement

I. Introduction   

In many industrial productions, such as chemical, petroleum, metallurgy, electric power, water treatment and other industries, it is often necessary to carry out position detection and control of materials stored in containers, storage tanks and warehouses during production, and the accuracy of material level detection. Timeliness is of great significance for achieving safe and efficient industrial production. Accuracy of measurement will affect the correct calculation and credibility of the company's cost accounting, energy consumption and other indicators. At the same time, the scope of application and maintenance of the test components are also related to the normal and stable production.

Generally, the way of measuring the level is differential pressure type, floating ball type, heavy hammer type, ultrasonic type, radar type and the like. With the continuous development of science and technology, the degree of production automation is getting higher and higher, and the requirements for measuring accuracy, reliability and continuity of the level are getting higher and higher. Ultrasonic level measurement is widely used at home and abroad by virtue of its non-contact method and its advantages of corrosion resistance, non-toxicity, maintenance-free, long service life and high cost performance.

Second, the working principle of ultrasonic measurement

Ultrasonic wave is a kind of mechanical wave, which is a kind of propagation process of mechanical vibration in elastic medium. It is characterized by high frequency, short wavelength, small diffraction phenomenon, good directionality, and can be directional and propagated as rays. Ultrasonic waves have little attenuation in liquids and solids, so they have strong penetrating power. Especially in solids that are opaque to light, ultrasonic waves can penetrate for several tens of meters. When they encounter impurities or interfaces, they will have significant reflection. Ultrasonic measurement The level is the use of this feature.

In the ultrasonic testing technology, no matter which kind of ultrasonic instrument, the electric energy conversion ultrasonic wave must be emitted, and then received and converted into an electric signal. The device for accomplishing this function is called an ultrasonic transducer, also called a probe. As shown in the figure, the ultrasonic transducer is placed above the liquid to be measured, and the ultrasonic wave is emitted downward. The ultrasonic wave passes through the air medium, is reflected back when it encounters the water surface, and is received by the transducer and converted into an electrical signal. When the electronic detection section detects this signal, it turns it into a liquid level signal for display and output.

It is known from the principle that the ultrasonic wave propagates in the medium. If the conditions of the medium pressure, temperature, density, humidity, etc. are constant, the propagation speed of the ultrasonic wave in the medium is a constant. Therefore, when the time required for the ultrasonic wave to be received until the liquid level reflection is received is measured, the path through which the ultrasonic wave passes can be converted, that is, the data of the liquid level is obtained. In the figure, D is the distance of the pool wall from the center of the ultrasonic transducer, A is the distance of the ultrasonic transducer from the 100% liquid level, H is the 100% measurement range, and B is the ultrasonic transducer from the lowest liquid level distance. General instrument manufacturers have restrictions and requirements for A, D, and A/B.


Third, characteristics and selection

At present, many companies at home and abroad produce various types of ultrasonic level detectors. Now, the Miniirangerplus ultrasonic level detector manufactured by Miaoshengli Co., Ltd. introduces its selection and use in the application of sewage treatment plants.

(1) Features of the Minirangerplus level detector

The Minirangerplus is an intelligent instrument that can be remotely connected to the appropriate type of transducer via a cable to measure a wide range of levels. Thanks to its unique intelligent software, it has the following features:

1. Under the same measurement range, the D value of the liquid level gauge is the smallest and the measurement blind zone is the shortest;

2. Applicable ambient temperature range from -40 to +95C with automatic temperature compensation;

3. Reliable performance and maintenance free. It uses Miaosheng's advanced sound intelligence software to effectively identify true and false echoes, which can satisfy many contactless measurement applications.

4. Unaffected by the agitator, the unique echo processing function ensures reliable operation and stable readings when using agitators and other liquid level surges or where there is a large amount of foam;

5. Easy to install and easy to operate.

(2) Selection

Choosing the right model according to needs and working conditions is an important condition to ensure accurate and reliable measurement results.

1. Maximum measurement range

Minirangerplus can be configured with different types of transducers such as 5m, 10m, and 15m. When selecting a transducer, it is necessary to consider the characteristics of the material being tested, the nature of the medium on the interface, the environmental conditions, and the operating conditions of the production system. The degree of attenuation and the reflection characteristics of the interface after ultrasonic transmission have a great influence on the measurement range. When the ultrasonic wave is attenuated quickly and the interface reflection is poor, in order to prevent the ultrasonic signal received by the ultrasonic probe from being too weak to distinguish from the noise signal, it is necessary to increase the transmitting power of the transducer.

In the process of reforming the liquid level measurement and control system, Yindingzhuang Sewage Treatment Plant selected different types of transducers according to the needs. For example, the coarse grille is 9.5 meters deep and the XPS-10 type transducer is selected. The maximum measuring range is 10 meters; the pool pool is 11.5 meters deep, the XPS-15 type transducer is used, the maximum measuring range is 15 meters; the fine grill corridor is 2 meters deep, and the single probe level gauge with the largest measuring range of 5 meters is selected. .

2. Temperature compensation   

Ultrasonic probes are made of materials with acoustic and electrical effects. When higher than a certain temperature, the piezoelectric components will lose their effect and cannot be used. Generally, the ultrasonic transducer is suitable under the working condition of -20-80 °C. In addition, since the ultrasonic measurement level is based on the sound velocity C in the medium transmitting the ultrasonic wave at the interface of the material to be tested, in practice, the sound velocity of the medium is determined by the temperature component, and is also affected by the temperature. As the season changes, the ambient temperature changes greatly. To ensure measurement accuracy, a temperature-compensated ultrasonic level gauge should be selected.

(3) Sound speed correction

The media properties and composition of the sound velocity transmitted on the material interface affect the measurement results. In order to adapt to different measurement media, Minirangerplus is equipped with a sound velocity correction system to correct the error caused by the composition change of the medium. The specific operation is as follows: In the Miniirangerplus program, the actual measured value is input. Minirangerplus calculates the sound velocity by comparing the actual measured value (distance) with its own ultrasonic measured value (time), and uses this sound velocity for subsequent measurement.

Fourth, the installation of the transducer

Whether the instrument can obtain accurate measurement data is not only related to the accuracy of the instrument itself, but also related to the installation and debugging of the instrument. In addition to obtaining more accurate test data, the correct installation can also ensure that the instrument works stably and is easy to maintain. Ultrasonic level detector should pay attention to the following points during installation:

1. The ultrasonic transducer should be installed in a place where it can only "see" the surface of the measured medium, so that sound waves can not be disturbed by objects such as baffles, agitators, and silo walls when they are transported in containers, storage tanks and silos. Measurement error;   

2. The ultrasonic transducer has a blind spot that cannot be identified by itself between the pulses it transmits and receives. For reliable measurement, the highest level of the measuring object should be outside the blind zone;

3. When measuring the solid material level in the storage bin, it is considered that the solid material is not flat at the interface. Usually, the material at the feed port is higher than the periphery of the storage bin. Therefore, the transducer should be installed at a position that reflects the average height of the material. ;

4. Another point of installing the transducer is to avoid zero reflections. When the ultrasonic transducer is aligned with the inclined wall of the tank bottom or the discharge port, the ultrasonic pulse is emitted, but the echo is not received, the detector alarms or adjusts to the maximum, and the interference echo is used as the level. The danger of echo reception. An effective way to solve this problem is to place a small reflector on the bottom of the liquid tank or install the transducer near the centerline of the tank.

5. Shielded cable connection should be used between the ultrasonic transducer and the detector, and both ends of the shielded wire should be grounded to avoid interference from electrical noise.

V. Conclusion

Due to the non-contact measurement principle of the ultrasonic level gauge, it is theoretically suitable for level measurement in various processes, but in applications it is subject to various factors such as mounting position, temperature, pressure, humidity, and The influence of foam, surge, etc. on the surface of the measured medium. In addition, the transmission distance and directivity of ultrasonic waves are related to the frequency of sound waves and are mutually restricted. Therefore, this measurement method is not suitable for long-distance measurement. In practical applications, the appropriate instrument should be selected according to the measurement requirements and working conditions to achieve the best measurement results.