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Far-energy ultrasonic flowmeters are used in urban sewage treatment plants:
In order to improve the increasingly serious water pollution situation, many cities have established sewage treatment plants, the sewage treatment rate has improved significantly, and the water environment has gradually improved. With the rapid development and popularization of computer technology, data acquisition systems have also been rapidly applied. Applying this system in the production process can collect, monitor and record process parameters at the production site, providing information and means for improving product quality and reducing costs. Data acquisition is to collect the analog signal output by the sensor and convert it into a computer-recognized digital signal before further processing.
Ultrasonic flowmeters are instruments that measure the flow of ultrasonic pulses by detecting fluid flow. Like the electromagnetic flowmeter, it is an obstacle-free flowmeter because it does not have any obstacles in the flow passage of the instrument. It is a type of flowmeter suitable for solving difficult problems in flow measurement, especially in large-diameter flow measurement. It is also one of the rapidly developing flowmeters in recent years.
2 principle of ultrasonic flowmeter
2.1 Composition
The non-contact ultrasonic measurement system consists of two parts: an ultrasonic signal transducer and a remotely mounted electronic transmitter. The transducer continuously emits a series of ultrasonic waves and receives reflected echoes returned from the liquid or solid surface being detected. The microprocessor in the transmitter processes the signal further and displays the data as an LCD digital display.
(1) Acoustic waves are a type of mechanical wave. Acoustic waves with a frequency exceeding 2 x 104 Hz are ultrasonic frequencies that cannot be heard by human ears. Its relationship between wavelength L, frequency f and speed c is
L=c/f
(2) Reflection and refraction of ultrasonic waves
When the acoustic wave propagates to the interface between two media, part of it can return to the original media, called the reflected wave, and the other part of the energy continues to propagate through the media surface to the other media surface, which is called the refracted wave.
Reflection Law: Sina/ Sina'==C1/C2
In the formula, the incident angle a, the incident wave speed C1, the reflection angle a′, and the reflected wave speed C2.
Refraction law: SINA/SINb==C1/C2
In the formula, the incident angle a, refraction angle b, C1 is the first medium wave velocity, and C2 is the second medium wave velocity.
2.2 Characteristics of Ultrasonic Flowmeter
The remarkable feature of ultrasound is its high frequency, which results in short wavelength, small diffraction, and good directivity. Can be called ray and directed propagation. In liquids and solids, attenuation is small, penetrating power is large, and there is significant reflection when impurities or interfaces are encountered. Ultrasonic sensors use these remarkable features of ultrasonic waves to obtain a wide range of applications in many scientific fields, namely industrial production.
2.3 Ultrasonic transducer
Ultrasonic transducer is also called ultrasonic probe. It is a kind of device that realizes the conversion between electric energy and sound energy. In ultrasonic detection technology, the ultrasonic device emits the ultrasonic wave and then receives it and converts it into an electric signal. The ultrasonic transducer is the ultrasonic transducer. The probe. There are various types of ultrasonic transducers used for detection, such as piezoelectric type, magnetostrictive type, electromagnetic type, vibration plate type, surface acoustic wave type, and the like. In the detection technology, the piezoelectric type is mainly used.
2.4 Analysis of advantages and disadvantages
advantage:
â—† Can do non-contact measurement;
â—† No flow obstruction measurement, no pressure loss;
â—† It can measure non-conductive liquid and it is a supplement to electromagnetic flowmeter without obstruction measurement.
â—† The biggest advantage of the ultrasonic flow class is that it is not necessary to cut off the pipe, and it can be used to measure the flow without obstruction. It is a flow meter that does not actually touch the medium.
Disadvantages:
â—†The time of propagation can only be used to clean liquids and gases; the Doppler method can only be used to measure liquids containing quantitative suspended particles and bubbles;
â—† Doppler measurement accuracy is not high.
3 Application
Ultrasonic measurement is a method of measuring container reserves by measuring the material level. Ultrasonic sensors send ultrasonic pulses from the top to the surface of the material and receive echoes reflected from the surface. It measures the travel time of the ultrasonic wave and calculates the distance from the reflective surface.
The propagation time is a visual dimension of the distance between the ultrasonic sensor and the material level. The propagation distance of sound waves is the product of propagation time and speed of sound. If the position of the ultrasonic sensor is known, then the level (and reserves) can be converted. The range of sound speed in the air varies widely. Therefore, the acoustic wave sensor needs temperature compensation. If it is used for gases other than air, it should be calibrated accordingly.
3.1 Measuring flow
In the flow measurement instruments of a sewage treatment plant, there are only two types of flowmeters classified according to the installation type: pipeline type and open channel type.
The open channel ultrasonic flow meter is a kind of flow meter widely used in sewage treatment plants. Therefore, we only briefly explain the open channel flowmeter here. The flow of liquid in the open channel has a certain relationship with the liquid level somewhere on the tank, ie:
Q=Kbhm
In the formula, Q is the flow, K is the flow coefficient, b is the width of the mouth, h is the height of the liquid level, and m is the index.
For a certain type of sink, where the values ​​of k, b, and m are constant, the flow rate can be calculated as long as the level h is measured. It can be seen that the ultrasonic flowmeter used to measure the flow of open channels is actually measuring the liquid level.
3.2 Measuring liquid level
Ultrasonic level gauge is a non-contact measurement, high accuracy, low maintenance, so it has been widely used in sewage treatment plant.
How Ultrasonic Level Gauges Work
As can be seen from FIG. 1, the ultrasonic transmitter/receiver is placed above the liquid, the ultrasonic wave is emitted downward, passes through the air medium, is reflected back when it meets the water surface, and is received by the receiver. According to the principle of ultrasonic propagation in the medium, if the conditions such as medium pressure, temperature, density, and humidity are constant, the propagation speed of ultrasonic waves in the medium is a constant. Therefore, when the time required for the reflection of the ultrasonic waves from the launching to the encountering of the liquid surface is measured, the distance through which the ultrasonic wave passes can be changed, that is, the data of the liquid level can be obtained.
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