At present, ultrasonic liquid level measurement technology has matured, and related measuring equipment is widely used in various industrial sectors, but its methods and principles are different, mainly because of the different needs in industrial fields, such as measurement in open environment, measurement in sealed environment and cost control requirements.Suitable ultrasonic liquid level measurement methods and suitable ultrasonic liquid level gauge products should be adopted according to different requirements.Therefore, it is very important to understand the advantages and disadvantages of relevant ultrasonic measurement methods and master the basic principles of ultrasonic.
In the field of industrial measurement, Especially in special measurement occasions, For example, the measurement of some liquids containing acid and alkali, Or when measuring toxic and harmful liquids and occasions with high danger, Ultrasonic measurement is widely used, mainly because it is non-contact measurement, does not come into direct contact with the measured liquid, and has certain safety.Besides, ultrasonic measurement is very flexible, and there are basically three ways:air medium measurement, liquid medium measurement and solid medium measurement.It is suitable for all kinds of liquids to be measured, and the three methods can be selected arbitrarily according to the measuring occasions and requirements.Intelligent ultrasonic level gauge the measurement system is also widely used in rivers, lakes and seas, sewage treatment plants and other liquid level warning alarm.

2.1 Fundamentals of ultrasonic liquid level measurement

2.1.1 Introduction to Ultrasound
Ultrasonic wave belongs to sound wave and is a kind of mechanical wave.Sound wave is the wave caused by the vibrating sound source in the surrounding elastic medium.All kinds of sounds heard by people in life reach people's eardrum because the vibration of sound source spreads through air medium, which causes the vibration of eardrum to stimulate people's auditory nerve, and people can hear the sound emitted by objects.Human perception of sound has a certain frequency range.Usually, the vibration of 20 ~ 20000Hz can cause human hearing.If the vibration frequency of an object is lower than 20Hz or higher than 20000Hz, the incoming ear can hardly hear it.Usually, we call sound waves below 20Hz infrasonic waves and sound waves above 20000Hz ultrasonic waves.Ultrasonic wave is mainly related to three physical quantities, namely wavelength nine, propagation speed C and its frequency f, and the corresponding relationship of the three physical quantities is c=f* λ
There are many methods to generate ultrasonic waves, such as piezoelectric effect method, magnetostrictive effect method, and electric effect method, etc.Usually, zui is used in piezoelectric effect method in industrial field.Why ultrasonic is so widely used in the field of liquid level and nondestructive measurement, it mainly has the following advantages:
1) Ultrasonic wave has good directivity.The frequency commonly used in industrial ultrasonic measurement is usually 0.5 ~ 10MHz, because of its short wavelength and good directivity, which is beneficial to detection.In addition, if its emission frequency is higher, the beam angle will be smaller and its propagation directivity will be better.
2) Ultrasonic wave has high energy.Under the condition of the same amplitude, the research shows that the vibration energy of a body is proportional to the square of its vibration frequency.The main reason is that ultrasonic waves have higher energy than other sound waves.

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