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technical article
Compare ultrasonic level gauge with radar level gauge
2012-2-15 10:33:53

和雷达液位计的区别我们一般把声波频率超过20kHz的声波称为超声波,超声波是机械波的一种,即是机械振动在弹性介质中的一种传播过程,它的特征是频率高、波长短、绕射现象小,另外方向性好,能够成为射线而定向传播。 The difference between an ultrasonic level gauge and a radar level gauge. We generally call sound waves with a sound wave frequency exceeding 20 kHz as ultrasonic waves. Ultrasonic waves are a type of mechanical wave, which is a propagation process of mechanical vibration in an elastic medium. Its characteristic is frequency High, short wavelength, small diffraction phenomenon, and good directivity, can be directional propagation. Ultrasonic waves have very low attenuation in liquids and solids, and therefore have strong penetrability. Especially in solids that are opaque to light, ultrasonic waves can penetrate tens of meters in length, and there will be significant reflection when they encounter impurities or interfaces. Ultrasonic measurement Level is the use of this feature. In the ultrasonic detection technology, no matter what kind of ultrasonic instrument, it is necessary to transmit the electric energy converted ultrasonic wave, and then receive it back and convert it into electrical signals. The device that accomplishes 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 transmitted downward. The ultrasonic wave passes through the air medium and is reflected back when it encounters the water surface. The electronic detection part detects this signal and turns it into a liquid level signal for display and output. From the principle of ultrasonic wave propagation in a medium, it can be known that if the medium pressure, temperature, density, humidity and other conditions 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 from the reflection of the liquid surface is measured, the distance through which the ultrasonic wave passes can be converted, and the liquid level data is obtained.


有盲区,安装时必须计算预留出传感器安装位置与测量液体之间的距离。 The ultrasonic level gauge has a blind spot, and the distance between the sensor installation position and the measurement liquid must be calculated during installation. 采用发射—反射—接收的工作模式。 The radar level gauge adopts the working mode of transmitting-reflecting-receiving. The radar level gauge's antenna emits electromagnetic waves. These waves are reflected by the surface of the measured object and then received by the antenna. The time from when the electromagnetic wave is transmitted to when it is received is proportional to the distance to the liquid surface. The relationship is as follows:


D = CT / 2


Where D-the distance from the radar level gauge to the liquid surface


C——speed of light


T——Electromagnetic wave running time


The radar level gauge records the time elapsed by the pulse wave, and the transmission speed of the electromagnetic wave is constant, the distance from the liquid surface to the radar antenna can be calculated, so as to know the liquid level of the liquid surface. In practical applications, there are two methods for radar level gauges, namely FM continuous wave and pulse wave. The liquid level gauge using FM continuous wave technology has large power consumption, and must adopt a four-wire system, and the electronic circuit is complicated. The level gauge using radar pulse wave technology has low power consumption and can be powered by two-wire 24V DC. It is easy to achieve intrinsic safety, high accuracy, and a wider range of applications. The biggest difference is the use of sound waves for ultrasound and electromagnetic waves for radar. And the penetration and directivity of ultrasonic waves are much stronger than electromagnetic waves, which is why ultrasonic detection is now more popular. Differences in main applications: 1. Radar measurement range is much larger than ultrasonic. 2. Radar has horn type, rod type and cable type. Relative ultrasonic waves can be applied to more complicated working conditions. 3. Ultrasonic accuracy is not as good as radar. 4. The relative price of radar is high. 5. When using radar, consider the dielectric constant of the medium. 6. Ultrasound cannot be used in vacuum, high steam content or foam on the liquid surface.


通过发射高能超声波,使其从被测物体表面反射回来。 The ultrasonic level gauge emits high-energy ultrasonic waves to make it reflect back from the surface of the measured object. The reflected signal is processed by an improved algorithm, which enhances the effective signal and better discards the invalid interference signal. These high-energy sound waves ensure minimal signal loss after passing through the medium. Thanks to these high-energy sound waves, the sound wave loss is less than that of traditional ultrasonic instruments. The stronger the transmitted signal, the stronger the reflected signal. The receiver circuit can identify and monitor low-level signals even in a noisy environment.
1. Liquid-medicated measurement method. The probe fixing device is at the lowest level in the liquid. The ultrasonic pulses announced by the probe are transmitted from the probe to the liquid surface in the liquid, and then reflected back from the liquid surface to the unified probe and accepted. With this measurement method, the temperature of the liquid is not required to change much, and there are fewer elements in the medium that affect the transmission of ultrasonic waves. Otherwise, the change in the transmission speed of ultrasonic waves in the liquid will have a greater impact on the measurement results. 2. It is a gas-based measurement method. The probe device is in the gas above the highest liquid level. Under this method, the accuracy of the liquid level measurement is affected The transmission speed of ultrasonic waves in a gas has a great influence. Under ordinary circumstances, you only need to agree with the premise of the device. This method is better, which is helpful for debugging and maintenance. 3, solid-media measurement method, a sound transmission solid rod or tube is pierced into the liquid, the upper end should exceed the highest liquid level, the probe device is on the upper end of the sound transmission solid, the formula H = vt / 2 is still used, but v Represents the speed of sound in a solid. The measurement methods described in 1, 2, and 3 belong to the single probe method and method, that is, the probe emits pulse sound waves, and then accepts them after transmitting and reflecting. Because the pulse needs to continue for a period of time during transmission, the echo and transmission wave placed in this period are not easy to distinguish. The interval corresponding to this period of time is called the measurement blind zone. When the head is installed, the interval above the highest liquid level should be greater than the interval between the blind spots. There is still a double-probe method for receiving and transmitting sound. The receiving and transmitting of sound waves are performed by the two probes on their own, which can greatly reduce the blind zone, which is very convenient in some extraordinary places where the device is relatively small.