Working principle and error calibration of ultrasonic level gauge (upper type)

In liquid level measurement, the use of ultrasonic level gauges is very common. The influence of various factors such as temperature, humidity, dust, and chemical composition of the liquid to be measured can easily lead to low measurement accuracy. Some errors that may occur in the measurement of ultrasonic level gauges are analyzed below, and corresponding compensation measures are proposed. 1. Working principle of ultrasonic level gauge Ultrasonic level gauge generally uses a ceramic ultrasonic transducer that integrates transceivers, and the transmission and reception of sound waves are completed by the same probe. The probe transmits an ultrasonic signal to the liquid surface to be measured, and the ultrasonic wave propagates from the probe to the liquid surface to be measured through the propagation medium, forms a reflection on the liquid surface, and the reflected wave propagates to the probe along the original path and is absorbed by the probe. The timing unit measures the time it takes for the ultrasonic wave to be received from the time it is transmitted to the echo. According to the propagation speed of the sound wave in the air, the distance from the probe to the liquid surface can be calculated, thereby obtaining the height of the liquid surface. 2. Common errors and calibration methods of ultrasonic level gauges 1) Reference sound speed accuracy error From the relationship between the distance value S, the sound speed C and the transmission time T, the formula S=C×T/2 shows that the propagation time of the ultrasonic wave is the level gauge. For the intermediate results of the measurement, the ultrasonic level gauge is used to measure the liquid level, and the propagation speed of the ultrasonic wave in the air also needs to be known. Therefore, the accuracy of the ultrasonic propagation speed value will greatly affect the measurement accuracy of the ultrasonic level gauge. •Temperature compensation In general, temperature is the main factor affecting the speed of sound. The temperature can be measured in real time by installing a temperature sensor on the ultrasonic level gauge, and the speed of sound value can be converted by using the relationship between the temperature and the speed of sound. However, in fact, the speed of sound is not only affected by temperature, but also related to many factors such as gas density, air pressure, humidity, and suspended matter in the air. Therefore, in practical applications, there are still many deficiencies in calibrating the speed of sound by using only the method of measuring temperature, and there will also be certain errors in the process of temperature measurement. Therefore, the temperature compensation method is only suitable for general applications, and cannot meet the requirements of high precision. Measurement. •Real-time sound velocity compensation Practice has proved that due to the influence of factors such as the complexity of the measurement environment and measurement methods, no matter what empirical formula and empirical data are used to compensate the sound velocity, new errors need to be introduced. Therefore, it is considered to be the most reliable compensation method to use the method of measured sound speed for sound speed compensation. As shown in the figure, a baffle is installed at the front end of the transmitting probe, and the baffle and the probe form a sound path interval with a fixed distance. This structure is called a sound path frame. When the probe emits sound waves, the baffle reflects a portion of the sound waves back to the probe. After the probe receives the reflected wave, it calculates the time from transmission to reception, and calculates the speed of sound. Use the measured sound speed method for compensation. Because the compensated sound speed is very similar to the environment where the measured sound wave propagation path is located, the environmental influences are basically the same, and the sound speed is usually relatively close, so this method is currently the most accurate sound speed correction method. . However, in the use of this method, the sound path frame should be made of materials with a low temperature expansion coefficient to avoid thermal expansion and contraction of the sound path frame due to changes in ambient temperature, which will change the sound path distance and affect the measured sound velocity accuracy. 2) Transit time error The acoustic wave is an elastic mechanical wave of longitudinal vibration, which propagates by the molecular motion of the propagation medium. Due to the absorption, scattering and diffusion of sound waves of the propagation medium, the sound intensity, sound pressure and sound energy are weakened, and sound wave attenuation occurs. And the measurement of the ultrasonic level gauge needs to form a sound wave reflection on the measured liquid surface, which will also cause the attenuation of the sound wave. The sound wave attenuates according to the exponential law of the propagation distance. When the liquid level is different, the transmission distance of the sound wave is also different, and the amplitude of the received wave will also be quite different. The system starts timing when the probe emits ultrasonic waves, and stops timing when the amplitude of the received signal exceeds the set threshold. When the liquid level changes, the amplitude of the received signal also changes. When the liquid level is relatively low, the amplitude of the received signal is relatively small, and the threshold may need to be reached at the fourth peak; when the liquid level is relatively high, the amplitude of the received signal is relatively large, and may reach the threshold at the third or even earlier. threshold. In this way, the time to stop timing is not certain, and this uncertainty will inevitably bring errors to the measurement accuracy of the system. If this error is applied to the oil storage tank above 1000m³, it will produce a very objective absolute error, so it must be eliminated. The simple way to eliminate the transit time error is to increase the time control circuit (TGC), and use the TGC circuit to compensate the attenuation of the sound wave during the propagation process, so that the amplitude of the received wave is basically the same under various liquid level conditions, so as to minimize the noise. Small measurement error. However, this method still has great limitations. This method needs to predict the propagation time of the sound wave at different liquid levels and the attenuation of the sound wave in this distance, and then draw a curve for the corresponding relationship between the two.And design a time gain control circuit that conforms to this curve equation. According to the previous analysis, propagation time and attenuation are two more important factors, which are easily affected by the on-site environment and cannot be well matched with the pre-prepared curve. Moreover, even if the fitted curve is very accurate, it is difficult to design a TGC circuit that exactly matches it.

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