With the rapid improvement of industrial automation, liquid level measurement, as an important measure and control means of liquid level measurement in industrial production, has been widely used in various containers, pipelines and water level measurement in reservoirs, rivers and canals. No matter what kind of place it is used in, higher and higher requirements are put forward for the measuring accuracy of the level meter. Ultrasonic level gauge is widely used in liquid level measurement. However, because the measuring accuracy of ultrasonic level meter is easily affected by temperature, humidity, dust, chemical composition of the measured liquid and other factors, the measuring accuracy is not high. In this paper, some errors which may appear in the measurement of ultrasonic level meter are analyzed and the corresponding compensation measures are proposed.

First, the working principle of ultrasonic liquid level meter

Ultrasonic level meter generally uses a transceiver integrated ceramic ultrasonic transducer, acoustic wave transmission and reception are completed by the same probe. The probe transmits ultrasonic signals to the measured liquid level, and the ultrasonic waves are transmitted from the probe to the measured liquid level through the propagation medium, forming reflections on the liquid surface. The reflected waves propagate to the probe along the original path and are absorbed by the probe. The timing unit measures the time it takes for the ultrasonic wave to be transmitted until the echo is received. The distance from the probe to the liquid surface can be calculated according to the propagation speed of the sound wave in the air, and thus the height of the liquid surface can be obtained.

Ii. Common errors and calibration methods of ultrasonic level gauge

1) Precision error of reference sound speed

From distance S and the relationship between the sound velocity C and transmission time T formula S = C * T / 2, the ultrasonic transmission time is the intermediate result of liquid level measurement, using the ultrasonic level meter measuring liquid level, also need to know the ultrasonic transmission speed in the air, so precision of ultrasonic velocity values will greatly influence the measuring accuracy of ultrasonic level meter.

• Temperature compensation

In general, temperature is the main factor affecting the speed of sound. A temperature sensor can be installed on the ultrasonic level meter to measure the temperature in real time, and the value of sound speed can be converted by using the relationship between temperature and sound speed. However, in fact, the speed of sound is not only affected by temperature, but also related to gas density, pressure, humidity, suspended solids in the air and many other factors. Therefore, in the practical application, there are still many deficiencies in the calibration of sound velocity only by using the method of temperature measurement, and there will be some errors in the process of temperature measurement. Therefore, the temperature compensation method is only suitable for general application, but cannot meet the high precision measurement.

• Real-time sound speed compensation

Practice has proved that due to the complexity of measurement environment and measurement methods and other factors, no matter what kind of empirical formula and data is used to compensate sound velocity, it needs to introduce a new one