The working principle of ultrasonic level gauge is based on the emission and reception of ultrasonic waves. The speed at which ultrasonic waves propagate in air is constant. Ultrasonic waves are emitted and then reflected back from the liquid or object surface. Timing starts from the emission and ends when the echo is received. The transmission time of ultrasound in the air is determined. Because it's a round trip, the actual distance is half of the distance. The depth of the pool or the height of the tank is fixed. The actual liquid level is the depth of the pool minus the distance measured by the ultrasonic level gauge, which is the height of the liquid or material level. Thus, ultrasonic measurement of liquid level or material level has been achieved.