The Ultrasonic Level Measurement is mounted on top of the vessel. Under the control of the electronic unit, the probe emits ultrasonic wave pulse to measured medium. The sound wave is reflected by the surface of medium, a part of reflection echo is received by the probe and converted into electrical signal. From the emission of the ultrasonic wave to the new reception, the time is direct proportion to the distance from the probe to the measured medium. The electronic unit detects the time and calculates out the measured distance according to the known sound velocity. Use subtraction to obtain the value of medium position.