(a.k.a. ultrasonic transducer, ultrasonic range sensor)
The basics: ultrasonic sensors measure distance using ultrasonic waves. They do this by sending out a high-frequency sound pulse then timing how long it takes for the echo of the sound to reflect back. Ultrasonic sensors are ideal for solid and liquid objects and are common for monitoring liquid in a tank. They are not affected by color or transparency of objects and can be used in dark, dusty or high-moisture environments.