It’s an Arduino controlled “RADAR” like project that uses the familiar Parallax ultrasonic sensor.

A low-cost (~$100), do-it-yourself (DIY) ultrasonic water level sensor/datalogger has been designed, constructed, and tested for use in education, citizen/community science, and research settings. The ...

Proximity sensors that use ultrasonic sound waves calculate distance to an object using the time it takes to receive the echo from an ultrasonic burst.