There are lots of information about the ping sensor, level measurement, distance sensor, and specifications are explained inside the Application Note. The Application Note contains explanation about ultrasonic distance, ultrasonic distance sensor, measurements, and ping sensor.
Inside this Application Note the reader can learn things about distance, sensor, humidity, and water level measurement. These are chosen from the Application Note:
Water Level Measurement with the Ping))) Ultrasonic Distance Sensor (#28015) General Description Parallax customers frequently ask about measuring water level with the Ping))) Ultrasonic Distance Sensor. The Ping))) sensor isn’t designed to be water resistant, and the specifications for the transducer don’t provide any details for humidity sensitivity. Therefore, customers attempting to measure water level are doing so at their own risk of damaging their Ping))) hardware. At some point the humidity and moisture will likely damage your Ping))) Ultrasonic Distance Sensor, but depending on your circumstances and need this may or may not be a problem.
Furthermore, the Application Note explains information regarding ultrasonic, transducer, measurement, and water.