I have developed for my old car fuel level sensor with ultrasonic.
Airoplanes used the same method.
The Transmitter and Receiver are the same piezo.
The piezo is placed on the bottom of the tank.
The piezo emittes a short wave and the wave is reflected from the surface of the liguid.
So you can stop the time....
Sensor ?!? from STETTNER WWW.stco-stettner.com