Ultrasonic sensors measure the height of the fuel in the tank by sending out ultrasonic signals, which are measured on the other side of the tank via another sensor. On the other hand, when capacitance sensors with driver fatigue monitor are used, the fuel passes through special vents when it consumed, causing the capacitance to change in the sensors, enabling the level of the fuel in the tank to be determined. This information is then passed on to the pilots via the onboard computing system.

http://www.fleet-sensor.com/dr....iver-fatigue-monitor