Gas/diesel is delivered to a station by truck and pumped into an underground tank, from which the station’s pumps access it. A pump has three main components: 1) a pump that actually pumps the gasoline out of the tank, 2) a positive Displacement Meter (PD) that the gasoline flows through and works a bit like a revolving door in that with every revolution of the door a fixed, accurate volume of gasoline goes through it, 3) the counter that takes the information from the PD meter and converts it into gallons and dollars and cents. If you are filling up the tank of your car, another system works that detects when your tank is full by sending small pulses of gasoline through the line. If there is still air in your tank, the pulses are dissipated in the tank. If your tank is full, the pulse is returned through the hose and detected and it stops the pump.