A gasoline pump has two parts: the mechanical unit which actually handles the pumping and a computer unit which controls the pump. The computer in most modern gas pumps is connected to a computer inside the gas station, allowing attendants to turn the pump on and off and process gasoline sales. It also monitors the amount of gas pumps. As the pump moves, it pumps gas at a constant rate. By watching how fast the pump moves, the computer can gauge how much gas has been pumped.
The Pump
Gas stations use two types of pumps: suction and submersible. Suction pumps are located above a tube that dips into the gas reservoir. They remove air, creating a vacuum inside the tube. Because surrounding air is still pushing down on the surface of the gasoline, the low pressure in the tube causes the gas to flow upwards and through the nozzle. Submersible pumps are actually lowered into the reservoir. A sealed motor in the submersible pump turns a turbine or screw which physically pushes the gas through the tube and out into the nozzle.
Stopping the Pump
Spilling gasoline is not only wasteful, it is also potentially dangerous. For this reason, gasoline pumps have a device built into them which automatically detects when the tank is filled up and stops the pump. Inside the nozzle is a small tube connected to a pump inside the gas dispenser, which constantly sucks air in through it. When the gas level in the car's fuel tank is below the level of the tube, the air flows easily. When the gasoline reaches the level of the tube, however, the air pressure inside the tube drops, since there is no longer air available for the tube to suck in. The pump detects this drop in pressure and shuts off the nozzle.
