A single click – and the gasoline stops flowing. A well-coordinated system ensures that the petrol pump is switched off when the tank is full, so that no dangerous situations arise when filling your car.
The petrol station’s pump system is based on the so-called venturi effect, which is caused by the pressure drop in a liquid as it flows from a larger pipe to a smaller one.
The pressure drop creates a suction, which the gasoline pump uses as a safety device.
By passing the gasoline through a constriction in the handle, a suction effect is obtained which draws air through a small tube, which opens at the end of the handle which is inserted into the car.
Electronic mechanism would be fatal
Once you start refueling, the air flows freely back through the small tube and allows you to lock the handle so that the gasoline flows by itself.
But when the tank is full, the gasoline stands so high that it blocks the small opening in the gasoline handle and causes the airflow to stop, causing the pump to shut off.
The system was invented in 1939 and is purely mechanical, so it does not contain any electronics that could pose a risk in contact with gasoline vapors.