When filling a can from a petrol station hand pump in preparation for the summer grass-mowing routine, I have noticed that no matter how hard I try, it is impossible to overfill the can.
Most petrol pumps have a small pipe inside the nozzle.
This pipe is connected to a vacuum at the other end of the hose and thus constantly sucks air from the end of the nozzle. When the tank is full, some petrol will be sucked by the vacuum pipe and create a sudden decrease in pressure, which triggers a valve and stops the pump. This creates a Venturi effect, a reduction in pressure, which draws air in through the sensing tube next to the mouth of the nozzle and over the top of the diaphragm while fuel flows. This lifts the diaphragm, which releases a spring-loaded poppet and pulls the valve closed, automatically shutting off the fuel flow. The mechanism also closes the valve if the ball of the safety cutout rolls back to obstruct the flow of air in the sensing tube.