How petrol pumps know when to turn themselves off

Summary by Krater

Steve Mould cuts a petrol pump nozzle in half and builds physical fluid models to explain how the automatic shut-off mechanism uses the Venturi effect and pressure drops to detect when a fuel tank is full.

From the video

Answers: How does a petrol pump nozzle know when to turn off automatically?

• petrol pump nozzle mechanism
• Venturi effect
• fluid dynamics in nozzles
• automatic fuel shut-off physics
• Bernoulli's principle
• pressure drop in constrictions

What it concludes

• The Venturi effect creates a pressure drop in the constriction of the nozzle tube, which draws air in through a small hole.
• When petrol reaches the end of the nozzle, it blocks the hole and liquid is sucked in instead of air.
• The heavier liquid reduces suction efficiency, creating a stronger negative pressure in the upper chamber that pulls up a membrane and releases ball bearings.
• The release of the ball bearings allows the valve mechanism to close and shut off the fuel flow automatically.