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This question already has an answer here:

Question has been bothering me a while.

When putting fuel at any gas station, when the tank is almost full, the pump just stops and makes a 'THUNK' noise.

I was wondering... how the hell does the pump knows the tank is almost full? There doesnt seem to be a probe that when the fuel touches it it stops the pump, and you can always put a little bit more and be ok...

So how does the pump know how to stop itself?

marked as duplicate by Rory Alsop, kyle_engineer, Bob Cross Dec 19 '17 at 1:48

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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Simple explanation:

Gas pump handles use air pressure to monitor when to shut off. There is a small tube in the nozzle that when foam or fuel blocks it causes the pressure to change and trips the shutoff.

Detailed explanation (with video):

How does a gas pump know when my tank is full?

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