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Speaker 1: Welcome to Brainstuff from the house stuff works dot com

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Speaker 1: where smart happens. Hi Am Marshall Brain with today's question.

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Speaker 1: How does a gas pump know when the fuel tank

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Speaker 1: is full so it can cut off automatically. This mechanism

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Speaker 1: has been around for decades, so it's safe to say

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Speaker 1: that there's not a miniature camera inside the nozzle looking

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Speaker 1: down into the tank and interpreting whether the tank is

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Speaker 1: full or not. This mechanism is purely mechanical and it's ingenious.

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Speaker 1: Near the tip of the nozzle there's a small hole

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Speaker 1: and a small pipe leads back from the hole into

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Speaker 1: the handle. Section is applied to this pipe using a venturi.

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Speaker 1: When the tank is not full, air is being drawn

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Speaker 1: through the hole by the vacuum, and the air flows easily.

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Speaker 1: When gasoline in the tank rises high enough to block

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Speaker 1: that that hole, a mechanical linkage in the handle senses

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Speaker 1: the change in section and flips the nozzle off. Here's

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Speaker 1: a way to think about it. You've got a small

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Speaker 1: pipe with section being applied at one end and air

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Speaker 1: flowing through the pipe easily. If you stick the free

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Speaker 1: end of that pipe into a glass of water, much

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Speaker 1: more section is needed, so a vacuum develops in the

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Speaker 1: middle of the pipe. That vacuum can be used to

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Speaker 1: flip a lever that cuts off the nozzle. The next

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Speaker 1: time you fill up your tank, look for this hole

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Speaker 1: in the small pipe, either on the inside or the

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Speaker 1: outside of the tip of the nozzle assembly. Do you

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Speaker 1: have any ideas or suggestions for this podcast? If so,

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Speaker 1: please send me an email at podcast at how stuff

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Speaker 1: works dot com. For more on this and thousands of

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Speaker 1: other topics, go to how stuff works dot com and

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Speaker 1: be sure to check out the brain stuff blog on

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Speaker 1: the how stuff works dot com home page.