How does a gas pump know when the fuel tank is full?

If you've ever put gas in a car, you've probably noticed how the gas pump shuts off when your tank is full. But how does it know to do that? Learn more about the clever mechanism that keeps your gas tank from overflowing in this episode of BrainStuff.

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Episode Transcript

Speaker 1 (00:00):
Welcome to Brainstuff from the house stuff works dot com
where smart happens. Hi Am Marshall Brain with today's question.
How does a gas pump know when the fuel tank
is full so it can cut off automatically. This mechanism

(00:21):
has been around for decades, so it's safe to say
that there's not a miniature camera inside the nozzle looking
down into the tank and interpreting whether the tank is
full or not. This mechanism is purely mechanical and it's ingenious.
Near the tip of the nozzle there's a small hole
and a small pipe leads back from the hole into

(00:43):
the handle. Section is applied to this pipe using a venturi.
When the tank is not full, air is being drawn
through the hole by the vacuum, and the air flows easily.
When gasoline in the tank rises high enough to block
that that hole, a mechanical linkage in the handle senses
the change in section and flips the nozzle off. Here's

(01:06):
a way to think about it. You've got a small
pipe with section being applied at one end and air
flowing through the pipe easily. If you stick the free
end of that pipe into a glass of water, much
more section is needed, so a vacuum develops in the
middle of the pipe. That vacuum can be used to
flip a lever that cuts off the nozzle. The next

(01:29):
time you fill up your tank, look for this hole
in the small pipe, either on the inside or the
outside of the tip of the nozzle assembly. Do you
have any ideas or suggestions for this podcast? If so,
please send me an email at podcast at how stuff
works dot com. For more on this and thousands of
other topics, go to how stuff works dot com and

(01:50):
be sure to check out the brain stuff blog on
the how stuff works dot com home page.