How Does A Gas Nozzle Know When To Shut Off?

How Does A Gas Nozzle Know When To Shut Off?

Hey, there.
I’m Josh Clark.
This is Brain Stuff.
Have you ever been
pumping gas in your car,
and all of a sudden, pssh, it
just shuts off in your hand.
Maybe it takes a finger off.
That kind of thing.
How does the gas nozzle
know when to shut off?
Well, I’m here to explain.
It’s actually
surprisingly complicated.
You ever heard of
the Venturi effect?
If you take a tube and you pitch
it in the middle, then fluid–
and remember, fluid can be gas.
It can be liquid.
It doesn’t just have to
be water or something.
After the pinch, after it
goes through the pinch,
it becomes compressed.
And as its velocity
speeds up, the pressure
on the other side of the
tube actually decreases.
Keep that in mind.
That’s the Venturi Effect.
If you look at a
gas nozzle, you’ll
see a little hole
toward the end.
And that hole actually
leads to a tube.
And that tube runs all
the way up the length
of the gas nozzle.
And it usually tees off.
On one side, you’ve
got a Venturi tube.
On the other side,
you’ve got a tube
leading to a diaphragm of air.
Now that diaphragm of air is
all blown up nice and big,
while there’s air running
through the nozzle sensor.
So while you’re pumping
gas, and gas is in the tank,
and you’re still
filling it up, air
is moving through this little
sensor pipe quite nicely.
The pressure on one end
of the Venturi tube,
which is on this
side, is nice and low.
And on the other
side, the diaphragm
is nice and puffed up.
Air pressure, no problem.
But as the gas starts to
rise, maybe it starts to foam,
or it starts to just
simply fill up the tank.
That sensor, that pipe,
starts to suck up gas.
And when the gas is sucked
up, the density of gas
is a lot heavier than
the density of just air.
The Venturi tube all of a
sudden changes in pressure,
which creates suction on
the other end of this pipe,
which sucks the diaphragm,
which collapses a little bit,
triggering an
automatic shut off.
Seriously, that’s what’s
going on in your gas nozzle
when you’re pumping
gas and it fills up.
That’s how it knows
your tank is full.
So I guess maybe
ruminate on this.
Chew on it.
Think about it for possibly
the rest of your life.
It is really complex,
but pretty cool.
It’s not magic, I guess
is what I’m trying to say.
I’m sure you like this video.
There are tons of other ones.
If you have a question that
you wanted us to answer,
you can leave it in the comments
section below on YouTube.
Like us.
Subscribe to us.
Just be all things Brain Stuff.
It’ll make you smarter.

35 Replies to “How Does A Gas Nozzle Know When To Shut Off?”

  1. This video shows the diaphragm connected to the thick part of the Venturi tube, not the thin part. This would cause gasoline to flow into the diaphragm. To maintain suction on the diaphragm and pull air in through the hole in the spout, the diaphragm needs to be connected to the thin part of the Venturi, as shown in and

  2. I've actually wondered about that for decades. I'm serious, I've read some of the comments. I found it interesting. Thanks

  3. maybe you also know why would a gas pump not shut off while fueling and overfill? I've had that happen after replacing fuel pump on a 2008 hyundai santa fe.

  4. In your fuel pump there's two tubes inside, an air tube that leads to a sack up top and a tube for outgoing fuel. When you pump your tank, air comes in and inflates the sack, fuel goes out. As the fuel level increases and less air coming in, which causes the sack to lose integrity until there is no more air to keep it inflated and it collapses. Once that happens, it triggers a shut off valve and then click, no more fuel.

  5. Ok that's full tank what about you fill tank from empty to half tank struggle gas won't get sucked up into the nozzle

  6. Why does my car a (2006 mustang) have a "hard fill" as described from Ford? It only lets the gas tank to fill up half way or a little more before it clicks off. I know I can fill it a few gallons past when it stops and its really irritating because if you don't know exactly how much is in the car when you start filling it then you don't know how far to push it.

  7. I need help with something idk if you can e-mail me or massage on on here it's a long story so I rather wait for your answer… I did subscribe and like 😉

  8. I don't understand that suction part. If the gas fills up the venturi pipe, why does that create suction so that the air pushes the gas out of the pipe again?

  9. Thx u soo much

    I didnt even know that was happening to me in the gas station

    Thx for your help man

    Btw my lvl of gas dosnt work

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