How does the pump at the gas station know when your fuel tank is full?

1. How Does a Fuel Pump Know
When to Stop?
How does that pump at the gas
station know when your gas tank is
full?
source: http://www.buyautoparts.com/blog/how-does-a-gas-pump-know-when-to-
shutoff.html

2. The Automatic Shutoff
• The automatic shutoff can be traced all the way back to
1939, so it is not the result of some modern
nanotechnology. It is a simple system that relies heavily
on vacuum. I am not talking about the vacuum that
comes by when you lift up your legs so it can suck up
the Cheetos lying beside your feet. The vacuum in a
fuel pump nozzle is a Venturi effect. The effect is the
reduction in pressure which results when a gas or fluid
flows through a pipe, or in this case a nozzle. The next
time you are at the pump look at the tip of the nozzle.
You will see a hole on the side somewhere. This is
where the Venturi port is located. It creates the suction
of the air in your gas tank.
source: http://www.buyautoparts.com/blog/how-does-a-gas-pump-know-when-to-
shutoff.html

3. Balancing The System
• When you get to the gas station your tank is
empty so there is air in it. When you fill up your
tank with gas this air must leave the tank. It goes
from your tank and into the gas station system,
keeping everything balanced. Once you put
enough fuel in your car for the fuel level to reach
the Venturi port it can no longer pull in air; it is
blocked by fuel. This then creates a pressure
imbalance. The imbalance flips a switch that
shuts off the pump, and you don’t get gas all over
your Nikes!
source: http://www.buyautoparts.com/blog/how-does-a-gas-pump-know-when-to-
shutoff.html

4. How Does a Fuel Pump Know When
to Stop?
The End
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source: http://www.buyautoparts.com/blog/how-does-a-gas-pump-know-when-to-
shutoff.html