How Does The Gas Pump Know When to Shut Off?

Have you ever wondered how a gas pump knows when your gas tank is full? I know I have.

You're driving along and your gas light comes on. Annoying, I know. You pull into the nearest gas station and pull up to a dispenser. You swipe your credit card, put the nozzle in, and squeeze the handle. You watch the numbers on the read-out climb and climb when suddenly *click* the handle disengages. Anyone who's pumped gas knows this means your tank is full. You return the nozzle to its cradle, hop in the car and go about your way with a full tank of gas.

But lets stop to think about what just happened. HOW did the nozzle know when to shut off? How did it know your tank was full? Its not a mystery or magic...just science.

The concept behind how the pump knows when to shut off is called the Venturi Effect. According to Wikipedia, the Venturi Effect is the reduction in fluid pressure that results when a fluid flows through a constricted section (or choke) of a pipe.

Near the tip of the nozzle is a small tube that leads back to an air diaphragm in the handle of the pump. When you first start pumping gas, the diaphragm is all puffed up and inflated, and air is flowing through the small tube. Once the tip of the nozzle gets submerged in gas (as the tank is filling up), gas starts getting sucked up into that little tube. When the little tube gets full of gas (which, remember, is more dense than air), there is a pressure change in the pipe. Suddenly, in an attempt to even out the pressure, the air from the diaphragm gets sucked down and out of the pipe. Once the diaphragm decreases in size, it triggers the automatic shutoff within the nozzle.

An easy at home way to see the Venturi Effect is with a drinking straw. Think of what happens when you suck on a straw that's not in a liquid. The air flows through just fine - this is what the little tube inside the nozzle looks like when you first start pumping gas. Now cover the end of the straw. The straw starts to collapse as a vacuum is formed - this is what happens when the gas covers and starts getting pulled into the small pipe. Its that vacuum that forms that pulls the air out of the diaphragm in the nozzle and triggers the automatic shutoff.

That's all it is. No diodes, no sensors, no magic wands. Just a little pressure change and a whole lot of science!

The video below is from a great Youtube channel called Brainstuff - HowStuffWorks. Check it out to see the Venturi Effect in action!