How do gas pumps know how to turn themselves off? You stick the gas nozzle in your vehicle's gas tank opening, squeeze the handle on the gas pump, walk away and the thing just knows when to stop. How does it know when your gas tank is full?

When operating properly, gas pump nozzles automatically stop the flow of gasoline so that your gas tank doesn't overflow spilling gasoline down the side of your vehicle and all over the parking lot creating a mess and costing you a ton of money. But how in the world does the nozzle know when to shut off?

Well, it's not magic. It's because of something called the venturi pipe. Usually found inside the gas pump nozzle or alongside it.

There are huge underground gasoline storage tanks underground at gas stations where you fill up. These tanks store thousands of gallons of gasoline. In order to get the gasoline out of these tanks and to your car, it takes a pump. Kinda like sucking your favorite soda through a straw. A pump is used to suck the gasoline out of those underground tanks. The thing that does the sucking, is the pump you drive up to. That's why it's called a pump.

So the pump draws the gasoline out of the tanks and up to the nozzle that you now have stuck in your car's gas tank. You've set the mechanism and now the gasoline is flowing into your cars tank. Meanwhile, you're in the store buying a snack trusting full well that the nozzle will cut off when your gas tank reaches full. So how does the pump know when to turn itself off?

This video will explain how gas pumps know when to turn off but you may find it complicated to understand.

So here's the simpler explanation. When the venturi pipe gets submerged in gasoline, remember it's several inches down into your vehicle's gas tank opening, the liquid chokes off the pressure. When there is no pressure to hold the nozzle handle in the open position, it releases. You hear that loud click and the gas stops pumping.

This is the venturi pipe. Look for it next time you fill up.

Venturi Pipe
Screenshot, Steve Mould

LOOK: See how much gasoline cost the year you started driving

To find out more about how has the price of gas changed throughout the years, Stacker ran the numbers on the cost of a gallon of gasoline for each of the last 84 years. Using data from the Bureau of Labor Statistics (released in April 2020), we analyzed the average price for a gallon of unleaded regular gasoline from 1976 to 2020 along with the Consumer Price Index (CPI) for unleaded regular gasoline from 1937 to 1976, including the absolute and inflation-adjusted prices for each year.

Read on to explore the cost of gas over time and rediscover just how much a gallon was when you first started driving.

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