How does a gas pump know when to stop pumping?


​

How does a gas pump know when to stop pumping?

This question came from a reader submission (thanks Derrick!). Has a curious question stumped you lately? Feel free to submit your own question here:
​

πŸ““ The short answer

A gas pump nozzle has a little hole leading to a pipe with a balloon (or similar device) at the end. When the tank has filled up, gasoline reaches this hole and causes a change in pressure that causes the balloon to slightly deflate. The balloon deflating triggers an automatic-shutoff of the flow of gas.

πŸ“š The long answer

It's not an electronic sensor. It's not a laser. A gas pump nozzle can sense when a tank is full and shut off because of physics and a clever mechanical device.

Let's venture into the physics first with ... the Venturi effect.

The Physics

The Venturi effect is most simply explained with a garden hose. If part of a garden hose is compressed, there is change in both the pressure and speed of the water. As the water passes through a choke point, the pressure decreases and the speed increases. You'll be familiar with this concept if you've ever put your thumb on the end on the garden hose in order to have the water flow out faster.

Pay close attention to the pressure effect here. When there is a choke point, there is a reduction in pressure on one end. Keep this in mind as we move on to the mechanical device.

The Mechanics

If you look up the nozzle of a gas pump, you'll notice that there is a tiny hole:

This hole is leading up a separate tube into a Venturi device in the gas pump nozzle. At the other end of the tube is a balloon (or another device but a balloon is the simplest way to explain this). Here's an overly simplistic diagram to explain how the device works:

When the tank is not full, gasoline flows into the tank and air begins to exit it. The air flows into the hole leading to the balloon. Because the entire pipe and balloon is filled with air, the pressure is even and the balloon remains blown up.

But once the tank is full, gasoline closes off the hole that was previously just taking in air. Since the density of gasoline is greater than air, the tube experiences a change in pressure. As a result, the balloon deflates. This is similar to how if you're sucking air from a straw and then put a finger on the end of it, your cheeks suck inwards.

The balloon is connected to an automatic-shutoff device. So when it deflates, it sets off a trigger to shut off the flow of gasoline.

​Sources​


πŸ“– Book of the week

​The Demon-Haunted World: Science as a Candle in the Dark by Carl Sagan and Ann Druyan​

Alien abductions, witchcraft, mass hallucinations. All of this is covered (and de-bunked) in this book. But this book is more of a manifesto for scientific thinking. This book was originally published in 1995, but its message still rings true today: Despite living in an information-rich era, there is a rising tide of pseudoscience that is not only leading us astray as a society but also threatening our democratic values. For a preview, check out this excerpt which often makes the rounds on social media because of its uncanny prophesying of the dilemmas we face today.


πŸ‘€ Catch up on other curious questions

P.S. πŸ€” Do you have a question for the newsletter? Click here to submit your question. (or reply to this email!)


Refer friends, earn rewards!

Have a curious friend who'd love to learn more about the world? Share the link below and you'll receive rewards for referring people to Today You Should Know.

πŸ“§ (1) Refer just ONE person and receive a special "Five Fast Facts" edition of the newsletter.

πŸ“£ (5) Refer five people and I will shout you out in the newsletter.

πŸ“š (10) Refer 10 people and I will ship you a free, surprise book on an interesting topic.

Thanks for spreading the word!

[RH_REFLINK GOES HERE]

facebook twitter linkedin email

P.S. You have referred [RH_TOTREF GOES HERE] people so far.

πŸ’‘ Today You Should Know

Learn something new every Friday. Join 2,000+ nerds and satisfy your curiosity one question at a time with topics including science, technology, history, and more.

Read more from πŸ’‘ Today You Should Know
a bird on a tree

Why don’t woodpeckers get brain damage? πŸ““ The short answer Woodpeckers avoid brain damage thanks to special adaptations in their anatomy. Their skull, jaws, eyes, and tongue are all uniquely designed to help absorb the impact of the constant pecking and prevent concussions and retinal damage. πŸ“š The long answer To us humans, banging your head against a wall sounds like a regrettable moment of frustration. To woodpeckers, it sounds like a Tuesday. Source: GIPHY Woodpeckers rapidly peck trees so...

A wooden spot over a boiling pot

Why does a wooden spoon stop water from boiling over in a pot? This question came from a reader submission (thanks Thomas!). Has a curious question stumped you lately? Feel free to submit your own question here: Submit your question πŸ““ The short answer Placing a wooden spoon across the top of a pot of boiling starchy foods, like pasta or potatoes, prevents spills by disrupting the bubbles as they rise. The wood's lower temperature and rough surface bursts the bubbles, keeping the foam from...

A bee is sitting on a yellow flower

Why do we say "make a beeline"? This question came from a reader submission (thanks Georgia!). Has a curious question stumped you lately? Feel free to submit your own question here: Submit your question πŸ““ The short answer The phrase "make a beeline" comes from how honeybees fly directly back to their hive after gathering nectar or return straight to a bountiful food source. πŸ“š The long answer In the Merriam-Webster Dictionary, you'll find this entry for the word "beeline":...