How does a gas dispenser know when to shut off?

(WHTM) — Pumping gas is an act so routine, drivers can do it on autopilot, zoning out while the gallons add up. But have you ever stopped to wonder just how that pump works? Or why your tank doesn’t overflow while you’re daydreaming?

There are large gas tanks underneath all the service pumps at a gas station. Gas stations have two or three of these large tanks, which can hold several thousand gallons of gasoline.

In order for the gas to come up from these tanks into the actual pump, it has to defy gravity. To do this, according to howstuffworks.com, most gas stations use a pump to get the gas from the storage tanks into the dispenser

How does the dispenser know when to stop pumping gas? It starts when the customer removes the pump handle from the dispenser. This action activates a switch that starts the flow of gas.

As the gas level rises in the car’s gas tank, the distance between the dispenser nozzle and the fuel grows smaller. A small pipe called a venturi runs alongside the gas nozzle. When the end of the venturi pipe becomes submerged with the rising gas, it chokes off the air pressure that holds the nozzle handle open and shuts down the gas flow.

Sometimes, this shutdown can happen before the tank is full as the fast-flowing gas backs up on its way to the tank, causing the handle to spring back before the tank is full. Pausing for a few seconds will allow the gas to continue into the tank and the pump nozzle to start pouring gas again. If you try to keep pumping without pausing, you run the risk of causing the tank to overflow.

Knowing more about how gas pumps work won’t change the price, unfortunately. The national average was $3.56 per gallon Sunday.

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