I was pumping gas in sub-zero temperatures at about six o’clock this morning, the wind whistling with spurts of snow on my face. 

As I waited for the slow-motion liquid gas to shoot into my husband’s small tank that has to be filled about every four days now while we share a vehicle, I was wondering how the gas pump knows that my tank is full.

I have always assumed that the gas probably hits the end of the pump and there is some sort of sensor that shoots the information to the machine, shutting off the disbursement of the gas.

I wasn’t far off on my guess but it’s not exactly correct. 

So what I found out is that it’s a concept called the “Venturi Effect” which is a reduction in fluid pressure that results when fluid goes through a constricted section of a pipe.

According to Optima Environmental Services, 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. 

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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.

Science. 

Ain’t it great?

Maybe someday the state and federal governments will give science a whirl sometime as they BS their way through the pandemic.