Question: How does a gasoline pump know when to shut off?

Answer:

I’ll explain it by making you wash my car.

If you were washing my car, I’d give you a hose, a bucket, and some soap. You’d start by screwing the hose into my outside spigot. Then, you’d turn on the water, and you get a steady stream coming out of the nozzle. You’d fill the bucket with soap and water, and...

... then you would stick your thumb over part of the water nozzle and spray my car down with a high pressure stream of water.

Then, you’d soap the car down, and...

... then you would stick your thumb over part of the water nozzle and spray my car down with a high pressure stream of water, again.

So, what do washing my car and gasoline pumps have in common? The Venturi principle! Giovanni Battista Venturi was a scientist in the late sixteenth and early seventeenth centuries who was very interested in how water flows through restricted channels (and this was long before garden hoses).

He realized that if a fluid flows through a wide channel (our hose) and it encounters a restricted channel (your thumb over the nozzle), pressure will build up behind the entrance to the restricted channel, and the fluid coming through that channel will move much faster.

Roughly a hundred years later, Clemens Herschel realized that when the fast moving fluid moves back into a larger tube, the pressure drops and a vacuum is created. Realizing this, he created an instrument called a Venturi tube. This tube was really nothing more than a large tube that is constricted at one point to a smaller diameter, and then opened back up shortly thereafter.

This little device is the key to gasoline pumps.

Gasoline vapors can be thought of as a fluid. The pump handle has a small Venturi tube inside it that connects to an open space within the handle. When you fill your tank, the vacuum created by the tube draws in gasoline vapors. So long as only vapors are passing through, everything is fine and good. Once liquid gasoline reaches the tube however, the amount of suction required to draw the gasoline through the tube is much greater than just the fumes. The vacuum increases in the handle, and that is enough to “trip” the handle and stop the flow of gasoline.

So now you know.

Thanks for washing my car.

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