A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

How does a gas pump know to shut itself off?

May 29, 1981

Dear Cecil:

Cecil, old buddy, even though I am receiving a doctorate this spring, the old adage that the more you learn the less you know still holds true. So tell me this: how does a gas station pump know when to turn off before spilling gallons of gas onto the pavement?

Cecil replies:

Pal, you're going to need a doctorate to understand the following, so cleanse your mind of distracting thoughts. In a gas pump handle you have two valves: the main valve, which is actuated by the oversize trigger you squeeze to make the gas flow, and the check valve, which lets gas flow out but won't let anything back in again, thus reducing fire hazard. In the seat of the check valve you have a little hole. To the backside of this hole is connected a Y-shaped tube. One branch of this tube runs down the nozzle and exits at the tip while the other runs back to a diaphragm connected to a release mechanism on the main valve. When you squeeze the gas pump trigger, gas running past the hole in the check valve sucks air out of the Y-shaped tube. (This is because of the Bernoulli principle: a moving stream of fluid tends to pull things in from the sides. Take my word for it.) As long the end of the Y-shaped tube exiting at the spout is unobstructed, air is simply pulled into the tube and nothing much else happens. However, as soon as the gas in your car's fill-up pipe gets high enough to cover the end of the tube, a partial vacuum is created therein, which yanks on the diaphragm, releases the main valve, and shuts off the gas. If the gas happens to be especially foamy one day, it may actuate the release mechanism prematurely, with the result that you end up with less than a full tank of gas. Simple, huh? Sure, just like nuclear fission. Stick with English lit.

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