'Nonsense Pressure' puzzler

The puzzler answer about how gas pumps shut off automatically is not correct. The modern pump handles use the Venturi effect to detect a change in air pressure when the gas in the tank has reached the tip of the nozzle. It has nothing to do with the “air pulses” coming out of the tank. Maybe the technology has changed since 1993 when the show was originally aired.


Yes I thought their answer was “BO-GUS” as Tom would say.

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I listened to that puzzler just today in fact, and was wondering. I presume there’s a small opening in the side of the nozzle that goes to a pressure sensor. Rapid flow out the nozzle will produce a large vacuum in that opening, but as soon as the flow slows (due to fuel backing up b/c tank is full or tank isnot venting through the charcoal canister properly), the vacuum in the small aperture lowers and is detected by the pressure sensor, shutting off the pump. Wiki article refers to a Venturi-pump, but I think that’s a misnomer. Seems more like it would be called a venturi pressure sensor. .Small pressure pulses from the pump seem like it could affect how it works I suppose.

The pump is not shut off, only the nozzle. It is a mechanical switching valve that stops the nozzle.

I remember in high school chemistry lab, if we wanted a vacuum to use for some experiment, we simply ran faucet water through a gadget that had a small hole & hose fitting on the side, and that would create quite a substantial vacuum. Wasted some water though. The faster the water flow, the bigger the vacuum. The force of that vacuum must be applied to a spring-loaded switching valve of some kind. As long as the vacuum is strong enough it overcomes the spring & keeps the nozzle open. That’s my story and I’m sticking to it …lol …

How gas pumps know when to turn themselves off - YouTube

The real answer is MAGIC, pure and simple.