Dear Tom and Ray:

How do automatic gas-pump nozzles know when the car's gas tank is almost full, and therefore when to shut off? This question has bothered me since I was 16 and gas was two bits a gallon -- neither of which is true anymore. -- Ross

RAY: Great question, Ross. The nozzle uses a simple mechanism that's been around for decades.

TOM: Basically, there's a little hole near the end of the nozzle. You can look for it the next time you fill up. And attached to that hole is a tube that's connected to the handle.

RAY: When gasoline is flowing freely (i.e., when the tank is not full), the moving liquid creates a vacuum as it pours into the tank, and air gets sucked freely through that tube. But as the tank gets full, the vacuum is reduced.

TOM: And there's a mechanical, vacuum-activated switch in the handle that -- get this -- senses when the vacuum reaches a critical low point and then switches off the gas flow.

RAY: This system is far superior to the previous method used to determine when the tank was full. My brother remembers using that system.

TOM: Yeah, when you felt the gasoline trickle down your pant leg into your shoes, you knew it was just past time to stop squeezing the handle.