. How does the automatic cutoff valve on a gasoline pump work? How is it able to shut off the gas before the nozzle has become immersed in the liquid? I don't see how the pump could be so sensitive to back pressure in the gasoline. — NG, Bloomsburg, PA
As you suspect, the pump isn't able to detect the change in gasoline pressure that occurs when the fill level reaches the nozzle. Instead, the nozzle uses several hidden components to shut itself off when the tank is full. There is a small hole near the end of the nozzle that becomes blocked by the liquid gasoline as soon as the fill level reaches that hole. Blocking this hole with gasoline is what shuts off the valve. There is actually a thin tube inside the main gasoline delivery hose that operates this valve system. That tube runs from the hole in the nozzle to a vacuum pump inside the gasoline-pumping unit. While the pump is dispensing gasoline into a partially filled tank, air flows easily into the nozzle's hole and the pressure inside the thin tube remains close to atmospheric pressure. But when the level of gasoline rises high enough, it essentially blocks the hole and the pressure inside the thin tube drops. This pressure drop is what triggers the valve and stops the gasoline flow. Look for the hole near the end of the metal nozzle next time you fill your car with gasoline. In most cases, it's easy to see.
. How do the automatic soda dispensers at fast food joints know when the cup is full? — MB, San Diego, CA
Those dispensers measure the volume of liquid they dispense and shut off when they've delivered enough liquid to fill the cup. They don't monitor where that liquid is going, so if you put the wrong sized cup below them or press the button twice, you're in trouble.