It's the pump fill nozzle, look closely, there is a second tube inside the fill nozzle, that tube senses the back pressure caused by the vapours rushing out of the tank, pull the nozzle back a little, don't shove it all the way in, and don't squeeze fully on the fill nozzle, when it finally does shut off, you will be full, don't squeeze any more in!
see link!
http://auto.howstuffworks.com/fuel-efficiency/fuel-consumption/gas-pump-full-tank.htm
Near the tip of the nozzle is a small hole, and a small pipe leads back from the hole into the handle. Suction is applied to this pipe using a venturi. When the tank is not full, air is being drawn through the hole by the vacuum, and the air flows easily. When gasoline in the tank rises high enough to block the hole, a mechanical linkage in the handle senses the change in suction and flips the nozzle off.
*Here's a way to think about it -- you've got a small pipe with suction being applied at one end and air flowing through the pipe easily. If you stick the free end of the pipe in a glass of water, much more suction is needed, so a vacuum develops in the middle of the pipe. That vacuum can be used to flip a lever that cuts off the nozzle.
The next time you fill up your tank, look for this hole either on the inside or the outside of the tip.
Or this link!
http://www.wonderquest.com/sleeping-birds-gas-nozzles-cold-hospitals.htm
A vacuum develops in the pump nozzle when the tank is full and that vacuum tells the pump to shut off.
Gasoline nozzle [Grenville Sutcliffe, Husky Corporation]
Next time you fill your tank, notice the nozzle. The nozzle "necks down" a short way past the handle and forms a narrow spout. Turn the nozzle over. There’s a small hole at the nozzle tip. Look closely. See how the "sensing" hole connects to a small vacuum tube that leads back up the nozzle. The tube connects the sensing hole at the nozzle tip with a diaphragm near the shut-off valve.
What you can’t see is a clever device (called a venturi) located also near the shut-off valve just before it necks down to spout, says Grenville Sutcliffe of Husky Corporation. It’s a ring with a channel through the ring. Gasoline must stream through the ring’s channel to reach the spout. The venturi ring also has four small conduits at right angles to the gasoline flow. These passages communicate flow pressure to the vacuum tube and a vacuum chamber above the diaphragm. The bottom of the diaphragm connects to the atmosphere.
Cut away diagram of a gasoline nozzle [Grenville Sutcliffe, Husky Corporation]
When you squeeze the handle trigger, gasoline pours through the hose into the narrow venturi’s channel, and picks up speed like a river running through a narrow gorge. As the gasoline stream moves faster through the venturi ring, its pressure drops and creates a vacuum. Like dominos: the pressure drops in the ring’s conduits, then in the vacuum tube, and, finally, at the tiny nozzle hole at the tip. Higher-pressure air outside the sensing port rushes in the small pipe and balances the pressure inside with out. The diaphragm stays in a neutral position. Gasoline continues to stream into the tank.
Eventually, the tank’s full. The gasoline in the fill pipe rises and covers the nozzle-sensing hole. Air can no longer rush into the nozzle hole to bleed off the venturi vacuum. The vacuum builds up in the vacuum chamber above the diaphragm. The higher atmospheric pressure on the underside of the diaphragm pushes the diaphragm up from the neutral position. This flips a switch in the automatic shut off. "Thunk!" The pump cuts off.
. Sucks cuz I end up having to check my gauge and have to be super carefull not to over fill.
