There is probably a very obvious answer to this question but I have not been able to find or dream one up.
How does the petrol pump know when my tank is full so that it can stop pumping?
Thanks in advance.
Cecil on the subject:
Mmmmhmmmmmm. Thanks for the link, beatle, but…
Why does the gas pump shut off 4 or 5 times every time I try to fill my tank? And not at the end, either. When I first start filling the nozzle will turn off repeatedly til my tank …well, I like to say ‘til my tank gets used to it’.
A WAG would be that the shape of the tube to your gas tank might have a bend that prevents gasoline from flowing to the tank as quickly as it’s coming out of the pump. That might allow a backup in the tube that gives a false “full” and causes the valve to shut off.
The little device that lets you set the valve open so you don’t have to hold it yourself usually has 3 stops, or settings. You might try using the middle setting instead of the all the way open setting.
They’re not pumps, they’re dispensers.
The pump (a submersible) is in the underground tank.
They’re called ‘pumps’ because in the old days they were. You used a manual pump to fill a glass beaker on top, then drained it into your vehicle tank.
Here, you can have one of your very own;
actual gas pump
Why anyone would even want one is beyond me, but there ya go.
Doing my part.
beatle’s right. Any gas that splashes on that little hole in the end of the nozzle will shut off the pump. I mean dispenser. 
Peace,
mangeorge
Also keep in mind that gas tanks need to have a “dead air” space for temperature-expansion purposes.
IE, a “full” tank isn’t necessarily “full”.
It’s not so much a problem in, say, California, but up here in Alaska, there’s definite safety reasons. For example, “Joe” leaves work after his car has been sitting out in the cold parking lot all day, at temps dropping to zero or below. On the way home, he stops in at the gas station and tops off the tank- and since he hates filling up in the dead of winter, he keeps “topping up” the tank 'til it’s filled the whole filler neck.
Then he drives a mile or two home, parks inside his heated garage and goes to have dinner. Over the course of the night, the gas in the tank warms up and expands. Without the ‘dead air’ space in the tank to take up the expansion, gas is forced out the tanks’ vent and/or filler cap and dribbles on the garage floor. Warm gasoline vapors build up 'til they reach the furnace or water heater’s pilot light.
If Joe is lucky, he’ll wake up on reentry and land in a snowbank.
Ever seen a little sticker or sign on a dispenser that says “Do Not Top Off”? There’s why.