How does a gas pump detect when to stop flowing fuel into your tank?

Question

A couple weeks ago, I had the gas pump goin' as fast as it could, the handle held all the way up and propped by the little thingy. I looked at the screen when it was around the limit for my tank, but it never stopped pumping. It just kept shootin' gasoline, and overflowed onto my hand and the ground. How is the machine able to detect when to stop pumping, and how might it have malfunctioned in my case? Thanks :)

Answer 1

On the end of the fuel nozzle at the bottom is a little hole.  This is a tube that runs to a lever.  Once gas begins flowing up the tube, pressure in the tube increases and it shuts off the nozzle.  Malfunctions will occur if:

1. The nozzle is upside down.

2. The tube is plugged.

C. The fuel flows past the end of the tube too fast, causing a venturi effect, which lowers the pressure in the tube instead of increasing it.

Answer 2

back pressure

Answer 3

In a gas pump handle you have two valves: the main valve, which is actuated by the oversize trigger you squeeze to make the gas flow, and the check valve, which lets gas flow out but won't let anything back in again, thus reducing fire hazard. In the seat of the check valve you have a little hole.

To the backside of this hole is connected a Y-shaped tube.

One branch of this tube runs down the nozzle and exits at the tip while the other runs back to a diaphragm connected to a release mechanism on the main valve.

When you squeeze the gas pump trigger, gas running past the hole in the check valve sucks air out of the Y-shaped tube.

(This is because of the Bernoulli principle: a moving stream of fluid tends to pull things in from the sides. Take my word for it.)

As long the end of the Y-shaped tube exiting at the spout is unobstructed, air is simply pulled into the tube and nothing much else happens.

However, as soon as the gas in your car's fill-up pipe gets high enough to cover the end of the tube, a partial vacuum is created therein, which yanks on the diaphragm, releases the main valve, and shuts off the gas.

If the gas happens to be especially foamy one day, it may actuate the release mechanism prematurely, with the result that you end up with less than a full tank of gas.