Q: How does the gas station pump know to shut off when your tank is full?— Steve, Sturbridge, Massachusetts

A: A vacuum develops in the pump nozzle when the tank is full and that vacuum tells the pump to shut off.

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.

Gasoline nozzle [Grenville Sutcliffe, Husky Corporation

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.

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.

By the way, when you next time jam open the nozzle with the hold-open clip — think of Grenville Sutcliffe’s mother. Hazel Sutcliffe invented that nice convenience that enables you to stroll around while the tank’s filling.