Answer Geek: How Gas Pumps Sense Full Tank

-- Q U E S T I O N: When I’m filling up my car at the gas station, how does the pump sense that the tank is full and then automatically shut itself off? — Kate S.

A N S W E R: After spending the last few weeks tackling such weighty topics as the human genome project, space travel, and, of course, why eating crunchy food appears to make your monitor flicker, this question comes as something of a relief.

Back in the old days, explaining how mechanical gadgets worked was your average Answer Geek’s stock in trade — it was all hydraulics or capillary action, how one kind of mechanical energy was converted to another or the way the gears meshed. Now, of course, it’s a whole different story. These days it’s all about microprocessors and scanner and barcodes, the storage and transmission of 1s and 0s or the flow of ionized particles.

Don’t get me wrong, the technology is fantastic, but when a group of Answer Geeks gets together for a beer after a long day of answering questions, you’ll often hear us lamenting the old days when we had the satisfaction that came with feeling like we had rolled up our sleeves and gotten our hands dirty.

An Old Technology

The automatic shutoff on a gas pump is definitely not one of the miracles of modern technology. But it is a very cool little mechanical device. When gasoline flows through the nozzle, it passes through something called a venturi, which is a tube with a narrow throat that changes the speed and pressure of the liquid passing through, creating a vacuum. The nozzle also contains a very small tube that begins just above the open end of the spout.

The next time you fill up your car, take a look at the nozzle and you’ll see a small hole. That tube runs back up into the fuel pump handle and as gas flows through the nozzle, the vacuum pressure created by the venturi causes air to be sucked up through the tube. As long as the tank is not completely full, air flows unimpeded up into the handle, and nothing much happens except that fuel continues to pour into your car.

As your gas tank fills, the fuel level rises until it covers the hole at the end of the nozzle. Suddenly, the flow of air stops and the vacuum pressure begins to build — think of what happens when you suck on a straw without putting it in water; if you place your finger over the open end, the flow of air stops and the pressure inside decreases until the straw collapses. In the nozzle handle, the vacuum pressure builds until it forces a small diaphragm inside the handle to move. That movement triggers a lever that pops the handle trigger, shutting off the flow of gasoline.

Pretty clever, eh? And it’s all done without transistors or sensors. No light-emitting diodes. No charge-coupled devices. Just levers and vacuum pressure. And a venturi. I tell you, my fellow Geeks are going to by psyched when they hear that I got to mention a venturi in this week’s column.

Todd Campbell is a writer and Internet consultant living in Seattle. The Answer Geek appears weekly, usually on Thursdays.