A few months back, a couple of engineering professors built a collaborative water Rube Goldberg machine at the American Geophysical Union fall meeting. This water science pop-up was demonstrated on the streets of San Francisco, and I was lucky to be one of the collaborators who was invited to design a piece of the display. It’s a quick project, but it came out working really well so I wanted to share it with you guys.
You’ve probably made a siphon before, and if so, you probably have at least a cursory understanding about how it works. The fundamental principle is based on hydrostatics, or the relationship between the height of a water column and it's pressure. At the highest point in a siphon, the pressure is actually lower than atmospheric pressure, also known as a vacuum. This allows the atmospheric pressure to push water up over the peak of the siphon so gravity can carry it the rest of the way. For most situations, that’s enough theory to describe your typical siphon. It’s a very convenient way to drain a reservoir or transfer a liquid without a pump.
But a siphon still requires some help to get started, also called priming. There’s the classic way of using your mouth to prime a siphon, but that only works for short, small tubes and with liquids that are safe for human consumption, not that that’s stopped countless people from ingesting a mouthful of gasoline trying to borrow some from a vehicle or drain the tank on a lawnmower. If you’re into brewing beer, you’ve probably used a racking cane which sometimes uses a plunger to prime the siphon. For larger applications like draining a pond, you have to get a bit more creative. The typical way to do this is to have a valve at the downstream end and a port at the crest of the siphon. With the valve closed, you can fill up the pipe with water from the port. Close the port and open the valve, and with any luck your siphon will pull out any air bubbles and start draining the pond.
But there are certain cases where it would be nice to be able to create a siphon without any intervention, a self-priming or automatic siphon: the next level of siphonry. And this project is an example of just that. This is a demonstration of a bell siphon which I built out of an acrylic sheet and a piece of clear pipe. A bell siphon has three basic parts: a reservoir, the bell, and a riser. Here’s how it works: as water fills the reservoir, the surface of the water is equally exposed to atmospheric pressure. Outside the bell, it’s exposed to the atmosphere from open top of the reservoir, and inside the bell it’s exposed to atmospheric pressure through the empty riser tube. As the water rises, it eventually forms a seal over the riser tube inside the bell, closing off the bell’s connection to the atmospheric pressure. As the water falls through the riser pipe, it creates a vacuum inside the bell, drawing more water from the reservoir up and out. Eventually the water level in the reservoir drops below the bell, allowing air to enter and breaking the siphon. As the remaining water drains from the riser, the pressure inside the bell returns to atmospheric and the process starts over again.
This is the same mechanism used in the Pythagoras cup, one of the oldest practical joke devices. But there are more pragmatic uses for the bell siphon too. Many septic systems use a similar device to dose effluent into a leach field which is more effective than just allowing it to constantly drain. Bell siphons are also common in hydroponics and aquaponics to create a cycle of wet and dry for plants. Finally, bell siphons are found in public restrooms. YouTuber Big Clive did a tear down of a multi-stage bell siphon used to automatically flush urinals.
A siphon is a classic example of science defying our immediate intuitions. Water flowing uphill with no pump or moving parts. And the bell siphon is a great extension to that with its ability to prime itself completely eliminating the need for any intervention at all. Thanks to Rolf Hut and Pete Marchetto for inviting me to collaborate with them on this project. Thank you for watching, and let me know what you think!