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.
I’m Grady and this is Practical Engineering.
On today's episode, we’re taking a look at an automatic siphon.
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 fluid column and its 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 it's
stopped countless people from ingesting a mouthful of gasoline trying to borrow some
from a vehicle or drain the tank on a lawnmower.
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.
Here’s a clip showing the entire process.
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.
“This is a lot more complex than I thought it was.
It is not immediately obvious how it works...”
I’ll link his video below if you want to check it out.
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 perfect 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!
If you’re new the channel and like what I’m doing, don’t forget to subscribe.
Visit my website at practical.engineering to read more about my projects or buy a t-shirt.
If you really like what I’m doing, you can support the channel on Patreon.
I use that funding to improve the quality and quantity of content.
You can also follow the day-to-day happenings in the shop on Instagram and Facebook.
Finally, all my past videos are sorted into playlists based on the different fields of
engineering, so go check them out if you haven’t already.
Thanks again for watching!
And let me know what you think.