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.
