The Better Bell Siphon

I’ve been using bell siphons for a few years in various grow beds. When I went to larger
beds, I ran into problems getting the siphon to stop properly. Here is an overview of how
I’m setting up the siphons in the new greenhouse, where the problems were, and how I resolved
it.
This design uses a trap to help start the siphon. When the water level rises, the bell
area becomes sealed by blocking the breather tube. As the water level continues to rises,
the trap blocks the air from escaping and the pressure inside the bell is slightly greater
than the atmosphere. The pressure keeps pushing against the water in the trap until it pushes
enough water out which “burbs” a few bubbles out. This reduces the internal pressure and
the water level inside the bell will quickly rise a bit.
This process continues until the water level reaches the standpipe height. The next “burp”
will send a rush of water down the standpipe and the siphon action will start. The air
inside the bell is evacuated and the siphon will also draw water through the breather
tube.
The water level will then start to go down until it gets to the opening of the breather
tube. When this happens a large amount of air enters the bell and stops the siphon.
The problem with large grow beds is that the water level goes down very slowly, so when
the air enters the breather tube, it doesn’t suck quite enough air and the water in the
siphon slows down and makes it get stuck in limbo and it will continue to trickle water
out at the same rate that the bed is filling.
To solve this problem, I simply placed a little cup under the breather tube. As the water
level rises the cup gets filled with water and sinks which seals the bell. The bell siphon
will operate normally and start up when the grow bed is full. As it drains, the water
level will get to the height of the cup.
At this point, the breather tube still has water being drawn through it and starts to
suck the water out of the cup. This makes the cup buoyant, forcing it to rise. Once
all the water is removed from the cup, the breather tube sucks only air, which does a
clean break of the siphon. Since the cub is buoyant, it isn’t able to flood with water
until the water level in the bed starts to rise again.
This is a close-up of the cup floating until the water level forces it to sink.
This shows the water level in the breather tube and displays how the pressure inside
the bell keeps the water level lower than the outside water. With this setup, the top
of the standpipe is about 2” lower than the external maximum water level. Typically
the entire bell can be underwater before it starts to siphon.
Here is the water level getting low enough so the water can no longer fill the cup. The
breather tube sucks the water out making it float, then it is forced to suck only air,
breaking the siphon.
Thanks for watching. Please leave a comment below if you if you tried the floating cup
solution or if you found this video to be helpful. I’ve added some raw video at the
end that shows all the parts I used for my bell siphon assembly.