This article gives some help with Bell Siphon Construction and Principles. A bell siphon is an “easy” way to drain a grow-bed in a aquaponic or hydroponic system. The idea is that in an ebb and flow system, you have a tank of fertilizer (for aquaponics its a fish tank) and a grow bed full of plants. You then pump the fertilizer into the grow-bed and the plants eat. The problem is that you cannot generally have the plant roots submerged in water all the time as the roots will rot. And you have to have a means to empty the tank so it does not overflow.
A bell siphon solves this problem very simply, with the added benefit of working without additional energy input. This is the coolest part about my system. A bell siphon works on the physics principles of atmospheric pressure and vacuums.
A bell siphon is basically three pipes. the first is a stand pipe that passes through your grow bed. When planning, it is important to understand that no water will get higher than the top of this stand pipe, and the fitting needs to be water tight. Coming out the bottom of your grow bed, you need to attach a 90 elbow to the standpipe. To this elbow, you need to glue a length of pipe of sufficient length to reach to your water tank. Finish this off with another 90 degree elbow pointing down. This is important, as you will not be able to get your siphon working properly without those two 90 turns.
The next pipe forms the bell, it is a little longer and twice the diameter of the stand pipe. One end of the pipe needs to be airtight. The simplest thing is to use schedule 40 PVC and just glue on an end cap. The other end of the bell pipe needs to have slits cut into it to allow water flow. The way I made my slits was that I leveled the saw on the end of the ball pipe, and made a short cut a little off center, and another the same distance off center in the other direction. Then I took a pair of pliers and snapped off the pcv between the cuts, leaving a small gap between the cuts. Because I leveled the saw on the pipe, I was able to make enough cuts at the same time that I had a “toothless” look on both sides of the pipe. I then turned the bell 90 degrees and repeated the process so I had 4 gaps to allow water flow into the bell. Don’t worry if this is confusing the video will show the bell.
Next I drilled a hole in the end cap and inserted a plastic 90 degree hose barb elbow. on the outside of the bell, I pushed a piece of aquarium air line on the other end of the barb. This allowed me to have an air hose running down perpendicularly to the bell. This hose needs to be fastened to the bell (I used flex ties) and have a completely airtight connection to the bell. The open end of the hose needs to be slightly above the top of the teeth cut into the bell.
After you set the bell over the standpipe so that it fits nicely over your bulkhead fitting, and the teeth are properly sized to allow water to flow into the bell, you need to create a gravel guard. If grow bed medium gets inside your bell or clogs the notches cut into it your siphon will not work. I used a PVC pipe cut bigger than my bell and slightly taller than it also. I then drilled several holes in the pipe, as water must be able to flow easily through it, but keep in mind the holes have to be small enough to keep out the medium.
Once you fill the tank you might need to tweak the system enough to get everything running, but once working properly this is a great way to drain your grow-bed.
For those scientifically inclined, this works because as the water rises in the grow bed, the air in the stand pipe is forced because the water is rising inside the standpipe by the same amount. Once the water level rises over the top of the standpipe the water begins to drain down. The 14 pounds (approximately) of atmospheric pressure presses on the water in the grow bed and push it down the standpipe. The closed end of the bell protects the water inside the standpipe from the weight of the atmosphere, which creates a low pressure zone.
At the same time, the weight of all the water running out of the standpipe, coupled with the two 90 turns keeps air from entering the wrong end of the standpipe. This creates a vacuum, and coupled with the high pressure outside the pipe, and low pressure inside the bell, the water in the grow-bed is sucked up into the bell, this allows the water to reach the standpipe even when the water level in the grow-bed is much lower than the water level inside the bell.
This vacuum exists until the end of the air hose attached to the bell is above the level of water in the grow bed. When that happens, air is sucked into the siphon with a loud slurping sound, and the low pressure is equalized and the suction stops.
As long as water is pumped into the grow-bed this will happen over and over.
If this is complicated and unclear, please watch the video and look over the attached PDF.