Understanding the Bell Siphon

Plants receive the most amount of nutrients and oxygen through slow flooding and rapid draining of water into the growbed. The water leaves the grow bed through a stand pipe and the rapid draining is achieved through a bell siphon. If the water was to leave the grow bed at the same rate it was entering, the roots of the plants would be continuously submerged in water and cause them to rot.

A bell siphon uses atmospheric pressure to control the rapid draining of water. A liquid always flows from an area under higher pressure to an area of lower pressure. The water flows up the bell siphon (an area of lowered pressure) and into the stand pipe, again lowering the pressure at the top of the tube.

Once the flow has begun, it will continue if undisturbed as long as the inside end of the tube remains below the surface of the liquid. The flow can be cut off by raising the outside end of the tube above the level of the surface of the liquid in the container.

The top of the stand pipe should sit around 25mm - 35mm below the surface of the medium. This will ensure the top of the medium stays dry and no insects or pests are attracted. The top of the stand pipe is how high the water will get to before the siphon is activated and it is drained out quickly. The highest slit determines how much water will be left in the grow bed once the siphon cuts off. 

More rapid draining can be achieved through a stand pipe that has a wider opening at the top. The downpipe also affects how quickly the water is drained. The configuration showed in the diagram below is one of the most effective ways of quickly activating the siphon.

Using a bell siphon over an electrical timer reduces the amount of parts using electricity and which can malfunction as well as applying physics in all its glory.