Aquaponics - Bell siphon

The bell siphon was a clever thing for someone to design, and as such you feel a bit of that "clever" rub off onto you when you make one. I made one, and feel a slight increase in my cleverness.

There isn't that much to a bell siphon, but what little there is tends to be very important.

A bell siphon consists of ...

- a standpipe (the bit that goes through the bottom of your grow bed)
- a bell (the bit that sits over the standpipe)

A bell siphon can also have...

- a siphon breaking air tube
- water inlet holes
- a media screen (cut off soft drink bottle with holes)

The bell sits over the standpipe.
The siphon breaking air tube is a tube that goes from the uppermost chamber formed by the bell, to a point around 1.5 cm above the bottom of the grow bed. If you dont have the tube, its possible for the siphon to continue draining the grow bed as the water is being pumped in. That is, when the siphon should have stopped and the bed should be filling again, it is possible that water will just keep flowing out at the same speed that its flowing in. By allowing a small stream of air into the top chamber you can break the siphons ability to continue suction when the water gets low enough to expose the pipe. When the bed is full and the siphon starts, this tube doesn't really do anything because its sucking water not air.  But as soon as the bottom of this tube is exposed to air, it brings the siphon action to an abrupt halt allowing the cycle to start again.

A bell siphon also has some cutouts to allow the water to flow in unrestricted. These can be cut outs or a series of holes drilled near the base.I used cutouts.

Make the standpipe not too tall (so water doesnt get blocked by the bell sitting right on top of it) and also wide enough to allow enough water to drain through it. The standpipe needs to have a diameter large enough to allow the water to drain faster than the pump is pumping water in. A large diameter standpipe is important, but dont go too large. If the standpipe is too wide the water will be able to run down on only one side and never form the seal required to start the siphon. If this happens the water will just flow out at the same rate the pump is pumping it in. To get the siphon to start, reduce the size of the standpipe or increase the flow rate of the pump until it's flow fills the tube.

Some things that will cause your bell siphon to fail are...

- air bubbles in the hose running back to your fish tank. (keep it free of kinks and always sloping down - the return hose should drain until it is completely empty at the end of each cycle)
- too small a diameter of standpipe for the flow (or too much pump flow)
- too large a diameter of standpipe(or not enough pump flow)
- gravel getting into your siphon (make a media screen)
- water entering your standpipe too much from one side (cut your standpipe so that its level to help get a rush of water all at once when the water level gets high enough - this helps fill the tube and start the siphon)
- having too small a gap at the top between your standpipe and the bell (make your bell taller)
- restricting the flow within the bell because there is not enough gap for water to travel up the bell (make your bell wider)

Even though aquaponics is a fascinating subject, when you make a bell auto-siphon those around you will tire of your company quickly. Try to include other topics of conversation when meeting friends. Other people wont find your auto-siphon as interesting to watch as you will.

[link from the future on calibrating a new bell siphon]


  1. Very Nice Article. Most Helpful.

  2. Thank you for a great article. I couldn't get the
    principles of the auto-siphon until I read this one.
    I finally know everything there is to know about everything.

  3. Just found this blog and it could come at a better time, I am just starting to get my system components together for my first Aquaponics system. This is a real help with the bell siphon. I look forward to reading more of your blog. Thanks

  4. why do the bell siphon don't drain atomically it just drain the same as the water is coming in.

    1. This comment has been removed by the author.

    2. Sorry I missed your question.

      If the growbed is full it probably means not enough water from the pump. Make the small pipe thinner.

      Have a look at this article I wrote on how to get it right...

  5. Do you have to setup your bell siphon in the middle of your grow bed? Or can you have on the ends of your grow bed if you have a long grow bed?

    1. You're actually better off not putting it in the middle. What you want is for water to flow through your system, so if you put the siphon in the centre, you should probably make the mater enter from all around the outside. This is to make sure you don't get dead spots where there is no flow and as a result no oxygen. With a long grow bed, I'd put water in at one end, and run the siphon at the other. I went a little crazy with the idea when I made my new grow bed and did this...


Popular Posts