Aquaponics - Bell siphon sizing not-so-magic formula

A "bell siphon" is a device that automates the flooding and draining of an aquaponics grow bed, even though the pump is adding water to that grow bed constantly. 

This post assumes you know lots about bell siphons. If you don't, you should start here on this post titled "bell siphon".

You probably should have already read this post titled calibrating a new bell siphon.

If you still don't feel you know enough, you might like to read this on flood and drain, look at this upgraded animation of a bell siphon working, watch this video of a glass bell siphon showing what's going on inside, or just skip this post and look at this youtube video of a panda cub sneezing. (personally I'd look at the panda)

It also assumes you don't mind reading something you think will eventually have an answer, but may disappoint.

Before you take anything I say to your design, you should know that my system is run with bits of string and bubblegum holding it together. 

My pump looks like this. I love wire.

Yes, those two opposing loops of stainless wire are the front bearing for my pump. Wire is the only thing supporting the impeller. It's been running that way for 6 months or so, but the output of the pump isn't really enough to run my system. The impeller is the wrong one salvaged from another pump, and doesn't really fit. 

There is no magic formula for weighing up the dimensions when designing or trouble-shooting a bell siphon, but there are some things you can do to make it a bit easier.

The problem, in a convoluted and sprawling nutshell, is this.

There are a huge number of variables in the way the water flows into and out of an aquaponics system. The type of pump you have is a big one, as is the length and width of your standpipe in the siphon itself. But there are other variables that can have dramatic effects. 

So, things that vary input...

  • pump output
  • height the pump has to pump up to (most mumps are labelled as if the user is going to pump water to nowhere. A pump labelled "3500 litres per hour - Max height 5 metres" might pump 3500 litres if there is no hose attached to it, but add a 5 metre hose and it will only pump 1 drop per hour. The amount it will manage at any given height in between are anybody's guess unless it comes with a graph indicating approximate values at different heights.)
  • restrictions to flow caused by corners and other bits that people use to distribute the water around their grow beds.
  • total length and diameter of hose used (there is a surprising amount of friction in shifting water through pipes)

And things that change output from the siphon, and how the siphon triggers might be...

  • width and height of bell
  • width and length of standpipe
  • freedom of water movement through the media and media screen (if you are drilling holes in the media screen, drill them all at the bottom - if the holes at the bottom cant drain enough, the ones at the top are not even going to be in play when the water level gets lower. The end result is that some kind of equilibrium can be reached where water coming in matches water going out through the siphon. In this case the siphon wont ever stop.
  • any obstructions to the outlet (uphill sections that might trap air or water, end of pipe being submerged  or anything else that might cause some back pressure)
  • changes in width of the outlet. Things like a flange that your stand pipe crews into may well restrict otherwise change the flow. Sometimes a bit of chaos in the standpipe can help create the seal required to trigger the siphon.
The result of all this, is that it's very difficult to say "this size siphon will work with this pump".

Even if you set up two systems with the same components, it would be a good idea to have a tap that will allow you to divert some water so you can fine tune your system. The irregular flow of water can mean some very small changes can have some big effects. 

So, the solution...

Of all these things, the easiest things to control, and thus the best things to start adjusting are...

  • Inlet flow. By adding a tap, and diverting perhaps 20% of your water back into the fish tank rather than the grow beds, you have some room to play. ie you have some spare water flow in hand, you can adjust your flow UP as well as DOWN to tune your siphon.
  • Outlet flow (Standpipe width) The standpipe is a very inexpensive component, and may even be sourced as an off-cut for free. This means it's easy enough to replace with a bigger one if need be, or filled with a smaller one.
My process for setting up a system would unfortunately involve a bit of essential trial and error.
  1. Buy a pump capable of turning over all the water in your fish tank ever hour plus 20% to divert. The diverted water aids in aeration so isn't wasted. (very important for your fish and filtration so pump selection should always be the starting point)
  2. Plug it in and look at the flow. 
  3. Get hold of a tube for a standpipe that you think will be big enough to dump more water than the pump puts in. My pump runs constantly and takes around 20 minutes to fill and 20 minutes to drain. That means my siphon can dump water at around twice the speed that the pump can deliver it. (see photo's below)
  4. Adjust accordingly. Its easy enough to adjust upwards, just drill a new hole and put in a bigger pipe. Downwards is also easy because a large number of hoses and PVC tubes fit the next size down inside them (I presume so you can join them). If you add an inner tube to reduce the width, you don't need to silicone it in place, because it doesn't matter if it leaks. Any leakage is just going into the original pipe anyway. I made mine bigger than I thought I'd need with this in mind. If you really feel the need, it would be easy enough to test it in a bucket. Plug the standpipe and fill the bucket, then let the water flow. If the pump cant keep up with the draining, then you will have no problems as you can always reduce it if you need to. 
That's the magic formula "Its easy to change the inflow and the outflow, so dont worry about it. Just have a go."

Just trust your judgement and as long as you have the tap between the pump and the grow bed, allowing adjustment, it will almost certainly work. If you are making your own system, you already trust yourself anyway, so just go that one step further, and do it. You'll be fine. And perhaps reread calibrating a new bell siphon.

Just out of something bordering on interest, my system runs like this. 

My inflow is from my very sad pump, and flows at around 50 seconds to shift a litre to the grow bed.

So I think that means I'm pumping 72 litres per hour. Which isn't really enough but that's the pump I have. 

I'm getting a new one. 

My standpipe was originally 19mm poly pipe but is now restricted to what my trade quality fridge magnet tape measure tells me is roughly 12 mm internal diameter garden hose.

The rate of outflow is really difficult to see in my real world system, because it drops straight down and doesn't photograph well, so I faked the output in my kitchen sink for the photo. It would look a little like this if I pointed my standpipe outlet to the side.

I warned you it might disappoint. 

If you are technically minded you might want to look at this 

I'm not, but I'm told it might be useful.


  1. Not dissapointed at all! =D

    Hoocked up my (un-gravelled!) system a few days ago and ran into equilibrium almost straight away, after just a few lucky drains.

    I think my outlet hoze needs to be a bit longer to not suck up air. The standpipe has an diameter of about 25/22mm, bell about 55/52mm, and drains the bed in about 2 minutes. The inflow is quite strong compared to yours to fill the growbed in about 10 minutes.

    The start is quite quick, but the problem comes when the siphon should stop. It cant manage to get enough air from the bottom of the siphon to break, and equilibrium happens!(?)

    I will try to turn down the inflow a bit, but i still want the bed to fill quite quick.

    Working on the second growbed right now!

  2. Nice to hear it's going relatively smoothly :)

    Once you put media in, it might change the equation a bit, but if you have the ability to change the flow, you should be able to adjust.

    I'm not actually sure if having media makes a difference. It would change the rate at which the water climbs in the grow bed, and that might has some impact, but I cant wrap my brain around whether it does or not. I like to think my head is too busy, but the reality is I cant work it out :)

    Keep us posted, and if you need help with anything, I'm BullwinkleII on the forum at backyard aquaponics...


  3. Me again! (cid228 btw!)

    I made some adjusments yesterday evening; I cut larger holes at the bottom of the bell, to make the water flow through the bottom of it without any resistance. Also; I changed the tube I had at the outlet from the GB to a 90cm rigid pipe with a 25-30° angle to my pond. The drop is about 45-50cm/1.4-1.6ft.

    At this point the growbed would fill and empty in about 15 minutes. But I had to assist the start and stopping of the siphoning with a slight tap on the bell.

    I'm thinking I might need to make some more tubulence in the standpipe to reliably get it started(?).
    The problem with equilibrium when the siphoning should stop was helped a little bit by cutting the bell, but it is still present and stops more by chance. Prolly because of the difference in pressure created by the small waves from the inlet. As I said above; a slight tap on the bell will also start and stop the siphoning. (I have a closed bell without a air-tube. Do I need one?)
    I also tried with narrowing the standpipe at the top, but I'm thinking the narrower part should be a few cm down inside the stanpipe.

    Before adding gravel, I'll have to manufacture a mesh/tube to protect the bell from the gravel. I'll get back to you with the results when I have done so!

    If you or anyone else got some idead on what could be the problem with the start/stop I'd be happy for any advice!

  4. At the top you are probably better off with a funnel shape. Lots of people use them, and claim they make the siphon more forgiving. There is a PVC fitting that will probably do the job.

    I think the funnel shape top works by getting the water to flow across the standpipe and make a seal too kick off the siphon.

    Another way is to create a short reduction inside the standpipe. (I think that's where you were going) The easiest way to do that is often to cut a short length of whatever you made the standpipe out of, and the cut it lengthways twice and remove a bit. That way it has a smaller diameter when squeezed into your standpipe. Put it around 1/4 of the way down from the top.

    I have a breather pipe on mine, but once you get the hang of adjusting your water flow, you shouldn't really need one.

    Have you read my post on calibrating a bell siphon? If not, it might help...

  5. It's also worth making sure the top of your standpipe is a clean horizontal cut.

  6. Hey again!

    Have'nt had any time to try further adjusments as I dont have the set up at my own home, but at a relatives.

    The reduction cut-pipe thingy is exactly what I did, though I think I placed it a bit too far up to get the right funnel-effect.

    Will read up on your link!


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