Aquaponics - Calibrating a new bell siphon

This post assumes you know something about bell siphons, and roughly how they work. It also assumes you have made one and want to get it to work. If you need a bit of an introduction to this device, it might be worth reading this post first - aquaponics - bell siphon

With anything that you are trying to calibrate, whatever it is, the most important thing to do is make sure you are changing only one thing at a time. 

It's possible to make a bell siphon work with all kinds of different approaches, but in its simplest form the standpipe drops straight down from the grow bed, and the water flow isn't interfered with in any way

By having your standpipe drop straight down into air the only thing you can change is the flow into your siphon. The easiest way to do that is to put a tap on the inflow to your grow bed, or a tap that diverts some water away from the pipe going to your grow bed, and dumps it straight back into your fish tank. Plastic taps cost about $2. Don't use a brass or galvanised iron one or anything metal because it might poison your fish.

Turn the single outlet from your pump into two by adding a T-junction between the pump and the grow bed.

Make one of your two new outlets go up a bit higher than the other. This one is your excess overflow, and needs to be diverted back to your fish tank. The reason we make it go up a bit higher than the other is so that it becomes the lest preferred way for the water to travel. Water hates traveling uphill, so it will prefer to go into the lower pipe with the tap attached. Only when the tap is turned off a bit will water flow through the higher pipe, and overflow back into the fish tank.

The other outlet gets the tap, and continues on to the grow bed.

before you start...

- make sure your breather pipe is 2cm away from the bottom of your grow bed.

- make sure the breather pipe is breathing fresh air when the water is low enough. Its possible, if the breather is a narrow enough pipe, and is hard against the siphon, to form a meniscus so that it sucks water even when it is above the level of the water. Check that the breather pipe is sucking air when it should be sucking air.

- make sure the water can flow through your media screen fast enough. If you have a screen made with holes drilled in it and there are not enough holes at the bottom, the flow through the media screen can reduce as the tide goes out. This can make for false stops. When the siphon stops, the sudden small amount of water that flows back out of the bell can create a localized increase in water depth. This can reseal the breather, and also kick off the siphon again if it hadn't completely stopped.

Steps in tuning your bell siphon might be (for a tap that is at the end of the pipe to the grow bed)...

1. turn on the tap to a midway position.
2. wait until the water gets to the top of the bell where it should trigger.
3. if it doesn't trigger turn up the tap and go back to step 2. If it does trigger wait until it empties.
4. if the siphon doesn't stop when the grow bed is empty turn the tap down a bit until it does.
5. wait until it fills and see if it triggers properly at the top. If it does, watch it cycle way too many times, drink beer and bang on about it to your friends (I recommend a glass bell siphon to this end). If it doesn't, adjust the tap so the flow is reduced a tiny bit and repeat.

The object here is to adjust the flow in ever smaller amounts until you narrow in on the correct flow. Once achieved, the flow should start within a few seconds of the first flow of water from the standpipe, and should stop quite suddenly. It will probably take 4 or 5 cycles to adjust.

If you still cant get it to operate, there is a fair chance your pump and siphon don't match. Try adding an inner sleeve to the standpipe to reduce its diameter.

Trouble shooting siphons

Any air in a bell siphon should just exit down the standpipe as the water rises and the pressure inside the bell increases. If it doesn't, then there is a fair chance the exit pipe isn't draining freely. With air in the bell, it's possible that your bell might float, and not trigger.

Some people say its a good thing to have some water in the exit pipe, but after a stack of experiments, I found that even though having some water in the pipe often fixes a problem siphon, a bell siphon will be much more reliable if you have a clear, straight down exit that is flowing into just ambient air pressure. That is, dump from your standpipe into fresh air, then catch the water in a drain to take back to your sump, or just drop it straight down into your fish tank if that's your arrangement. At the very least, make sure your drain is running downhill all the way to the end. It makes it a lot easier to get the flow right if your are only dealing with one variable.

The more simple your system is, the more reliable it will be.

Getting your flow right is a much better solution than adjusting your drain by putting bends in it, adding uphill sections, or submerging the end. All of these things can get a siphon to work that otherwise might not, but dumping to ambient air pressure, and adjusting the flow until it works, will make for a very reliable siphon because this makes for only one variable, and with a tap, you have total control over that one variable.

Keep your system simple, and it will reward you with reliability.


  1. Is there some kind of bell siphon mathematic formula that gives accurate amount of water suckage per length of standpipe, height of bell, length of drain, diameter of standpipe? Unfortunately I have to go uphill a little bit but if it's a true siphon, it shouldn't matter as long as the end is lower than the top of the standpipe, right?

  2. The problem with uphill bits, can be when the siphon stops. Sometimes if a siphon gurgles and splutters it will stop and start. If a siphon doesn't break decisively, it can just keep running out at the same pace that its being fed by your pump.

    You will probably get away with it, but it's best avoided.

    I think I'll answer your first question as today's post, because it's quite a question :)

    and it'll be a little late...

  3. Dont forget you can put your siphon anywhere. It doesnt need to be in the grow bed. All you need is a hose connected to a bucket, and your siphon mounted through the bucket. So in chefthechef's case it might be better to put the siphon over the place you are draining into.

  4. Heya¡­my very first comment on your site. ,I have been reading your blog for a while and thought I

    would completely pop in and drop a friendly note. . It is great stuff indeed. I also wanted to

    there a way to subscribe to your site via email?

    Pressure Calibration

    1. Not that I know of, but there is such a thing as "Google reader" that does something like collate all the blogs you read (if you read more than just mine) so that might be worth a look.

      And there is something else called "RSS feed readers" that allow you to subscribe to blogs and (I think) be notified of new posts.

      I don't use either of those things, so sorry I cant tell you more.

      I'll ask a couple of readers what they do and post back here in reply to this within a week if I get any info.

    2. Try "google feedburner".

      I just looked it up and it looks like it might do what you want. (thanks mike)

      It looks like it might be in the process of dying, but there will be something to take it's place.


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