Backyard Aquaponics
A place to discuss aquaponics

I am just in the process of converting my somewhat disappointing DWC system to a flood and drain system using LECA as the growing medium.

As you can imaging this is taking a considerable amount of re-piping, including building bell siphons. Before I filled the beds with media I tested that the siphons were working. They have a stand pipe height of 24cm, designed to leave about 4cm dry zone at top of the 30cm deep beds. They all triggered as expected. Now, having filled the beds with LECA and re-tested, I found that the water level in the beds rose until it was just overflowing out of the bed before the siphons triggered. This means that I would have no dry zone at all were I to leave the system like this.

As I am new to the operation of flood and drain beds is this a simple matter of slightly shortening the height of the stand pipes (bit by bit) to make them trigger at a lower water depth? Or am I missing something more fundamental? Would having a shorter pipe with the same height bell siphon outer sleeve be more or less effective, as the air gap will be larger between the end of the pipe and the top of the bell. One thing that is lacking in all of the info I have seen is any fundamental data on the size of the effect of the air space above the stand pipe in the bell. There is lots about 2:1 ratios between stand pipe diameter and bell diameter, and size of pipes suitable for different sizes of grow bed, and even a bit about the bottom of the bell 'cap' being level with the top of the pipe but this depends on the style of cap available (mine are not domed). I cant get my head around is a small space or a large space best

Having researched quite extensively the design of bell siphons before building mine I am quite happy with the basic design, as they start, flow and stop ok, just now not starting at the right point.

Any help or thoughts gratefully received!

Graeme

I have no idea about bell syphons, however filling the GB's with media would have taken up a fair bit of space originally filled with water which suggests that your flowrate into the GB's is too great.
I could be completely wrong of course but try reducing the flow.

good point as the media will take up about 60% of the space i.e. the water is only about 405 of the total volume. i was using a fast flow rate just to fill the bed and test it so it is possible that this was part of the problem. Will turn it down a bit and try again

Shorten your standpipe?

What size standpipe and what size bell are you using? How much longer is your bell than your standpipe? What does the piping look like under your bed?

Media will sometimes cause issues with the siphon not breaking because it slows down the water movement. I could also see it stopping the siphon too early if the media didn't allow water flow very well. It sounds like the air is getting stopped in the bell and not allowing the water down the pipe.

Standpipe height is 24 cm and bell is 28 cm. Piping coming out of the bottom of the growbed incorporates a trap to make the siphon trigger more effectively than it was doing using just a 90 degree elbow 9it was tending to dribble rather than siphon). Will shorten standpipe and gauge the effect

If your water isn't going down at all, I'd blame the trap (even though I'm not sure what that is).

Remove it, and tweak it to make the siphon work. If the elbow is on a slight up angle, it won't work properly, and I mean slight. Make sure it's angled down, then just tweak the flow to get it to work properly.