This is an "ebb and flow" aquaponics system, and I originally thought a timer would be the simplest way to go. Most resources recommend 15 minute intervals. The way I understood that, the pump would fill the grow bed to the proper level over 15 minutes (while the bed was simultaneously draining), then the timer would turn it off, and bed would finish draining, and then the timer would turn the pump back on so the bed would fill again. It sounds so simple. The timer I had on hand offered 30 minute intervals. I tried to time things out so that the pump filled the grow bed to an appropriate level while the grow bed drained slowly enough.
There was some trial and error. By the end of the day the Bell Siphon was looking good. In a Bell Siphon system the pump runs continuously. It fills the grow bed to a certain level, at which time the siphon kicks in and drains the bed. The height of the "stand pipe" in the Bell Siphon (generally an inch or two below the level of the grow medium) determines how full the bed gets.
During my initial research, the Bell Siphon had seemed complicated. Maybe I just didn't understand it.
The truth is, you can make a Bell Siphon in about 15 minutes, though perfecting it's operation in your particular aquaponics system may take some time for trial and error testing.
The old bathtubs I'm using as my grow bed and fish tank have 1.5" drains. So I made the first Bell Siphon from 1.5" pipe (so that it fit right into the drain hole of the tub -- that is, a 1.5" piece of PVC for the stand pipe and 3" piece for the bell pipe. That was too big for the little system I'm working with. In the end, I bought an 1.5" to 3/4" PVC adapter for the tub drain and made a 3/4" standpipe and 1.5" bell pipe. This system is perfect. The grow bed fills in...I don't know...about ten minutes (with the pump turned to it's minimum flow), at which time the Bell Siphon kicks in and drains the bed, then stops to allow the bed to refill.
Typically of yours truly, I didn't document building the Bell Siphon, so I have no photos of it's construction. I used this website http://www.ctahr.hawaii.edu/oc/freepubs/pdf/BIO-10.pdf) for instructions. I didn't follow them exactly, but close.
My grow bed is approximately 130 gallons. My standpipe is a 5" piece of 3/4" PVC pipe. My bell pipe is 6.5" 1-1/2" PVC. The four "teeth" I cut at the bottom end ( using a Dremil cutting tool) are about 1" tall. I used a 3/8" piece of clear vinyl tubing as the snorkel tube, and a product called Liquid Tape to seal around it.
My grow medium consists of some large gravel left on our property when we moved here and some "river pebbles" purchased at Lowes. The river pebbles are really too small in that they could clog the Bell Siphon system. To combat that, I made a gravel guard of harware cloth wrapped in nylon window screen.
We're leaving the pump on a few days to make sure we've got it all together enough to buy some fingerlings for the fish tank and plants for the grow bed.