This video will show you the details on how to build a bell siphon. This variation is a bit unique and has been coined the "Torcellini Siphon" on some of the forums. This model has couple of adaptations I did to a standard bell siphon which made it operate better with various sized grow beds and various flow rates. To see some of the physics on how it works, I strongly recommend you go back and watch the original video. I currently have six of these running in the greenhouse with no issues and if you read through the comments in the original video, the testimonies speak for themselves. I'm not saying this is the only way to build a siphon...I just know it works very well in our system. All the measurements that I give are based on a 12" deep grow bed wall. The media is about 11.5 inches deep and the water floods to about 10 inches.
First, I wanted to detail the media guard even though it has little bearing to the bell siphon. I like to use a 6" pipe since it provides enough room for the bell assembly plus allows me to reach my big hands down inside. You can usually find 6" pipe at most of the large home improvement stores and it's usually a light green color - I happen to have some scrap PVC pipe from a construction site.
I cut the length of the guard to be at least 1 inch above what the media level will be, in this case, it will be 13" long. This is so I can grab the top and spin it to cut off the roots that grow into it. If you're using a soft liner like dura-skrim or EPDM, the bottom of the pipe should be sanded smooth so it doesn't cut the liner while you're turning it.
I like to use a table saw to cut slits into the pipe so the water can easily drain from the stone. I will set the blade around 5/8" high so it will cut a nice long slit. I mark the top edge of the pipe to divide it into quarters and use that as a guide along the saw's fence so I can keep the slits uniform...not that it really matters since it's buried in stone. Ignoring all safety measures, but still keeping all 10 fingers, I cut each slit, then adjust the fence by 1/2" and continue cutting more slits until I get to around 3/4 up the pipe. If you don't have a table saw, you can make the media guard by drill a lot of holes in it, but I really like the slits because they sort of create a sharp edge which slices through the roots easily.
To get the siphon drain through the grow bed, I like to use bulkhead fittings. They make a nice secure connection to the bed and can handle the bumping of the plumbing without risking damage to the bed and liner. These are 1-1/4" threated fittings, which will need reducers to get down to the 3/4" drain pipe. If you can find a slip-fit 3/4" fitting you won't need any adapters. I've seen them listed on the McMaster-Carr web site - but they can be a bit pricey. It's best to sandwich the liner between the base of the bed and sacrificial board so when the hole-saw bit hits the liner, it is held in place. Otherwise the teeth from the hole saw will pull and tear or distort the liner. Once you're through the liner, you can remove the top board and finish cutting.
To install the bulkhead fitting, remove any debris where the gasket will be and thread on the large nut. You want to tighten it just enough so the gasket compresses slightly. If it's too tight, the gasket can deform and leak.
The stand pipe and drains are all made from 3/4" thin-walled PVC which can be found in the plumbing section for potable water. The trap is important to get the siphon started if the fill rate is very slow. As shown in the first video, it's used as a burping system to create a rush of water to go down the stand pipe and start the siphon action. It operates best if the fittings are as close together as possible. All the trap pieces are cemented together. Because of the 1-1/4" bulkhead fitting, I needed to make two identical adapters to connect to the 3/4" pipe. These are made from a 1-1/4" treaded to 1" slip-fit, and a 1" to 3/4" coupling adapter. A little liquid Teflon or petroleum jelly works well on the treads to create a leak proof connection into the bulkhead fitting. I then attach the trap to the coupling and I don't cement the trap the fitting in case it needs to be removed. A dab of petroleum jelly works well to slip the pieces together. When finished, the entire assembly should be as close to the bottom of the bed as possible.
This was the first time I installed the trap so I actually made the pipe too long and cut it down to keep the trap close to the bottom of the bed.
For the bell housing, I use a section of 2" PVC pipe and cut it 9" long. I need to cut some legs into the bottom so the water can get into the bell. Again, not using the table saw how it should be operated, I set the blade to about 3/8" high and quarter the pipe for the first edge of each leg, then move the pipe back from the blade a bit and carefully side-blade material out of the pipe until there are 4 nice slots. Next I cement a 2" cap on to the pipe.
Another important component is the breather tube which is used to get the siphon to stop properly. The tube needs to be connected into the bell at about the level of the standpipe to insure that water is being sucked through it; essentially it's like sucking the water from the cup like a straw. If it goes in through the top, usually there isn't enough suction from the siphon to draw water all the way through the tube. I like to fasten a 1/4" NPT to 3/8" barbed fitting to the side of the bell. I purchased a 1/4" tap on eBay and it works well for threading these plastic pipes. After tapping the hole, I add a little petroleum jelly or Teflon and screw in the fitting so it's tight. It's important to make sure air can't leak in since it would make the siphon not work properly. Next I attach a section of 3/8" I.D. tubing and cut it so it will be about 3/4" from the bottom of the grow bed. It looks a bit long in the video, but the bell sits on top of the bulkhead fitting and the cup will set directly on the bottom of the grow bed. Basically, the tube needs to be long enough so it's still inside the cup when it's resting on the bottom, but short enough so the cup can rise a bit when it becomes buoyant. I fasten the tube to the size of the bell with a few zip ties and provide a spacer to hold the tube away from the bell. This little space is needed so the cup can fit between the bell and the tube. I also cut a notch the bottom of the tube to make it uneven. When it's sucking the water, if it's flat it can suck against the bottom of the cup causing it to plug the breather, not allowing it to suck any more.
The last part is the standpipe. If you have a trap, the top of the pipe should be cut to be about 1 inch below the maximum water level. I just slip it into the fitting so it can be removed and cut to different lengths to adjust for the desired height. That's about it. If you found this video helpful, don't forget to give it a thumbs-up and leave a comment on how your siphon is working. Thanks for watching!