Here is an external bell siphon which is reliable and easy to build. A great feature is that there is no shaving of pvc fittings or parts required.

Notice that is appears to be a simple upright ‘tee’ section coming out from the side of the growbed half-drum. The bottom of the tee is the drain as you might expect but there is a secret hidden inside the part you see. Below is a picture of all the parts you will need.

Assemble Your Bell Siphon

Start your assembly by fully inserting a 1-1/4″ to 1″ slip fit adapter into your 1-1/4″ cap as shown above.

Connect a 1/2″ to 3/4″ adapter to one end of a 12″ section of 1/2″ pipe.

Next, you will want to wrap the 1/2″ pipe with duct tape or better yet, electrical tape until it fits snugly inside the 5-3/8″ piece of 1″ drain pipe. Place the tape about 2-1/2″ to 3″ inches from the end without the adapter. It will fall roughly near the middle of the 5-3/8″ pipe when assembled as shown in the photo above. The ends of the 1″ and 1/2″ pipes will match at the end.

The following picture shows the silicone seal which must be made between the 1″ and 1/2″ pieces of piping. Be absolutely certain your siphon is completely sealed. If it leaks, your siphon will never break and your growbed will never fill properly. EDIT: Unfortunately, we have found that the silicone seal will fail after time. A much better solution is to use water proof epoxy material. Water Weld has worked well for us so far.

Finish the assembly by inserting a 3-1/2″ piece of thin wall 1″ pipe into the Tee section. This type of tube is intended for low pressure systems. The I.D. is larger so it provides more space for water flow past the 3/4″ adapter. You may use this type of tubing for the drain section above but the standard thicker wall pipe will require less tape. Dealer’s choice.

Firmly install the 1-1/4″ cap assembly on top of the siphon and insert the 1/2″ pipe assembly up through the bottom of the Tee. When properly put together the top of 3/4″ adapter will be touching or nearly touching the underside of the 1-1/4″ cap. There should be little to no space for the water to drain. The reason for this will be clear enough soon.

Installing and Testing Your Siphon

Install your External Bell Siphon by connecting it to the outlet pipe on the side of your growbed. You can let the siphon drain into a trough or tank but I ‘tee-d’ into a common drain pipe and connected several growbeds to my sump tank.

Allow your growbed to fill. It should NOT drain because your cap was clamped down pretty tightly to start. (Hope you remembered to install an overflow tube!)

When the water level is clearly high enough, Gently, slowly, carefully twist the cap about 1/8″ and see what happens. If the siphon doesn’t start, turn the cap more to increase the cap height slowly until the siphon runs. Wait until your bed empties and be sure the siphon breaks! It may take 2-3 sets of gurgles until it finally burps to let you know it is finished. If you see a whirlpool or dimple on the water’s surface, it has not yet completely broken the siphon.

To ensure a good break, I found that it helps to drill 3 to 5 small 1/8″ holes in the upper surface of the drain pipe INSIDE the growbed. You can also try to reduce the inlet flow, but this will greatly affect your flood and drain cycle time. You might also try making the top 1″ pvc sleeve longer and replacing the 1-1/4″ cap assembly with a simple 1″ cap. If all else fails, go back and check your silicone seal. You may have damaged it when you jammed the cap onto your siphon.

Unfortunately, there is no easy way to adjust the trigger height for the drain cycle. The dimensions given work well for my growbeds but if you find you need, say, another inch of fill, you must start with a 13″ length of 1/2″piping. Don’t forget to add the same extra length to the 3-1/2″ section of 1″ inch pipe!