4 Steps to Understanding Bell Siphons
May 19, 2017
Bell Siphons are one way to intermittently fill and drain a container such as for an ebb and flow hydroponic system. While they are not particularly complicated devices, they are easier to understand if simpler siphons are considered first.¹
1. Simple Siphons
A simple siphon will cause water to flow up and over the lip of a reservoir, and down to a lower reservoir.²
A drawback to this design is that the siphon tube must be primed by filling with water. If the siphon tube is a flexible hose, this can be done by inserting the hose into the water of the first container until the entire hose is submerged, plugging the near end, and then placing the end into the lower container before unplugging.³
As an alternative, a pump or suction may be used to start the initial flow, but are not required to maintain it once started.
2. Self-Priming Siphon
If instead of the siphon tube going over the lip of the container, it is moved to pierce through the side, it can be primed by filling the container with water to the top (or past) of the siphon tube. The siphon won’t start until the container has a water level high enough to crest the curve at the top of the siphon tube, but once the siphon has started it will continue on its own until the water has drained to the bottom of the siphon.
If a continuous stream (of compatible flow) of water is added to the container, it will first fill, and then drain to the level of the bottom of the siphon tube, and then the cycle will repeat. This action can be used as an alternative to using pumps to raise and lower the nutrient level in an ebb and flow system.
A variation with a short submerged siphon tube can be used as a failsafe to prevent the container from overflowing.
3. Greedy Siphon
This is similar to the self-priming siphon above, but instead of the siphon tube going through the side of the container, it comes up through the bottom.
The same principles apply, if the container is filled enough then the siphon will prime and drain. Under a continuous flow, the cycle will repeat.
The classic example of a greedy siphon is a Pythagorean cup.
If the cup is filled to a moderate level, the siphon doesn’t prime, and the cup can be used to drink from. If on the other hand, a “greedy” person fills the cup full enough to prime the siphon, the siphon will empty the cup out of a hole in the bottom.4
4. Bell Siphon
In a bell siphon, the curve at the top of the siphon tube is replaced by a “bell. The lower portion (drainpipe) is the same as with a greedy siphon, but instead of the upper portion of the tube curving off to one side, it is cut off right at the start of the bend, and replaced by a loose fitting larger tube closed at the top (the bell) that cups over the drainpipe, but does not seal to the bottom or stop flow at the top.
The path created between the outer bell and the drainpipe replaces the path of the bent tube in the previous examples (and gives the bell siphon its name). Since if the system drains too fast it can break the flow, a turn is frequently introduced below the drainpipe to slow the flow. Other modifications may include strategically placed holes intended to prevent an undesirable stable state where the inflow and outflow match instead of the fill and drain cycle.
- Although it depends on what one means by “complicated” and “understand”. While siphons are easy to make, and the basic principles of how they behave are fairly straightforward, exactly why they work is still up for some debate as it is likely a combination of complicated factors that aren’t yet (at least commonly) well understood.
- A bendable straw in a full glass can act as a siphon. Instead of bending the straw upwards to drink, one can bend down and take a sip from the end of the straw while the free end of the straw is below the level of the drink in the glass. If done carefully, a siphon can be created and a mess made as the drink siphons out of the glass through the straw until the level drops to the free end of the straw.
- This pipette like the method of priming a siphon is preferred when the liquid being siphoned is toxic or where there is cause not to suck on the free end of the tube.
- Pythagoras is best known for his work with triangles and the Pythagorean triple, and apparently was the sort of practical joker one doesn’t wear good clothes around.