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A siphon is a mechanism for drawing water off from a higher container to a lower container. In its simplest form, it is a length of hose. To see how it works, one can place a hose into a tank of water, remove all the air out of the hose, block off one end of the hose (with one’s thumb) and bring the blocked end out of the tank and below the water level in the tank. When one removes one’s thumb, water flows from the tank until either end of the hose is above the water level. Then air enters the hose and siphoning action ceases.

A simple siphon is traditionally used for such things as cleaning fish aquariums. However, siphons have also been adapted to control flooding and draining of growbeds in an aquaponics setup. In this useage, they are called autosiphons.

Contents 1 Autosiphons 1.1 Looped Autosiphon (also U-Bend or Gooseneck) 1.2 Bell Autosiphon (Also Pipe-in-a-Pipe) 1.2.1 Air Tube 2 Conclusion

[edit] Autosiphons

An autosiphon is simply a siphon that can start and stop itself in response to changing water levels. They are used to control the Flood and Drain cycles in a growbed. There are two implementations that are currently being used in aquaponics: a Looped Autosiphon, and a Bell Autosiphon.

[edit] Looped Autosiphon (also U-Bend or Gooseneck)

This is the easiest autosiphon to construct and is easily conceived from a basic siphon. It is constructed as a loop of flexible tubing, with the crest of the loop below the surface of the growing media. A Looped Autosiphon can be mounted internally or externally to the growbed, with the outlet below, directly through the base of the growbed or out the side.

The water begins draining out of a Looped Autosiphon once the water level in the growbed is above the lower level of the horizontal part of the loop of hose. To remove all air from the Looped Autosiphon and start it working effectively, the water level needs to rise to the top of the hose. So if the hose is 40mm diameter, then water level needs to increase by 40mm. Once the looped autosiphon is fully engaged, all air will be expelled from the hose, and water drains from the growbed. When the water level has dropped to the level of the intake of the looped autosiphon, air is again introduced to the hose. This may cause a slurping sound as the siphon breaks. By adjusting the rate that the pump fills the bed, the amount of time spent slurping can be greatly reduced.

Looped Autosiphons work best when the intake of the hose is horizontal, the loop is smooth and round, and the outlet ends in a vertical drop. Care should be taken in choosing the flexible hose. Hose with thin walls will collapse over time, decreasing the flow of water.

Auto Siphon Gallery

Modified U-Bend Auto Siphon

Setup of pipes and air tube configuration

Auto Loop Siphons

[edit] Bell Autosiphon (Also Pipe-in-a-Pipe)

The bell autosiphon consists of 1. A vertical standpipe inside the growbed extending through the base of the growbed, 2. A larger pipe (siphon pipe) placed over the standpipe. The siphon pipe is freestanding and taller than the standpipe, and is fitted with an endcap. Additionally, the base has pieces cut out to allow free movement of water. 3. A small air tube to assist in breaking the siphon when the water level has dropped low enough.

Water flows up between the walls of both pipes then down the inside of the stand pipe, creating the siphon. When the water level drops to the intake end of the air tube, the siphon is broken.

Pipe Sizes and Calculations Tests were carried out using 40mm (1.5 inch) standpipe, with a gap height of 12.5mm (0.5 inches) and a growbed area of 350cm2, these figures will be used and calculated for different size pipe configurations in the following table:

• Pipe – standard pipe size in metric (mm)
• Area – cross sectional area of stand pipe
• Circ – circumference of pipe Height – minimum clearance needed for sufficient volume of water to flow into standpipe unimpeded (higher means more air, means more water flow (lph))
• Outer pipe – minimum size of outer siphon pipe needed for sufficient volume of water to flow between standpipe and siphon walls unimpeded
• Max flow – maximum flow of pipe, for continuous pump flow, the maximum input should not be greater than half this figure.
• Min inflow – minimum flow rate into the GB
  • on tests, the flow rate worked down to 360lph before false starts were occurring, 10% was then added to this figure
  • minimum inflow is calculated on lph required to flow into a GB area of 1 cubic metre at minimum height clearance

• the outer siphon pipe size should be a minimum of 50% greater then the stand pipe size so as not to impede water flow
• Cut outs are the pieces cut out of the bottom of the siphon pipe to have leg supports and to allow sufficient water flow
• 3 legs are used for supports at all time and the depth is set for 15mm on each leg
• (20/55) legs are 20mm wide, distance between each leg is 55mm (these figures are approximate)

To work out how long to make cutouts:

1. wrap a piece of string around the pipe
2. lay the string down flat and measure the distance
3. deduct the width of 3 legs (width depends on pipe size but not critical)
4. remainder divide by 3, this is the distance between legs
5. mark on pipe with pencil to make sure before cutting

[edit] Air Tube

Air is required to break the siphon seal and allow the water trapped inside the siphon pipes to fall back into the growbed or down the outlet pipe. To get air into the siphon is quite simple and there are many places that have proved to work:

1. put a hole and tube on the upside of the siphon.
   
2. run the tube up to the top of the upper horizontal part of the siphon, a hole is in the end cap where the air tube could be fitted 
   
3. it can be placed on the GB wall (below the high water mark) and joined to the outlet pipe outside of the GB

The main point is that when the water falls below the air tube, air is then sucked in – this air will flow with the water until the GB water level falls below the siphon end pipe. Water flow in the siphon then ceases but air is still sucked through the tube filling the siphon with air and breaking the seal, it is important to have the end of the air tube at least 10mm above the lower height of the siphon pipe. In pic1 the air tube is 25mm above the base, the support legs are 15mm deep.

The air tube need not be there at all, the hole would have been sufficient but with air flowing with the water, water velocity decreases, test results: 40mm pipe will displace 6,000lph, by adding air into the flow the output decreased to 5,000lph.

[edit] Conclusion

Autosiphons are a very useful mechanism for controlling the Flood and Drain cycles in a growbed.