Page "Siphon" Paragraph 44

Gas in the liquid is a concern in large siphons.

The gas tends to accumulate at the crest and if enough accumulates to break the flow of liquid, the siphon stops working.

The siphon itself will exacerbate the problem because as the liquid is raised through the siphon, the pressure drops, causing dissolved gases within the liquid to come out of solution.

Higher temperature accelerates the release of gas from liquids so maintaining a constant, low temperature helps.

The longer the liquid is in the siphon, the more gas is released, so a shorter siphon overall helps.

Local high points will trap gas so the intake and outlet legs should have continuous slopes without intermediate high points.

The flow of the liquid moves bubbles thus the intake leg can have a shallow slope as the flow will push the gas bubbles to the crest.

Conversely, the outlet leg needs to have a steep slope to allow the bubbles to move against the liquid flow ; though other designs call for a shallow slope in the outlet leg as well to allow the bubbles to be carried out of the siphon.

At the crest the gas can be trapped in a chamber above the crest.

The chamber needs to be occasionally primed again with liquid to remove the gas.