siphoning

Siphoning is a method of transferring liquid from one container to another using a tube, driven by gravity after the liquid column is primed. The flow depends on atmospheric pressure and the hydrostatic pressure difference between the source and the destination. The outlet must be lower than the source surface to sustain flow, and the highest point in the tube acts as a temporary energy barrier. Under sea-level conditions, atmospheric pressure can support a continuous column of liquid up to about 10 meters for water, with the maximum height varying by liquid density.

Operation typically involves filling a flexible or rigid tube with the liquid, placing one end in the source and the other in the receiving container, and priming the tube to remove air. Once the liquid forms an uninterrupted column and the outlet is lower than the source, gravity maintains the flow. If air enters the tube, the siphon breaks and flow stops. Friction and headings above the source can slow or halt movement.

Common applications include transferring liquids in households and warehouses, refilling aquariums, draining or filling water features, irrigation, and certain laboratory experiments. Siphons can move water, fuels, or other non-viscous liquids, provided safety considerations are observed. The technique is not suitable for lifting liquids uphill and is sensitive to air leaks and unsuitable liquids.

Variations include self-priming siphons, which are designed to initiate and maintain flow with less manual effort, and anti-siphon devices used in plumbing to prevent unwanted backflow. Siphoning has a long history, with references to siphon devices dating back to ancient engineers, and the term derives from the Greek word siphōn meaning tube or pipe.