For the carbonation device, see soda siphon. The word siphon (from “pipe, tube”, also called syphon) is used to refer to a wide variety of devices that involve the flow of liquids through tubes, see siphon terminology. By definition the word refers specifically to a tube in an inverted ‘U’ shape which causes a liquid to flow upward above the surface of a reservoir, with no pump but powered by the fall of the liquid as it flows down the tube under the pull of gravity, and discharging at a level lower than the surface of the reservoir whence it came. Note that while the siphon intake tube must be below the surface of the liquid in the higher reservoir, it need not touch the liquid in the lower reservoir and indeed there need not be a lower reservoir; a siphon can discharge into mid-air so long as the exit is below the surface of the upper reservoir. Siphons are generally governed by Bernoulli’s principle. This is simply a restatement of conservation of energy principle in that the sum of all energy at any given point in the siphon flow is constant. Because the reservoir generally has a constant energy per unit volume (the sum of pressure energy and gravitational potential energy is constant), the outlet must also have the same energy. The outlet has lower gravitational potential energy due to the height difference and increased kinetic energy (or dynamic pressure). Siphons share many characteristics with other fluid systems such as manometers. Specifically, all the principles regarding hydraulic head, including the limitations on lift of the working fluid, are valid (i.e. the fluid pressure can’t be negative at any point in the system so there is a maximum energy tradeoff between fluid pressure and gravitational potential energy). Unlike a manometer, which operates as a measurement of two pressures in static balance (often the atmospheric pressure and vapor pressure of the liquid), a siphon is in balance when the inlet and the outlet are at the same level. The gravitational energy, pressure energy and flow energy are the same at inlet and outlet when they are in static balance in a siphon whereas in a manometer, gravitational potential energy and pressure energy are exchanged until static equilibrium is achieved. When the outlet of a siphon is lowered from the static case, gravitational potential energy decreases and is converted into a fluid pressure difference and velocity of the fluid to satisfy conservation of energy. The flow and energy change at the outlet propagates back through the fluid at the acceleration of gravity in the fluid as the energy at any point must still be equal. The conditions at the inlet are unchanged, so all the energy changes in the siphon fluid are in pressure of the fluid and its velocity. Consequently, a siphon can be described as the sum of various types of head that represent the total energy in the system. The equation is written between any two points a and b in a system that contain the same fluid. : where: p = pressure of the fluid γ = ρg = density·acceleration of gravity = specific weight of the fluid. v = velocity of the fluid g = acceleration of gravity z = elevation = pressure head = velocity head