Concept

Hydrostatics — where it appears

The mechanics of a fluid at rest, in which pressure varies with height alone and nothing depends on a velocity. It is the right model for a siphon before it starts and the wrong one after, which is where several durable confusions about siphons begin.

Named by 3 essays across one field — each of them below, with the objects they name alongside it.

The one calculation in which the atmosphere really does push. How high a partial vacuum inside the tube will raise the liquid, against the absolute pressure achieved inside it. The lift is (p_atm − p_inside)/ρg and it is capped at the barometric height of 10.11 m, because below the vapour pressure the liquid boils and pulling harder buys nothing. The horizontal lines are five crown heights: a crown below a line's intersection with the curve can be primed by suction and one above it cannot, at any pump. This is the process the running siphon's argument explicitly excluded, and it is the one where the atmospheric account is the mechanism rather than a limit.

The one place the atmosphere pushes

The height of a siphon's hump is not in its flow rate, and the atmosphere holds the column together rather than driving it. That account excludes one process by name — starting. That is the process the atmospheric account describes correctly, and it is the only one in the whole device.

misconceptions · Siphon

The source falls 6 m and the crown pressure does not move. The pressure at the crown of a draining siphon, against time, as the source level falls from the top of the tank to the end of the run, a drop of 6 m. It is flat to 1.5e-11 pascals — not nearly flat, exactly flat, because the two effects of a falling source cancel identically. Losing a metre of level shrinks the drop, which slows the flow and raises the crown pressure by half a velocity head; and it grows the rise, which lowers the crown pressure by ρg per metre. Those are the same number. So a siphon that starts will not break as it drains, however far the level falls, and the run ended because the level reached the outlet instead.

The siphon that does not break

A draining reservoir shrinks the drop and grows the rise at the same time, and the siphon's own coupling — a metre of extra drop costs a metre of hump — says a siphon should break as it empties. It does not. The two effects cancel exactly, and the crown pressure of a draining siphon is a constant that does not contain the source level at all.

misconceptions · Siphon

100 metres of water, −1.98 MPa absolute at the top. The absolute pressure up a transpiring column 100 metres tall, with the sap rising at 0.25 mm/s through conduits 40 µm across. It starts at 1.3 kPa at the root, falls by 9.79 kPa a metre for gravity and 10.02 for friction, and reaches −1.98 MPa at the top — below zero, which is not a low push but a pull. The pale line is the same column with nothing flowing. The floor is not the vapour pressure but the pore a gas bubble could be drawn through: −2.81 MPa for a 50 nm pore, which this column would reach at 142 metres. A suction pump lifting the same water from a free surface stops at 10.1 metres, because it offers the water somewhere to boil.

Where a liquid does pull

A fluid cannot pull, and the essays that settle what suction is are right about every gas and every liquid with a free surface near it. A liquid with nothing in it to boil on is another matter. Every tree taller than ten metres depends on the difference, and the floor under it is set by the size of a pore rather than by the vapour pressure.

misconceptions · Suction

Named alongside it

The objects these essays reach for when they reach for this one.

Absolute pressure Barometric height Cavitation Misconception Model limit Vapour pressure Initial condition Siphon Suction Energy equation Liquid tension Poiseuille flow

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