The debate over the mechanisms responsible for the flow rates of simple tube siphons (Fig. 1) has received much attention in the physics education and general physics literature in the past decade. Particularly with regard to the driving mechanism for water siphons, some suggested explanations emphasize contributions, or lack thereof, from the atmospheric pressure on the tank supply-side, while other sources argue for, and against, the pulling effect of the weight of the water in the long side of the siphon, in a chain-like action via intermolecular forces, as the dominant mechanism driving the siphoning action. What is more, the atmospheric model has recently become the principal competitor to the chain pulling model, neither of which in our assessment supplies the essential explanation of the dominant driving mechanism of simple tube siphons, nor in any way accounts for their near steady-state flow rates.

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Siphon uses atmospheric pressure
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A.
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and
P. M.
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A.
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S.
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J.
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The height limit of a siphon
,”
Sci. Rep.
5
(
2015
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[see
P. A.
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,
F.
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,
M. C.
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)]. And what is more, there are two liquid phases of water—the high density normal phase (HDL) with low cohesion and high diffusivity, and a low density (LDL) high cohesion low diffusivity phase, the latter of which is dominating at normal temperatures and negative pressures
[see
A.
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and
L. G. M.
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6
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8998
(
2015
)]. These reports strongly suggest that exotic conditions exist in extreme height water siphons that challenge their explanation employing the fundamental principles of classical fluid mechanics. We see no need to venture out into what is today a no man’s land of fluids transport theory to explain normally operated siphons.
6.
We identify normal water tube siphon operation as having ambient pressures of 0.5 atm or greater throughout, which allows for a siphon height above the supply tank of 5 m at sea level. Note that water siphons do indeed operate continuously under more exotic pressure conditions, particularly at or below the edge of the HDL phase transaction (see Ref. 5 discussion). An example of such water siphon behavior is reported in
S.
Hughes
and
S.
Gurung
, “
Exploring the boundary between a siphon and a barometer in a hyperbolic chamber
,”
Sci. Rep.
4
(
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R.
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,
D.
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Negative pressures and the first water siphon taller than 10.33 meters
,”
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4
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2016
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Siphonic concepts examined: A carbon dioxide gas siphon and siphons in vacuum
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(
July
2011
). (Note: The neglect of the intermolecular forces, and hence the eliminating of a chain pulling model in CO2 gas, as put forth in Ramette and Ramette and here, is justified by the fact that the correction to the pressure term in the van der Waals equation of state for CO2 gas at NTP, which is reflective of the effects of intermolecular forces, is less than 1%).
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W. D.
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and
B.
Stanchev
, “
Towards explaining the water siphon
,”
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52
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10.
In addition to the Borda’s Mouthpiece referencesin Ref. 9, see:
A.
Jenkins
, “
Sprinkler head revisited: Momentum, forces, and flows in Machian propulsion
,”
Eur. J. Phys.
32
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2013
(
Sept
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2011
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366
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A.
Fenghour
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W. A.
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V.
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The viscosity of carbon dioxide
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44
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1998
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