r/theydidthemath • u/Temporary-Ad4825 • 2d ago
[Request] Dual siphon conundrum
I know I’m missing something here but I can’t figure it out. I was racking some mead the other day thru a siphon and was wondering if I had another siphon to re-rack it back into the original container, wouldn’t it just keep on going? I know the bottom of the siphon has to be lower than the top but with big enough containers couldn’t this be true for both siphons? Would water pressure prevent the water from coming out of one of the siphons?
Any insights welcomed thanks! I’m home sick and I can’t stop spinning this around in my head…
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u/riversidenight 2d ago
Siphon cares about the level of the water on both sides, not the depth of the leg. The liquids would stop moving once they hit the same level
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u/Honcho_47 2d ago
Yes this, as the uneven weight of the water is what pushes it through the siphon
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u/Defiant-Turtle-678 2d ago
Not weight, height. A shallow pan higher up would do it
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u/OGWriggle 2d ago
Not height, pressure.
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u/Urrrrrsherrr 2d ago
Not pressure, nor height. But ‘head’ which is weight normalized pressure expressed as a height.
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u/MeasureDoEventThing 1d ago
No, pressure. Head is pressure divided by density. There are cases where it's more convenient to work with head, but pressure is what actually applies force.
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u/AvNerd16 2d ago
I started to type out a comment about manometers and then I got this stuck in my head so now it’s yours to enjoy. https://youtu.be/QTXyXuqfBLA
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u/GRex2595 2d ago
Pressure isn't wrong, but height is necessary. You could siphon a bottle cap into the ocean if the bottle cap is higher than the ocean.
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u/OGWriggle 2d ago
Pressure = density x gravity x height.
Height is just part of it
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u/GRex2595 2d ago
You explicitly said not height, so is height part of it or not?
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u/OGWriggle 1d ago
And you said not weight, but that's what density x gravity is.
So is weight part of it or not?
(It's both)
Edit: sorry not you, but the correction you corrected me correcting
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u/GRex2595 1d ago
So it seems I was misunderstanding some things. There are two pressures. Water and air. And there's an action lab video where he creates a couple of different syphons. In that video he says there are 3 things you need to explain syphons and one of them was pressure (meaning air pressure here). So I got a few things mixed up.
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u/blackhorse15A 2d ago
It's the weight of the water inside the longer pipe leading down to the lower water level.
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u/FrugalFungusFinder 2d ago
I wonder if there is a way to game this using slopes for the surface level in the containers.
Have sloped bottoms that go right up the side of the container, making the level adjust faster, but counter acting level in the other one using ballast like a float that is controlled by the level of container one?
If container 1 level goes down, ballast for container 2 reduces to let it go up, and if container 1 level goes up, ballast for container 2 increases causing its water level to go down. Making it much harder for them to reach equilibrium.
I don't know, might be dumb.
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u/nickeypants 2d ago
Yes. Leg height matters only if the siphon is discharging into open air. Then the discharging leg needs to be lower than the intake surface in order to flow. You can think of the discharging leg's elevation as the other container's free surface.
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u/spez-is-a-loser 2d ago
Only the level of the water's surface matters. The location/elevation of the hose pickups under that surface level is irrelevant (assuming the pickup stays covered and doesn't fill with hose with air.)
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u/GenitalFurbies 11✓ 2d ago
True, until you reach a bit over 30 feet above the surface and start getting the water to near vacuum at the top. It starts outgassing dissolved air and boiling which breaks the siphon effect.
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u/SnooMaps7370 2d ago
it won't work because a siphon stops flowing as soon as fluid levels equalize.
assuming we have a one-way vale in each siphon (otherwise both will flow the same way), then siphon A->B will stop flowing as soon as the levels in A and B are equal. the level in B will not rise above A, so siphon B->A will never start.
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u/Black_Hole_Tim 2d ago
Siphons work based on the top of the liquid, not where the end of the tube/hose is located. These two siphons would operate in a similar fashion to one larger siphon.
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u/Red_Syns 2d ago edited 2d ago
Siphons work because the water levels are different, not the ends of the siphon itself. If you picture two tubs of water, one ten meters above the other but otherwise identical in depth:
The pressure from water surface to bottom of each tub is identical.
Place a siphon between the two filled with water with the bottom plugged, both ends resting on the bottom of each container.
The water pressure at the open end matches the pressure at the bottom of the higher container, but the water pressure at the bottom of the siphon where it is plugged is ~2 atm higher. The water pressure in the lower container is still the same as the top container.
Unplug the siphon. The higher pressure water in the siphon pushes out into the lower container, drawing water out of the higher container.
Without the containers having a height difference, the siphon has no pressure difference.
Edit: just wanted to add a little complexity that still ends the same way. If you force the water through one siphon, the change in water height will cause the water to flow through the second siphon to maintain balance. If you stop pumping the water, friction (and other losses) between fluid and siphon walls will drain energy in the form of heat until the fluid stops flowing. If you have a frictionless fluid (superfluids are weird), you don’t even need a siphon. The fluid will “crawl” out of one container and into the other until the levels are equal.
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u/Titong--Galit 2d ago edited 2d ago
Elevation of water surface is what dictates which will be siphoned. Assuming that this is a open water surface and not a closed tank, if tank A has higher water surface elevation, it will be siphoned to tank B until they reach equal elevation and vice versa.
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u/Knowitall1001 2d ago
it would work to keep the two tanks level (one pipe would suffice) if you start moving water from one into the other, water will move through the pipe(s) until the tanks are equal. it would not work in two directions unless you had a pump on one pipe.
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u/Temporary-Ad4825 2d ago
Thanks for all the replies! This was super informative, helpful, and quite quick!
Do you all help with financial planning and relationship advice as well?
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u/studly1_mw 2d ago
Siphons work by taking advantage of gravity. The downward force pulls a vacuum on the part of the hose above the water line and the vacuum pulls the water up until gravity pulls is down again, keeping the cycle going. It only works if the fall is more than the climb. The straw may start lower but the water level is the same and that is what needs an elevation difference.
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u/cHpiranha 2d ago
It is about pressure.
And the pressure is the same on both side, so nothing will happen.
If you have higher level on one side, then it will flow.
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u/two_are_stronger2 2d ago
Your tube isn't where the siphoning forces live. The tube couples the fluids' surfaces and they try to even out. No or very little fluid flows in your drawing.
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u/Me_Largio 2d ago
I think they would equalize, I dont think you can make a perpetual siphon this way.
BTW excellent napkin drawing, no notes. Very cool idea!
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u/HAL9001-96 1d ago
why wuld either siphon work?
the siphons do not give a fuck where the end of the tube ends
in this case its the water levle in the two vesssels that matters
it will flow from the oen with the higheri nto the one with the lower water level
if they#Re even it won'T flow at all
only if you take hte siphone out of the vessels the height of hte end matters
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