overflow and syphon is there a limit? - The Planted Tank Forum
Old 05-02-2008, 10:52 AM Thread Starter
Guest

Join Date: Feb 2008
Posts: 317
overflow and syphon is there a limit?

Hi there,

with a DIY overflow is there a limit as to what height the siphon will work to? was going to use one in my wifes frog tank in which case one side of the overflow would be about 30-40cm into the tank and the rear well to be honest I don't know where to put the back one. So will the siphon still work?

Regards Darren
duzzy is offline

Old 05-02-2008, 02:30 PM
PT Biologist

Join Date: Oct 2003
Location: Maryland USA
Posts: 3,220
What kind of overflow are you talking about? If it is the PVC constant level siphon then, yes, theoretically, it would work. Practically, the siphon height of the overflow would have a 'stronger vacuum' which would cause gases in the water to come out of solution and form a bubble in the tube which would impede and could eventually cut off the flow.

You might be better off having the tank drilled at the level you want the water to be at.

Sean

Aquascape? I'm a crypt farmer.

It's a fine line between fishing and standing on the shore looking like an idiot.

That IS an aquascape, it's titled "The Vacant Lot".
SCMurphy is offline
Old 05-02-2008, 07:05 PM
Planted Tank Guru

Join Date: Sep 2007
Location: Hiawatha, IA
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The higher the back piece the less it will flow due to gravity. I've made plenty of DIY overflows from PVC and the lower the exit on the outside the better the suction and flow will be. Flow is also equal to the diameter of the pvc and the power of the pump pumping.

Craig
Craigthor is offline

Old 05-02-2008, 10:58 PM
Planted Member

Join Date: Feb 2007
Location: NW Oregon
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The flow through a siphon is determined by the difference in height between the water surface in the tank, and the height of the outlet. It's the difference in atmospheric pressure between these two points that "pushes" the water through the siphon. The outlet must be lower than the water surface.

It doesn't matter how far below the tank surface the inlet is placed. Nor does it matter how high the apex of the siphon tube is above the tank surface, to a point: atmospheric pressure cannot push water higher than about 10 meters above the tank surface.
PDX-PLT is offline
Old 05-03-2008, 06:12 AM
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Theory.
Quote:
Originally Posted by PDX-PLT View Post
The flow through a siphon is determined by the difference in height between the water surface in the tank, and the height of the outlet. It's the difference in atmospheric pressure between these two points that "pushes" the water through the siphon. The outlet must be lower than the water surface.
Practical application.
Quote:
Originally Posted by PDX-PLT View Post
It doesn't matter how far below the tank surface the inlet is placed. Nor does it matter how high the apex of the siphon tube is above the tank surface, to a point: atmospheric pressure cannot push water higher than about 10 meters above the tank surface.
Only problem is that the weight of the water in the down tube causes the suction which is greater, the longer the drop. I.E. if you have a siphon with a max height of 5 inches over the upper reservoir, but change the down tube length from 1 foot to 2 feet, because of water's cohesion you increase the weight of water pulling down and increase the flow in the tube. This effect also has a maximum like the height. The siphon will form a slight reduction of atmospheric pressure inside the tube leading to gas degassing from solution and forming a bubble. The bubble takes up space at the top bend and impedes flow. If it gets too big in a longterm controlled siphon, like the constant level siphons, it can stop the flow. Air pressure forms the siphon, gravity runs the siphon.

An analogy to understand siphons is to imagine a long, frictionless train extending from a plain, up a hill and then down the hill into a valley below the plain. So long as part of the train extends into the valley below the plain, it is "intuitively obvious" that the portion of the train sliding into the valley can pull the rest of the train up the hill and into the valley. What is not obvious is what holds the train together when the train is a liquid in a tube. In this analogy, atmospheric pressure holds the train together. Once the force of gravity on the couplings between the cars of the train going up the hill exceeds that of atmospheric pressure, the coupling breaks and the train falls apart.

Sean

Aquascape? I'm a crypt farmer.

It's a fine line between fishing and standing on the shore looking like an idiot.

That IS an aquascape, it's titled "The Vacant Lot".
SCMurphy is offline

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