Backyard Aquaponics
A place to discuss aquaponics

Dictionaries describe siphons incorrectly:

http://www.physorg.com/news192688538.html

I think that his correction is also wrong

"
Dr Hughes said that in fact water falling down one side of the tube pulled the water up the other side.
"The column of water acts like a chain with the water molecules pulling on each other via hydrogen bonds," he said.
"

They must be magic bonds, because air gaps in the fluid don't stop a siphon as long as there is enough water below to pull the gap through.

And there is another error in the definition as well. The length of pipe legs has nothing to do with it. The input doesn't need to be shorter than the other, I have used siphons that are the same length in and out, they just stop working when the head runs out.

I always thought things like dictionaries were infallible somehow.

I tracked him down and sent him an email. It might be nice to have a siphon dude on hand.

I would have thought "low pressure" would have something to do with the definition???

From the researching i did when I wrote an article about siphons for the BYAP magazine, it's really a combination of many factors including atmospheric pressure, but this only plays a fairly minor part in the forces which make siphons do their thing.

If you sealed off the source container it would certainly stop after a while, or gurgle as air came back up the tube like an upturned bottle of water.

No doubt the molecular attraction plays a part as well, but if you get a length of dry hose, put a pin hole in it, and lower it into a bucket of water so that water rises up both ends almost to the point they touch, then seal the hole, you have an easy test for molecular forces.

just start a siphon with it keeping you finger over the hole. The two sections of water have dry space with dry walls between them but you can still get the siphon going as long as your air gap isn't too big, and there is enough water below it on the exit side to drag it through. I get air in siphons all the time when I do the "jiggy jiggy thumb on hose" method to siphon fuel or something that doesn't taste so good.

Is there any other method....?

We've actually got some hoses with one way valves in them (read ball bearing). Makes siphoning really easy..

u a closet pertrol theif joel??

Not any more, it's out in the open now....

awww sorry!

yeah cool. I saw those once at a market. One of those ideas I really wish I'd come up with. I love the simple ones. Not that they made any real money out of it or anything, but it's just one of those inventions that makes you think "Arggh! Of course! What was I thinking with my 19 coat hanger hinged stirling engine petrol nicker alternative. The simple ones are always the best.

If you're talking about an air gap in the stream then maybe the bubble stretches and the vacuum formed in the bubble pulls the water behind, so the water stream is still connected with a stretchy bubble connecting it.

I like to think of the water as being like train cars connected by magnets, they will try to stay together but there is a limit if you put a wedge between the magnets (like air bubbles). Also it's easier to explain why you need either a longer train car below a hill to get it going and keep it going, or pressure from gravity above to push the train over the hill, or the train has to be already moving to keep it going if there isn't a pressure difference (from head pressure).

Continuing my train analogy maybe you can think of the water bonds as magnets and the air bubble as springs - if you put too long of a spring in there the momentum will collapse.

That's the way I've always understood it as well, but if the air gap is being stretched, it might just be a partial vacuum that's sucking the water up. And if that's the case the major contributing factor might just be localized lower pressure as DéjàVoodoo mentioned. That would put it back at least partly in the camp of the atmospheric air pressure folks.

I would say something like => gravity causes water to fall in the pipe causing a slightly lower pressure*, behind the falling water which in turn sucks more water through. Some friction and molecular forces contribute to the sum of stuff happening.

There appears to be quite a debate opened up with the parties being pro gravity or pro pressure.

I cant see it working any way other than gravity causes a low pressure. Low compared to atmospheric. We can see that as either sucking or blowing, because its really about difference caused by gravity.

So gravity causes pressure difference in the pipe, shifting water.

Or so I think

For some reason, I love siphons.


*than ambient atmospheric pressure.

the way i understand it, gravity or some other force eg. you sucking on the hose, primes the siphon. once water falls down the pipe sufficiently it creates a vacuum, which will immediately try to reach equilibrium with the outside atmospheric pressure.

so you have 2 forces at work, gravity with will continue to pull the water down the pipe, but also atmospheric pressure which is why you see a sudden burst of water when the siphon starts - gravity alone won't push down the pipe like that.

once the pressure inside the siphon is equal to the outside pressure, it stops (more or less).

fyi while some places state cohesion makes a siphon work, it only plays a very minor role. i think they sometimes confuse cohesion and hydrostatic pressure, which would also make a siphon work in the case of the 2 buckets 1 hose full of water experiment.