The height limit of a siphon | Hacker News Reader

The height limit of a siphon

nature.com

49 pointsmonort17 comments

A longer summary: the maximum height of a siphon pumping water, at sea level, predicted by barometric pressure is about 10m.

But the tensile strength of water is nonzero (a surprise to me) and that is enough to allow much higher siphons if you first remove dissolved gases from the water.

The paper speculates that several hundred metres may be possible, perhaps even more. The paper also describes an actual experiment where they showed that a 15 metre high siphon works in practice.

What are the practical implications of higher siphons?

IANA fluid dynamics expert, but it seems to me that for certain designs which move water by creating a lower pressure at some point inside of a pipe you've created a "siphon" in a general sense. If that's true, this could affect certain water pump and plumbing designs, albeit this degassing process may be prohibitively expensive.

Closed circulation central heating systems are degassed. Though, the water is hot there so boils much more easily. Maybe closed circulation cooling water systems...

Its more than this - its that the model of balancing the weight of water with the vacuum pressure being the siphon limit is not correct.

This paper shows an alternative model is more likely to be correct. It is that the water boils easily at low pressure, and that is the limit of siphon height.

By careful preparation (degassing), boiling is prevented. Which here allows the column to be raised about 10m.

A 'law of physics' has been modified. This is pretty cool!

Relevant xkcd what-if: http://what-if.xkcd.com/143/

Except he has gotten it wrong hasn't he?

Which I guess is why this is trending.

His note [5] links to https://www.youtube.com/watch?v=8F4i9M3y0ew which is a demonstration of a siphon in a vacuum, so he's covered :)

But working "a little bit" as the note says is a good way off from working an order of magnitude or two better.

> Sodium action figurine collection

Randal surely has some interesting friends.

My summary: the limit is much higher than can be predicted by barometric pressure because of the tensile strength ("surface tension" of water)

So this is why I couldn't siphon (gray) water from my bathtub on the third floor to water plants in the yard, not lack of moral fortitude. Great!

No, unless the water had to rise 10m from your bathtub before descending. If it only had to rise enough to climb through a window before descending to your yard, then that is not enough to produce the problem in the article.

The problem described is that water cannot be lifted up more than 10m (due to air pressure pushing on the bath side not exceeding the weight of 10m of water in the hosepipe), not pulled down on the far side.

yet but no..

1. bottom of siphon outlet has to be sealed by the surface of water held in a container.. and

2. and height of long side must less that atmospheric pressure in height from said surface of water

or water column on down side will break and draw will be lost

But the bottom side of the siphon doesn't have to be the bottom of the system. Run the tube up 1m to the window and back down out the window perhaps 3m to a bucket on the porch roof - an easy, simple, working siphon. From that bucket, you can use a regular positive-pressure bottom drain to go down as far as you like.

I love this classic science. Theoretically this experiment could have been performed in the 19th century!

This is why water is pumped from below pushing up rather than pulling from the top. But perhaps this will open up new ways to make water pumping more efficient.

Reply on news.ycombinator.com