Syphon or Siphon - but the dictionary is wrong anyway. 6

[Artisi]

An interesting news item today in Australia's Sydney Morning Herald.
I wasn't sure where this would be best placed but as this forum is read by many who read and contibute to other forums guess this is as good as any.

http://www.smh.com.au/entertainment...oes-unnoticed-for-99-years-20100510-uoh2.html

 

[katmar]

The article was interesting enough, but the comments make even more interesting reading. It is sad to see how non-technical people regard the search for accurate knowledge as a waste of time and money.

According to Wikipedia, it is the "tensile stength" of liquid water that makes a water siphon work in a vacuum. I will have to think about that one. But somehow this "tensile strength" won't allow water siphons with a rise of 15m. Very confusing.

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[racookpe1978]

Ah yes. The ancient woe of facing two bad choices: Like the Greeks trying to sail the strait and narrow between Syphon and Charyllis ...

Either can sink a wit (er, ship.)

 

[25362]

It is generally assumed (Bernoulli) that the minimum pressure is at the summit of the siphon or just downstream, and if it equals the vapor pressure of the fluid in question the priming may be lost limiting the height of the siphon.

 

[IRstuff]

Sure, it's a form of tensile strength in play, when you suck the fluid up the piping to prime the siphon in the first place.

And it's related to the vapor pressure of the fluid at the temperature of operation, so if you had a fluid with a high vapor pressure, sucking on the piping would cause the fluid to vaporize, thereby preventing priming and siphoning.

TTFN

FAQ731-376

 

[hydtools]

Once again having a PhD does not mean you know what you are talking about or even understand how to research to gain knowledge.

Ted

 

[BigInch]

The tensile strength of water is rather important in nature: the height of trees is determined by water's tensile strength. Otherwise, trees would not exceed 33 ft in height.

Enjoy this,

http://www.aip.org/pt/feb00/maris.htm

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[hydtools]

Sucking does not pull the water. Sucking creates a pressure lower than atmospheric pressure at the other end, the unbalanced force then pushes the water up the pipe. Try pulling water uphill through a partially filled pipe.

Capillary action in trees depends on the surface tension of the water to pull the water up. When to tube is too large, the weight of the water is greater than force the surface tension can pull.

Ted

 

[BigInch]

Actually nobody said "sucking pulls up" the water.

**********************
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[hydtools]

Then I mis-read IRstuff.

Ted

 

[BigInch]

I know he wasn't implying that.
... nothing to worry about, I'm sure.

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch

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[BigInch]

trees on high mountains would only be 16 ft high.

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[25362]

Think about chemical potential as the driving force.

 

[btrueblood]

"tensile strength of water"

That is an analogy I don't think should be promoted (reworded: that's dumb!). Keep it up and we'll have people asking us if they can heat treat their water to improve its tensile strength. And then people telling us not to drink from pumped water systems, since the cyclic loading from the pump "fatigues" the water...

 

[BigInch]

You'd rather talk about "chemical potential" then?

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[btrueblood]

Oh yeah. Water has, after all, the chemical potential to be converted to, or mixed with, EtOH.

 

[jte]

No wonder I've been feeling tired! I've been drinking fatigued water! Back to beer for the energy boost I need!

jt

 

[cvg]

or mix water with CH3CH2OH, chill and enjoy

 

[BRIS]

A siphon works by the difference in pressure between atmospheric pressure (gravity) pushing on the water surface and the negative pressure caused by the water column on the downstream leg. It has nothing to do with water tension - a siphon will operate with air pockets. The dictionary is correct.

 

[katmar]

What drive the liquid through the syphon is the difference between the column pressures due to the different heights of the two legs, ie the well known

Pressure = [ρ] x g x h (Density times gravity times height)

So it is gravity that drives a syphon. The role of the atmospheric pressure is simply to ensure that the pressure at the apex of the syphon does not go below the vapor pressure of the flowing fluid. The atmospheric pressure is equal at the inlet and outlet of the syphon, so it cannot cause the flow.

I do no buy the "tensile strength" argument. I am not a botanist, but I suspect that the way a tree gets water to heights above 10 meters has more to do with the xylem capillaries working as millions of little pumps in series than to do with "tensile strength" of a liquid. I have seen that when you half fill a syringe with water and close the outlet with a finger you can easily withdraw the plunger further - all that happens is that the water boils and there is no "tensile strength" in evidence.

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