Physicist unmasks 99-year-old mistake in English dictionaries

The Register ^

[Gomez]

An Australian physics prof has discovered a 99-year-old error in the Oxford English Dictionary - repeated in most dictionaries worldwide - and is having it corrected.

The error is in the definition of the noun "siphon", a tube used to draw fluid from a higher location to a lower one - as when emptying a vehicle fuel tank, an aquarium or other vessel difficult to empty by other means.

Liquid is, of course, drawn up the shorter limb of the siphon by the weight of that in the longer downward one: thus the operating force is gravity. However most dictionaries follow the OED in stating that atmospheric pressure drives the process.*

Dr Stephen Hughes of Queensland University of Technology noted the error after visiting a massive siphon project in South Australia which was being used to transfer gigalitres of water into a depleted lake.

On returning, the prof decided to write an article about the siphon for use by school science teachers, and discovered to his dismay that most dictionaries described the process wrongly.

"An extensive check of online and offline dictionaries did not reveal a single dictionary that correctly referred to gravity being the operative force in a siphon," grumbled the physicist.

The OED currently says:

A pipe or tube of glass, metal or other material, bent so that one leg is longer than the other, and used for drawing off liquids by means of atmospheric pressure, which forces the liquid up the shorter leg and over the bend in the pipe.

"The OED entry for siphon dates from 1911 and was written by editors who were not scientists," explained Margot Charlton of the Dictionary's staff. Amazingly, it seems that in 99 years nobody had ever queried the definition.

The next edition of the OED will be corrected.

According to Hughes some encyclopaedias - though not the Encyclopaedia Britannica - repeat the error. The doc has written a paper with more detail on siphons which the interested can read here.

Bootnote

*This may be true during the process of starting the siphon off, which is usually done by creating a temporary suction on the outflow end of the pipe so as to draw fluid up and over the hump. This works by the action of atmospheric pressure on the surface in the to-be-emptied vessel: but once the siphon is flowing this force is countered by atmospheric pressure at the other end of the pipe.

[UnbelievingScumOnTheOtherSide]

There are places in the country where that aquifer is under pressure, and you don’t even need a pump to get the water.

That's called an Artesian well.

[BitWielder1]

OK, I’ve thought about your drawing a bit. The pressure is required to keep the tube filled with water, but it’s not responsible for the flow of the siphon.

Once the water splits, it will stop flowing.
Same thing can happen you get too much air into the hose.

[Cyber Liberty]

And I bet if you propped the center of the hose over 33 feet, it will split the water and stop the flow. So...I have to agree pressure is required to keep the water from splitting, provided the tube is large enough for capillary action to make the water “stick” to the tube.

My point is, the pressure is not required for the water to flow. But I can see how you need it to keep the water from splitting into the two sections of the tube.

There are two things at work: Keeping the water in the tube, and making it flow from a higher container to a lower one. So, odd as it seems, we’re both correct.

[bitterohiogunclinger]

“So a siphon requires both gravity and atmosphere.”

Not according to scientists that have experimentally done it in a vacuum. Everyone keeps confusing atmospheric pressure and hydrostatic pressure. The reason the siphon breaks down if you exceed the magic “33” feet has nothing to do with atmospheric pressure pushing anything up the tube, it has to do with degassing of dissolved gasses breaking the siphon.

[Cyber Liberty]

provided the tube is large enough for capillary action to make the water “stick” to the tube.

provided the tube is too large for capillary action to make the water “stick” to the tube.

[Richard Kimball]

Alternately, one Atmosphere is roughly equal to 34 feet of water. Try to create a siphon that can lift water higher than 34 feet before it makes it's turn downwards. If the siphon action stops when the lifted column of water exceeds 34 feet, then atmospheric pressure is the root cause of the siphon action.

To take water up a tube that does not have an effective prime requires drafting. Drafting is driven by atmospheric pressure. The maximum theoretical lift at sea level is roughly 33.8 feet. A column of water 1 foot tall exerts .434 psi at the base. 14.7/.434 gives us the 33.8. Siphoning, OTOH, is not dependent upon atmospheric pressure, but on the vacuum created in the hose when water drains out the other end.

If you had to draft to lift your water to start the siphoning process, there would be a maximum theoretical lift. I do not know what the results would be if a 100 foot hose were plugged and lifted over a 35 foot wall then back to below the level of the water in the original container, but I suspect it would still siphon.

[Cyber Liberty]

I wonder if that would happen if I used distilled water?

[LonePalm]

A siphon WILL work in a vacuum provided that the tube is filled prior to flow beginning.

Garde la Foi, mes amis! Nous nous sommes les sauveurs de la République! Maintenant et Toujours!
(Keep the Faith, my friends! We are the saviors of the Republic! Now and Forever!)
LonePalm, le Républicain du verre cassé (The Broken Glass Republican)

[PeterPrinciple]

Who gives a crap and who uses a syphon?

---

might be useful information when the revolution starts..............

[BitWielder1]

Not according to scientists that have experimentally done it in a vacuum.

I'd like to see that study. Do you have a pointer to it?

Everyone keeps confusing atmospheric pressure and hydrostatic pressure.

We are talking open containers so the HP starts at AP and decreases as we go up the tube.
In the case of water, HP can not go negative, we will get a low pressure steam bubble instead and the liquid will stop flowing over the bend.

[Yo-Yo]

If you had to draft to lift your water to start the siphoning process, there would be a maximum theoretical lift. I do not know what the results would be if a 100 foot hose were plugged and lifted over a 35 foot wall then back to below the level of the water in the original container, but I suspect it would still siphon.

Take a 100 foot hose, fill it with water, and plug one end. Lift that one end, and by the time you have the entire 100 feet of hose in the air, you'd have 34 feet of water in it, and 56 feet of collapsed hose. Can we agree on that?

How does the water flow in this example (shameless ripped off from Wiki) when the plug at D is removed?

What moved the water up the short leg? We then agree that atmospheric pressure had something to do with initating the siphon effect, and the tensile strength of water had zero effect in initating the siphon effect.

Therefore, if the atmospheric pressure effect was so powerful to initiate the siphon effect, it still has that same power to maintain the siphon effect. In most practical siphons, the atmospeheric pressure force greatly overwhelms the tensile strength force in making the siphon work.

Another example of this, also ripped off from Wiki, is the siphon where the short up leg has a diameter much greater than the downard leg diameter, yet the siphon still functions.

Since the weight of water in the shorter up leg exceeds the weight of water in the longer down leg, it is not tensile strength and gravity that is moving the water, it is atmospheric pressure moving it, again pointing to the main force in most practical siphons.

Yet 300 foot sequoia trees prove that tensile strength can move water.

It's a candy. It's a breath mint. Wait, you're both right!

[Oztrich Boy]

OED: A pipe or tube of glass, metal or other material, bent so that one leg is longer than the other, and used for drawing off liquids by means of atmospheric pressure, which forces the liquid up the shorter leg and over the bend in the pipe.

Is correct, Siphon would not work in a vacumn, or if the shorter leg was more than 34' at normal Atmospheric pressure.

[Eaker]

100 feet of hose in the air, you'd have 34 feet of water in it, and 56 feet of collapsed hose. Can we agree on that?

Nope.

[Yo-Yo]

Why not, and please then explain to me how a barometer works.

If you had a 100 foot rigid pipe, fill it with water, cap both ends, stand the pipe straight up, remove the lower cap, all but 34 feet of water would come out of the bottom of the pipe.

Increase the atmospheric pressure, you can increase the height of supported water. Decrease the atmospheric pressure, you decrease the height of the atmospheric pressure.

We haven't even gotten to the siphon part yet, and we've limited ourselves to supporting a column of water equal to the atmospheric pressure on the bottom of the column of water.

[Eaker]

56 + 34 = 90.

[Yo-Yo]

LOL, that’s my public edumakation showing!

[Eaker]

LOL!

[sima_yi]

I stand corrected. I hadn’t considered the molecular attraction between the water molecules. That’s what I get for posting before thinking.

[BitWielder1]

A paper written by Hughes: http://eprints.qut.edu.au/31098/8/31098a.pdf

Ah, a Chain Siphon. That's different. It would work in vacuum.
If we had a liquid that could stay intact at negative pressure, that would work too.
Except for a relatively tiny amount of molecular attraction, I don't think we have that.

[Ditto]

Who gives a crap and who uses a syphon?

I do it all winter to get the water off the top of my swimming pool cover. ;~))