back to article Physicist unmasks 99-year-old mistake in English dictionaries

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 …

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Pint

Damn

Why didn't I think of that. I could've been famous, won a Nobel Prize, been immortalised in science books to come, had huge amounts of money, and finally proven that you don't have to be a scientist to be intelligent. Right, I'm off to browse the dictionary to find other errors and have them corrected.

Beer, cos I would like to use gravity to siphon it into my liver as the little ascii diagram below deomntrates

Beer

Table ---

| < pipe

Me lying down

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Anonymous Coward

Yes But.....

Does it explain how to get the taste of petrol out of your mouth?

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Coat

taste

How about dookie?

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But would it work in a vacuum?

Apart from a bit of surface-tension, atmospheric pressure is an important part of the process. because as the liquid travels down the long tube, it creates vacuum behind it. Air pressure on the liquid reservoir encourages the liquid up the short tube to fill the vaccum.

Another interesting test is whether it is possible to siphon water up a hundred feet (that's 100ft above the surface of the reservoir), over a wall and down a thousand feet the other side. I bet it would manage to get up about 33ft.

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My thoughts exactly

...no siphon on the Moon I would think.

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Anonymous Coward

how about

I start the siphon off with a pump instead? The liquid exitting into the lower (destination) reservior will have directly pulled more into the tube from the source without any requirement for air.

I doubt the gigalitre siphon in South Australia has got some bloke sucking on a tube on the promise it's beer at the other end.

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I bet it would manage to get up about 33ft

Lucky guess or did you do some research?

The limiting factor in terms of how high you can raise the water from the upper reservior before the drop down the long lenght of tube is the vapor pressure for the liquid.

The higher the siphon goes, the lower the pressure of the liquid in the tube at it's highest point, if the pressure gets low enough, bubbles form in the water and the siphoning effect is lost.

For water, the vapor pressure point when at normal atmospheric air pressure is about 10meters/33ft.

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Happy

Work in a vacuum? - Yes and No

The first question you should ask is "what is the vapour pressure of my liquid?" and this should tell you whether it would evaporate (or more likely freeze and then sublime) in the vacuum. Most liquids would be creating a localised "atmosphere" as they evaporated away into the (near) zero pressure of the vacuum which is likely to be below the vapour pressure of the fluid at whatever the local temperature is.

The force driving the siphon is gravity, surface tension is more likely to be important in a capillary tube, these can achieve spectacular lift heights without any siphon at all and are popular in nature.

The vapour pressure leads to the answer about how high a siphon tube you can use, the 100ft high pipe would be likely to reduce the local pressure of the liquid at the top to below the vapour pressure at which point the water would "boil" inside the tube. This would cause the siphon to fail by effectively air locking the syphon. The process is that the masses on each side of the trapped gas section would end up balanced against each other as if they were on a pair of scales. Only when the fluid passes over the top with sufficient density (i.e. generally not as a gas) does gravity do the job for us.

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Water in a vacuum

You can't have water in a vacuum, it would boil. Same for a 33' column of water, it would boil.

Atmospheric pressure is an important part of the process, it keeps the liquid from boiling.

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Anonymous Coward

What if...

... you have a 1100ft flexible tube, start the siphoning with the bend just above liquid level, then slowly raise the bend up to 100ft?

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No guess

It's just one of those numbers I happen to remember. 33ft underwater the pressure is doubled, and I remember the old barometer stuff from when physics lessons had real physics in them.

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Re : Water in a vacuum

Separate the thermodynamics from the mechanism

Gravity drives the thermodynamics - the potential energy lost is the product of the mass of liquid moved and the height difference between the reservoirs.

The mechanism requires that the 'hump' over which the liquid has to climb is such that the liquid will not form a vapour bubble at the highest point - this point will depend on the atmospheric pressure. With a high-boiling liquid and a low hump the 'outside' pressure could be quite low. Practically this would require the entire app. to be a partial vaccum chamber

In a normal environment, of course, atmospheric pressure is related to gravity.

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What if... no luck

When you got close to 33' the water would boil and the siphon would fail.

Unless you were in a VERY big diving bell over 66' down...

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Boffin

Nonsense!

Of course it requires atmospheric pressure!

There is no way for the water in the downhill slope of the siphon to 'pull' water upwards in the initial up-hill part, and for that part you NEED the external atmospheric pressure to instigate a pressure gradient in the initial part of the siphon.

Sure, it's gravity in the 'downhill' part of the siphon that exerts a partial vacuum in the upper part of the siphon, but without the external pressure it wouldn't work.

The linked document is also full of rubbish. It insinuates that the cohesion required to 'pull' the water up the hill comes from hydrogen bonds! (snort!)

It would be very easy to install a manometer in the topmost part of the siphon and show how the pressure changes.

Also, it's childs play to show that you can't suck water more than 10m uphill, a figure that maybe just happens to coincide with the atmospheric pressure? Please!?!

Even the wikipedia gets it right, unlike this Oz PhD. (Wonder what mail-order university he got his certificate from.)

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Aquarium

Forgive my ignorance, but submerging a hose then just sticking your finger over the end until the hose is below the tank water level works just as well as sucking.

Wouldn't that be a mechanical process rather than atmospheric?

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Anonymous Coward

Incorrect

You're talking about the mechanism for lifting water to the high point of the siphon prior to its operation, not the action of the siphon itself. If you are lifting the water by suction, atmospheric pressure pushes the water up the siphon as the pressure at the other end is reduced. But that isn't what the article is about.

Note that is is not necessary to start a siphon in this way. If the column of water in the siphon is entirely submerged and then one end is moved away to a point lower to the surface of the liquid (keeping the column sealed), the siphon will start working irrespective of the height differential.

And the reason the reason the siphon works - as this article is pointing out - is entirely due to gravitational force. In simple terms the mass of water in the column on one side exceed the mass of the water on the other side, and hydrostatic pressure at the top (caused by the gravitational force on both columns) will draw the water up the shorter column.

BTW - Could we all aim to be a bit gentler in our postings?

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Nope, still atmospheric

Nope, it's still atmospheric. When you lift up the tube you are applying a force to the tube to lift it, but you are not applying any force to the water inside the tube. It is atmospheric pressure that is lifting the water as you lift the tube.

If there was no atmospheric pressure the water would just run out of the tube as you lift it.

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Anonymous Coward

Don't see the connection

A key point seems to be whether a liquid can act as a chain, i.e. can you pull something with it. It seems to me that you can. Note how a syringe is used to draw liquids from containers. Imagine two syringes fused end-to-end with quantity of liquid in each of them (forming a continuous column, i.e. no air bubbles). If you pull on one syringe out, the other will be sucked in (and vice-versa). If two people pulled at both ends, the stronger would pull the weaker forward. The enclosed liquid is acting just like a chain, and air pressure will have no bearing on this process whatsoever.

The siphon is working in the same way. Gravity is pulling on both columns of water (like a syringe at both ends) and the highest column will have the greatest pull.

I'm guessing there is some limitation, however - if the pull is great enough, presumably the liquid will change state to a gas. Comments?

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Right...

"""When you lift up the tube you are applying a force to the tube to lift it, but you are not applying any force to the water inside the tube."""

So you're saying that you can move a container by applying a force to it, but that force doesn't transfer to the contents, a generic 'atmospheric' force just happens to cause all that mass to accelerate in exactly the same direction you move the container?

Everyone seems to be an engineer today... Atmospheric pressure exerts force on open surfaces of a fluid, normal to the surface of that fluid (happens to be parallel to gravity in most cases.) Atmospheric pressure doesn't change much over distances that you could reasonably use a siphon (partial pressure of water would limit you to 10m, as mentioned above,) so the atmospheric forces on both bodies of water (one higher and one lower) is the same, and they cancel out.

And hydrogen bond /are/ important (someone argued that point earlier,) otherwise water would have a significantly lower partial pressure, and likely not be a liquid at any sort of reasonable temperature.

Now if you haven't taken a fluid statics class, please stop sharing your special versions of physics.

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Two syringes

Suction is an illusion caused by the push of atmospheric pressure. In your example, if you pull the end of one syringe out, that reduces the pressure inside the syringes, allowing atmospheric presssure to push the other end in. Do this in a vacuum and the other syringe end won't move.

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Anonymous Coward

Re: Two syringes

Quote: "Suction is an illusion caused by the push of atmospheric pressure [...] Do this in a vacuum and the other syringe end won't move"

This will still work in a vacuum (assuming the syringe is strong enough to maintain its structural integrity). A syringe doesn't draw liquid because the atmosphere is pushing it. If you're in any doubt, part-fill one, put the end against your skin, and pull. What you will is suction from the syringe (pulling), not pressure from the atmosphere on the other side (pushing). Remember, atmospheric pressure will be constant on all sides of the syringe irrespective of movement, so it has no bearing on its action. Also, bear in mind that the atmosphere is a gas, which (unlike liquid) can readily expand or compress.

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Syringes and suction

"put the end against your skin, and pull."

Ah, but it's still pressure. This time, it's the pressure of the inside of your body pushing into the decreasing pressure inside the syringe. Your body's internal pressure is normally balanced against the pressure of the atmosphere outside. As you draw the air out of the syringe, it's internal pressure becomes less than this, so your skin is pushed in, by your own body.

This effect can also be seen if you seal a container of hot air and pour cold water down it's outside. The container will constrict as the air inside has a lower pressure when it cools, so the air pressure outside is no longer balanced, pushing in on the container.

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MnM

bootnote

as the credit expert gazillionaire would say, 'they know thaaat!'

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Shorter OED seems OK

My Shorter (two-volume) OED says:

"A pipe or tube used for conveying liquid from one level to a lower level, using the liquid pressure differential to force a column of liquid... etc". It doesn't specifically say gravity, but that's what causes the pressure differential, right?

It never occured to me that the Shorter would use different definitions for a word than the Compact...

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New Oxford American Dictionary

The NOAD (2nd edn), which is bundled with Mac OS X, has the following, correct, definition:

siphon |ˈsʌɪf(ə)n| (also syphon)

noun

a pipe or tube used to convey liquid upward from a container and then down to a lower level by gravity, the liquid being made to enter the pipe by atmospheric pressure.

• Zoology a tubular organ in an aquatic animal, esp. a mollusk, through which water is drawn in or expelled.

verb [ trans. ]

draw off or convey (liquid) by means of a siphon.

• figurative draw off or transfer over a period of time, esp. illegally or unfairly : he's been siphoning money off the firm.

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Paris Hilton

Siphon...

...Paris could put the theory into practice...

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Headmaster

Or perhaps...

... nobody else really gave a monkey's...!

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You're oversimplifying

If a siphon doesn't depend on atmospheric pressure, why can't you siphon water up more than 10m? Clearly gravity is the source of the energy, but it works by the weight of the long side reducing the pressure at the top of the siphon so that the liquid on the short side is pushed up by atmospheric pressure.

It is reportedly possible to siphon in a vacuum if the liquid is thoroughly degassed, relying on the tensile strength of the liquid, but this is not the main mechanism in normal siphoning.

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A tissue

"It is reportedly possible to siphon in a vacuum if the liquid is thoroughly degassed, relying on the tensile strength of the liquid".

This is why you can siphon over small heights with a tissue. The tensile strength of water causes capillary actions between the fibres which will "suck" the water up a little bit, hopeful enough to get over the barrier and to allow siphoning to begin.

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Joke

@Richard Tobin

Oh how I wish gravity was a source of energy, no need to burn nuclear or fossil fuels, it'd be just great!

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isn't atmospheric pressure also necessary?

Gravity makes the fluid flow out of the longer leg. But why doesn't a vacuum form inside the tube, above the level of the fluid in the upper container? Isn't that because of external atmospheric pressure?

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Anonymous Coward

Surely...

As the liquid falls out of the longer part of the tube, it creates a vacuum / lower pressure in the tube, and it is in fact the air pressure pushing the liquid into the tube from the top tank?

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Not strictly a dictionary...

"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,"

...but read all about it on WikiPedia: http://en.wikipedia.org/wiki/Siphon

Also answers why 10m for water is the limit, under what circumstances a siphon will work in a vacuum and what role exactly gravity and atmospheric pressure play.

I know it's in to bash the wikifiddlers around here, but give them some love when it's due. :)

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Bollocks Reg

You're insulting engineers by publishing this.

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I don't know why

...but the word "gigalitre" makes me chuckle. It just sounds dirty.

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Anonymous Coward

Pretty sure

My physics teachers said it was atmospheric pressure too. Atmospheric pressure pushing the liquid up into the vacuum created by it falling away down the other side.

it would be interesting to see if you could siphon a liquid out of a completely sealed container... I'm guessing not, but i don't have a handy experiment to check.

Would it work without gravity? Probably not, but that doesn't mean that it's the main principle at work.

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Excellent counterpoint...

If atmospheric pressure was not a key factor in siphon action, then it would be possible to siphon out of a sealed container wherein the only possible inlet/outlet for the fluid would be the tube (example: a soda bottle with a tube running to the bottom, run through a hole in the bottle cap which is then caulked and screwed tight onto the bottle). Willing to wager a drink the siphon action doesn't work all the way because internal pressure decreases (because volume increases as the liquid flows out and no more air can come in) external air pressure becomes great enough to stop the flow.

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Nope...

"""If atmospheric pressure was not a key factor in siphon action, then it would be possible to siphon out of a sealed container wherein the only possible inlet/outlet for the fluid would be the tube"""

Another non-engineer then?

A siphon isn't going to be able to create a vacuum, as it isn't a pump. There's no way to remove the fluid from your container without replacing it or creating a vacuum.

By adding a closed container, you actually cause atmospheric pressure to become relevant - by removing it's effect from one body of liquid. In your standard siphon, both bodies of liquid experience pressure from the atmosphere, and that cancels out, but when you seal the container, the atmosphere cannot exert force on the surface of the liquid, which means that it will not cancel out, and your siphon will break, as you expected.

Has anyone even drawn a force body diagram of a siphon? I suggest you try and see just how nuts this atmospheric pressure suggestion really is.

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Badgers

Australian

Everyone knows Australia is upside down being on the opposite side of the world, of course their water works differently.

And apparently the only thing keeping them from drifting off into space is alcohol.

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Simple experiment...

Fill a beer glass with water and invert it under the surface in a filled sink.

Gradually lift the glass and marvel out how the level of the water in the glass is above that of the sink.

Now tell me how atmospheric pressure has no impact on the siphoning process.

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Coat

I failed

I got as far as "Fill a beer glass"

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Happy

Yes, this is an experiment to test vapour vs atmospheric pressure

What the (quite revealing) beer glass experiment is doing is finding the equilibrium point in the system you establish.

As you lift the beer glass you are lifting the contained water (that is above the main water level), this requires force proportional to the amount of water you lift (how far out of the water the glass is).

Meanwhile we have the atmosphere pressing down on the exposed water surface (not the water in the pint glass though as the glass is in the way).

As you lift the pint glass up you will find that any trapped gas in the top of the glass seems to expand (do this in hot water to see it better). This is due to the pressure reducing in the pint glass as some of the atmospheric pressure is offset by the weight of the lifted water trapped in the pint glass. The trapped gas is not just expanding as the pressure is reduced though, that air is a mixture of air and water vapour, as you lift the glass and reduce the pressure more of the water turns to vapour and your gas appears to further expand. Do this with a tall enough pint glass (10m or more) and you'll find that there is a point at which further lifting of the glass simply results in more gas at the top and no further rise in the water level.

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Typical lying Ozzie

As any fool knows, the operative force is a chav sucking the petrol out of your tank.

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Boffin

PhDs all round for Reg readers

Of course siphons need atmospheric pressure. The maximum height you can have bend above the inlet is even proportional to the atmospheric pressure. No atmosphere, no siphoning.

As for the good doc, this extract from his paper demonstrates a truly mind boggling degree of stupidity:

"The column of water acts like a chain with the water molecules pulling on each other via hydrogen bonds"

Perhaps he should correct his own misconceptions before addressing everyone else's...

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The Map is Not The Territory

Sadly, I can't get past the pay-wall to see the article, but the summary suggests that a "chain model" is the best way to mathematically model a siphon. Now, I can't comment on that - and for all I know, as a model of the flow rates, etc. of a siphon, it may be the best - suitable for video game simulations, etc..

But I can say, from practical experience using old garden hoses to drain puddles at my childhood home, that atmospheric pressure isn't irrelevant. If you have a leaky hose or hose-coupling between the water level you are trying to drain and the top of the siphon,. you'll loose your siphon effect - and even if you have a leak on the lower end, you'll find a reduced siphon effect.

Any physics explanation of the process that claims that atmospheric pressure is unimportant is flat-out bone-headed wrong. Sure, surface-tension (or at least the electro-static attraction between different sides of the H2O molecule) might help maintain a siphon in a vacuum - but as any pump maker/operator knows, there's a limit to the "drawing" power of a pump.

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But...

...isn't atmospheric pressure a side-effect of gravity?

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Naw...

...atmospheric pressure comes from all the gas molecules that are piled near the surface of the earth as they are drawn towards the center of the planet by....oh, wait, never mind.

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Go

Atmospheric pressure

The atmospheric pressure controls how high above the surface of the upper liquid the siphon tube can pass before it descends. I think you'd have trouble siphoning mercury above a metre wall.

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Stu
Joke

Hummm

It seems we have a bit of a connundrum here, nobody on these hallowed sci expert forums ;-) seems to be able to come to a concensus whether gravity is really involved or not.

I call for a scientific experiment as part of El-Reg's PARIS project, entitled "Water siphoning experiment 1A in a microgravity environment - is air pressure or gravity the main culprit?"

Just what is the cargo limit onboard the paper aeroplane experiment anyway? ;-)

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You do need gravity

You do need the gravity because the weight of the water in the long tube is what drives the process.

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