Siphons

Cecil's Columns/Staff Reports
1

From http://www.straightdope.com/columns/010105.html

I pondered this bleakly for a while. Then a thought occurred to me. Al, I said, the highest you can raise water in a siphon is around 34 feet. By curious coincidence, the maximum height that water can be drawn in a tube sealed at the top (a water barometer) is also around 34 feet.

With sincere respect to your work Cecil (and I’m a long time fan), I agree with Uncle Al, the tensile strength of the liquid is what keeps the fluid together provided the vapor pressure of the liquid is less than the ambient pressure.

The 34 feet limit is no coincidence, and it isn’t constant. Cavitation (a.k.a. boiling in pure fluid systems) occurs when the vapor pressure of the liquid (a thermodynamic property of the substance) equals the ambient pressure. The water vapor pressure is related to the temperature. At room temperature (25 C), the water vapor pressure is 3.166 kilopascals (ref 1 below), while atmospheric pressure is 101.3 kPa. The pressure differential is 98.1 kPa, giving plenty of room for a negative suction pressure.

If you apply the familiar hydrostatic pressure equation, dP = rhog/gch, where dP is the pressure differential, g is gravitational acceleration, rho is the density of water (1000 kg/m3), gc is the conversion constant from mass to force in Newton’s second law, h is the height, you see that (98,100 N/m2)/(1000 kg/m3)/(9.81 m/s2)*(1.0 kg m/N s2) = 10 meters, or 32.8 feet.

When you go higher than that in a barometer (meaning you draw water up a tube with suction), the pressure at the top of the fluid column is near 3.1 kPa and the water boils. Increase the temperature of the water to 80 deg C, making the vapor pressure 47.34 kPa, dP is 53 kPa, and the height is 18 feet. These calcs are for pure water, gases and such affect the vapor pressure, but have only an indirect influence on siphoning.

Now, let’s assume you have two expandable bladders with no air, each filled to half capacity connected with a filled tube at equal elevation. Drop the elevation of one below the other, and all the water in the top one will go to the lower bladder without the influence of the atmosphere. There would be no driving force from the bladders themselves, no atmosphere, indicating gravity is pulling the water to a lower energy state. The water remains liquid because thermodynamics says it’s phase remains liquid at those conditions.

I was about to discuss the capillary action argument brought up by Al, and then after some homework decided he knew what he was talking about, which often seems to be the case when talking science with “Uncle Al.”

I hope this helps, all comments and criticism gladly welcomed.

Regards,

Dr. Dennis Hussey
Research Engineer
Chemical Engineering
Oklahoma State University

  1. Smith and Van Ness, Introduction to Chemical Engineering Thermodynamics, Appendix C Steam Tables.
2

This is an interesting question. How about an experiment?

First, would a siphon work if the hose emptied into another container? That way, the air pressure would also be pushing equally on both ends.

Second, would it work if the hose emptied into a closed container? That way, the air pressure in the container would increase as (if) it fills.

3

Mike,
First question, yes, and your observation of the pressure working on both sides is correct (assuming the pressure differential from point 1 to 2 caused by air density is negligible, which is 1/1000 of water, probably a fair assumption).

Second question, it does drain until the pressures balance. That’s a good equilibrium problem, you’ll need the volume of the container, height differences, fluid densities, the ideal gas law and hydrostatic pressure equation to solve it.

Good ideas,
dfh

4

A simple experiment might be telling. Place the end of the tube that is doing the “sucking” in two different locations. One: at the top of the tank being drained - thus minimizing the “pushing force” of air and water above it. Second location: at the bottom of the tank - increasing the pushing force. Does the rate of water flow change? My gut feeling is that it won’t, at least not much. I think the vacuum, forming at point B, is pulling harder than the water is being pushed upon. Maybe poking a small hole in the top of the tube at point B, so as to let air in (thus negating the pull of the vacuum), but not to let water out would put the burden entirely on the pushing force.

5

Here’s a home science project on siphons:
Take meter length of paper towels, soak it.

Next take a big salad bowl of water and put it on your kitchen counter.

Put one end of the paper towels at the bottom of the salad bowl, leaving something heavy so it won’t float away.

Hang the other end of the paper towel over the counter by at least a few inches.

Observe.
Questions:

Does it appear from this that air pressure is necessary for siphoning to occur?

Does it appear that cavitation is necessary for siphoning to occur?

Will this work to siphon uphill?

Does this effect result from a combination of gravity and surface tension in the water?

Further work:

Do all liquids form into drops? If so, why, if not, why not?

6

Interesting column and topic.

My personal experience with siphons… as a teenager, removing gasoline from the car gas tank to another container for use in the lawnmower. I’d seen my dad do it. The problem is you can’t just put the hose in and lower the outside because you have to get some of the gasoline into the hose to start the siphon. The answer, you don’t suck the gas into the hose. Rather, you blow air into the gas tank until the pressure pushes the gas up the hose. Then as the gas is flowing, you invert the outer arm to lower than the tank and into the catching container. Viola - siphoned gas.

But that only explains how you prime the pump, so to speak, not the process that maintains the siphon.

DPWhite, a complication presents itself, in that is a roll of paper towels really a siphon? It is using capillary action to pull the water along the roll. Hmmm.

There is also this thread on siphoning:
http://boards.straightdope.com/sdmb/showthread.php?threadid=54026

7

After reading this particular SD column, I realized there are some dumb physicists far and wide.
After checking the message boards it’s obvious that Cecil did not consult with any more physicists than Uncle Al. Any physics buff would know the first and only equation you need for this question is Bernoulli’s. Before even plugging in numbers, the density of the fluid cancels out, and the air pressure cancels out. The resulting equation is the change in elevation (ie gravity). Argh. I’m getting irked just writing this. I don’t even know why this column was printed. There’s no upside. Couple years ago, Cecil did a bit on how it’s possible to suck up spaghetti with your mouth. That was explained with air pressure. Am I spewing? NO. Just in search of the truth.

  1. The chain metaphor - Consider when it comes to air pushing on the water. If the water is a chain, then how do you push a chain? Chains are only pulled.
  2. “A siphon will work in a vacuum” - That’s Cecil’s deduction. The encyclopedia does not state that.
  3. “Maximum hieght that water can be drawn in tube sealed at the top…” - siphons don’t have sealed tops (apples vs oranges)
  4. “…is 34 feet” - True, but only at Earth’s atmospheric pressure and with water. Increase the air pressure and increase the height. Why 34 feet? Because the sum of the forces is equal to zero. At 34 feet the pressure of the atmosphere equals the weight of the water.
  5. “in practice it depends on air pressure” - start a siphon and slowly raise the outlet to a point above the water level. The siphon stops; did the air pressure stop too?
    OK, now I’m spewing. Done
8

Water can be under large negative pressures, of many atmospheres. This is an unstable situation, akin to superheating where you have a liquid above its boiling point, but because there’s no nucleation site where bubbles can get above a critical size where they start to grow out of control, it remains liquid. If there are no nucleation sites (dust, rough spots on the inner surface of the pipe (and a gaping hole in the pipe definitely counts as a rough spot)), etc, then cavitation can be avoided even at extreme negative pressures. So yeah, with care, you can get a siphon working without the help of atmospheric pressure, and the 34 foot limit is not a hard limit. If you’ve got a lot of nucleation sites, on the other hand, you won’t be able to get the water to any appreciable negative pressure, and it’ll cavitate. In other words, it depends on the siphon.

The general rule is that the water is propelled by a pressure difference. Whether that’s a zero pressure at the open end and negative pressure at the top, or atmospheric pressure at the bottom and zero pressure at the top, it still works that way.

Siphons work because they’re set up to avoid cavitation–the pressure differences don’t work out if you get a gap in the liquid. This can be achieved either by assuring positive pressure at all points, or by avoiding nucleation sites.

9

First. Anyone who has answered this string, and claimed that either pressure OR gravity was the acting force driving a siphon, is wrong. The people who claimed gravity was the culprit are mostly wrong. The people who claimed air pressure is the culprit are only slightly wrong. The idiots are wondering, right along with Cecil, how this could be.

The answer is simple; it is both gravity and pressure. What? For the simpletons, I will make this as easy as I can, and it has been a while since my last physics class so I don’t remember the equations. However, I have not forgotten simple physics and material properties.

A siphon is nothing more than a straw that creates its very own pressure difference that allows the higher pressure, at point D in Cecil’s diagram, to push the liquid up the straw. As the water is pushed past D along BC it creates a vacuum that is continually filled in by more water pushed in the tube by the pressure difference that causes more water to travel past point D which causes… Cecil was almost correct about this part except the end of the siphon, point C, only needs to be slightly lower than point D not A. This is the basic definition of a straw; only, the water is creating the pressure difference instead of our mouths or a pump. The reason why water can only go so high, point D to B, in a column is because at some point the water being pushed up the tube has a gravitational force that eventually equals the pressure/force at point D, and since these two forces are equal no movement occurs. Without gravity the column could be as high as you want it, though, I have no idea how you could determine height without some mass around to get a reference point which then causes gravity to be around so… However, this definition is slightly wrong. Because, the other force acting on the liquid is so small compared to the pressure/force that most physics explanations disregard this as being negligible. (Side Bar: Whenever your professors claim something is negligible it means they don’t understand it and that means it is the most interesting point of the lesson.) They INCORRECTLY disregard this because this is not the full explanation.

Water and all liquids have a tensile strength. This is one of the basic differences between a gas and a fluid. Water has a very very small tensile strength. Meaning it doesn’t take very much force to pull it apart. But, it does take SOME force to pull it apart. We must all remember surface tension, right? Though, surface tension is slightly higher than the tensile strength of the molecules inside the liquid. For more info on that go to college, or back to college as is the case with Cecil. Therefore, a siphon will work in a vacuum with a gravity field. However, the height of the column is now determined by how high the tensile strength is for that liquid. This also applies to the height of a column in an atmosphere; the force acting against the weight/force of the water is pressure/force + tensile/force. If the tensile/force is greater than the weight/force of the liquid then the siphon will work. If the weight/force is greater then it won’t work. If you are thinking ahead then you will know that this is also determined by the gravitational field the siphon is in. The weaker the field the higher the column can be, but without a gravity field the water won’t fall down the other side. So, no siphon without some gravity or pressure. I am going to leave out one more very crucial point in how to make a purely gravity driven siphon which only the brilliant people will be able to get. I bet Cecil doesn’t see the last variable in making this kind of siphon work continuously, but he isn’t really brilliant so I won’t hold it against him.

10

How did you become a professor of anything? Do you even know the tensile strength of water? Look it up and please tell me if it comes even remotely close to being able to hold onto 34 feet of water in Earth’s gravity. Think of it this way smarty pants. How much weight do you think you can hold onto? Maybe 500 lbs. Well if you were water then in realtion to your 500 lbs. the 34 feet would be between 50000 and 500000 lbs. That’s pretty amazing water you are talking about. Next time ponder much harder.

11

OK, we welcome different perspectives here at the SDMB and we appreciate people who take the time to explain the reasoning behind these perspectives. Having said that, it appears that you really need to get over yourself, “The Brain”. Your insults are gratuitous and make you sound like spewing pompous ass. If anything, they detract from your credibility, such as it is.

Your little theory may be correct (or not) but there’s no need for your sniping.

Carry on.

12

Let’s suppose you have a small tank of water (A) that has a surface area of X square units that is a couple of centimeters above a much larger tank (B) with a surface area of 1,000 X square units (in other words A is higher than B by an exceedingly small amount and B is very much larger in surface area (and volume) than A.

If air pressure were the sole determining factor in siphonization (a made-up term…cute, eh?)A should not siphonize its contents to B at a measureable distance. You can calculate the forces from the Force=Pressure/Area equation, or at least you could design an experiment to show the parameters that would result in siphonization or prevent it.

If gravity and molecular bonding of water were the driving force, you could set up an experiment that would prove that mechanism. If atmospheric pressure were the driving force, you’d get different results (re tank height differential and surface area differential) that would differ from the results if molecular cohesion and weight (gravity) were the driving forces.

You’d get “in between” answers if: 1) both were significant factors; and/or; 2) you had failed to consider OTHER factors, such as capillary (liquid/solid surface) interactions.

Another interesting experiment would be to use a siphon tube that was non-uniform in diameter, e.g. wider at the inlet from tank A to the outlet at tank B (and vice versa).

I’m afraid that I can’t offer fancier equations than F=P/A. I’m just a dumb MD. I took a few years of calculus, physics, and chemistry in college many years ago, but that was because I had to in order to get into medical school. It may be that my “thought experiment” is stupid and neglects some physical reality that other more savvy posters will bring to my attention.

13

As is often the case, this topic as been covered by Jearl D. Walker in The Ultimate Reference of ddball Physcs, “The Flying Circus of Physics”. It’s qustion 4.105, and in his edition wth answers he notes that, indeed, a siphon will wrk in vacuum, and confirms a number of the other assertions given above. His references are:

“The Siphon” by A. Potter and F.H. Barnes" in “Physics ducation” Vol. 6, p. 362 (1971)

“The Siphon” by M.C. Noakes in “School Scence Review” (Great Britain) Vol. 29, p.233 (1948). It is also reviewed in the American Journal of Physics vol. 16, p. 254 (1948)
I note that, at least until recently, the Boston Museum of Scence had an exhibit called “Fluidica” on such topics, which included a mechanical analogue of the siphon. It consisted on a beaded chain (like the one on light bulb pulls) stretched over a freelly rotating pully wheel between two reservoirs. If you raised one “reservoir” the chain caused the whel to rotate, moving more of the chain into the lower reservoir. Obviously, atmospheric pressure isn’t causing a chain to move – it’s gravity and the interatomic orces keeping the cain together. (The exhibit may still be up, but moved. They have an exhibit on Everest in that space now.)

14

Thanks for this example, Dr. Hussey, because it makes things clearer for me. I think you deserve an acknowledgement from Cecil.

Here’s my non-technical understanding of the whole matter (I hope it’s right).

Atmospheric pressure is a factor in water’s “cohesiveness.” Greatly lowered pressure would cause the water to boil, losing its “cohesiveness.”

Then a water siphon (like the one above) would probably not work in a vacuum because the water would be boiling.

If we used some liquid that would not boil in a vacuum (can such a liquid exist?), then the siphon would work.

The above would also apply to a siphon over 34 feet tall because the lowered pressure at the top would also cause water to boil.

So the siphon operates because of water’s “cohesiveness,” and the “cohesiveness” is dependent on atmospheric pressure.

15

It should be obvious, but apparently it isn’t. The tensile strength of water won’t come into play on the left (inlet) side of the siphon until the column of water is 34 feet high. Up to 34 feet above point D, there is positve pressure on the water, so even if there were zero tensile strength, the siphon would still work if point B is less than 34 feet. Any tensile strength to the water would allow the siphon to work with B greater than 34 feet, because now the column of water above 34 feet can pull on the rest of the water.

I’d suspect how high above 34 feet the siphon will work will depend not just on the tensile strength of water, but also on how far below D the outlet C is. If the discharge side is into a second tank which has a water level C’, then I expect the maximum height of the working siphon will be approached as C’ approaches D. Of course, the siphon is barely transporting water in this case.

If the maximum height a siphon works is truly not much more than the water height in a water barometer, then tensile strength isn’t the dominant mechanism.

Hussman, in your expandable bladder experiment, are you bladders at zero pressure? If so, will the water really remain liquid? If not, how can you say there’s no influence of the atmosphere?

16

big Al Gore sigh;
ok, i’ll toss in my two cents.
there is a lot of arguement because, to some extent, both sides are right.
can a siphon work on the moon??
Answer: yes.
does air preasure matter??
Answer: hell yes.

a siphon can work as long as the combined air preasure and capillary force can hold the column of water up to to the peak of the tube.
if you imagine a fluid with no capillary force ( i.e.: the molecular atraction of the fluid to the sides of the tube is equal the atraction of the fluid to itself ), then you could not siphon such a fluid on the moon.
if you had a fluid with a positive capillary force, then a siphon could work on the moon, depending on the hight of the hump and the diameter of the tube. the capillory force would have to be sufficient to support the column of fluid. ( a smaller diameter could make it over a taller hump).
For water (or gasoline) in a tube the size of your standard garden hose, capillary action ain’t doin’ much, and it’s all air preasure (so you can’t siphon gas on the moon). for water in a tree, the vessels are very thin and air preasure isn’t so necessary. for (almost) the same reason, wicking (siphoning with the paper towels mentioned above) WILL work on the moon.
on a side note: for very wide hoses, even air preasure can’t really do it, since the air and water can pass side by side, allowing air to the top of the tube breaking the siphon.

-luckie

Only drink with your best friends or complete strangers.
Your friends will understand when you make an ass out of yourself,
and the stranger wont know who that asshole was.

17

As my collague Alphagene has mentioned, this forum includes no place for name-calling.

So those of you thinking of personally insulting another poster, just hold your breath while you run over to «The BBQ Pit» forum where you can then release those cuss words. Or, an even better solution IMHO, go find a nice, quiet, private area, indulge in some primal scream therapy, and re-join us once you have achieved “Serenity Now”.

moderator, «Comments on Cecil’s Columns»

18

Excellent (Say it like Mr. Burns.)

Modestly I admit, I did not think I would not be SO successful at being just like Cecil on my first try. Why are you so accepting of Cecil’s diatribe but not the rantings of another? I suppose it must be a hero worship that causes you to not realize that the veiled insults that Cecil makes, presumably for a laugh, with the support of all of his followers, is much more negative than anything my post could create. Mr. Adams for the longest time has enjoyed ridiculing the “masses” by regurgitating the brilliance of other people’s minds. Is it that you just can’t take it when his limited intelligence is revealed? Or do you not understand? In any event, the “masses” who don’t understand, that Cecil is insulting them with an arrogance gained from other people’s work, need someone to keep their minds open. I may not be very good at helping them. But, someone needs to stand up to the over-inflated ego of the great Cecil.

To Alphagene: RELAX already.
To hussamn: I did get carried away in my zeal to show up Cecil. It should not have carried over to you. Sorry.
To Wink: You are absolutely right

19

About Mr. Hussman’s Bladder Experiment:

Mr Hussman is correct that his bladder experiment would work. However, it is not a siphon and does not explain the process of siphoning. His bladder experiment is exactly like popping a hole in the bottom of a bucket and watching the water spew out. And, in a conservation of energy approach the siphon would “look” very similar to this. It is the raising of the liquid from point D to B that everyone is speaking about. Closing off the surface of the water does not change the effects of the siphon in a bladder. In a ridged sealed container it would not work like Mr. Hussman describes.

20

Like I said I Will get better at this.

Mr Hussman make a vert silly comment that I feel the “masses” need to think about.

When he comments thermodynamics says water will not change phase he is taking a mental approach to this that is dagerously wrong. (Ask Mr. Newton how dangerous this can be.) Water exists and, like all matter, reacts to universe as it exists. Water has not a care in the world for our incomplete discription of how that interaction works. Thermodynamics does not control the process of water or matter or anything else except by closing the box on human thought. Thermodynamics is a human discription of a very small part of the processes in the universe. It is only thought the matter and its interactions are what is. In other words, water tells thermodynamics that it will not change phase or that it will.