Question about siphoning

[Street Deep]

Why is Cecil pulling his punches in his otherwise admirable article “How does a siphon work” on the question of how a siphon works? Cecil is absolutely right in saying that air pressure explains the operation of a siphon, but he lets the Encyclopedia Britannica off too lightly.

According to Cecil, the encyclopedia says: The action of a siphon depends upon the influence of gravity (not, as sometimes thought, on the difference in atmospheric pressure —a siphon will work in a vacuum) and upon the cohesive forces that prevent the columns of liquid in the legs of the siphon from breaking under their own weight.”

The encyclopedia's statement has so, so much wrong with it, as I shall demonstrate.

No one is saying that a siphon doesn’t depend on the influence of gravity and it isn’t “sometimes thought” that the action of a siphon depends on the difference in atmospheric pressure. Everyone accepts that without gravity, a siphon cannot operate. As for the difference in atmospheric pressure, that will hinder the operation of a siphon, because atmospheric pressure increases as you go lower, which everyone knows. In any case the difference in atmospheric pressure is of such a small magnitude as to be negligible when water is being siphoned. It’s a straw argument. What really is sometimes thought, and correctly, is that an ordinary siphon depends on the presence of atmospheric pressure. By “ordinary siphon” I mean a siphon using water found in the environment on earth, rather than some other fluid, such extremely pure water found only in a laboratory.

There’s no way an ordinary siphon depends on cohesive forces, because for cohesive forces to play a role the water has to be under tension, and the water in it is never under tension. It’s under pressure. Every part of the water in the siphon is under pressure to some degree. If it were not, it wouldn’t be able to exist in a liquid form. It would start boiling at room temperature (so much for a siphon working in a vacuum). Air pressure is what keeps the water under pressure in the tube, keeping it in the tube and in one piece, keeping it liquid, and pushing it through the tube towards the lower end.

I don't think the encyclopedia can explain the observed bubbles that, if not too large, can be carried through a fast-moving siphon. In many cases the bubbles are large enough to completely separate the water, and yet the siphon still works. So much for the importance of the water in the legs of the siphon from not breaking under their own weight.

The encyclopedia ignores the fact that a dense enough gas can be siphoned, though not in a vacuum. Clearly there is no cohesiveness of a gas. For example cold air is dense enough to be siphoned in air at room temperature.

I have read that extremely pure water, unlike ordinary water can exist even under negative pressure, that is to say, the water is pulling on the containing walls, and this negative pressure can be several times atmospheric pressure in magnitude. It's hard to believe, but it's also hard to believe someone just made it up, so I guess there's some truth to it, and ultrapure water can really be under tension. But this doesn't mean the water is under tension in an ordinary siphon. For that to happen you'd need to be siphoning ultrapure water, and you'd need to have a height of more than about 34 feet.

[questionable]

I have a question about Cecils column on siphoning water http://www.straightdope.com/columns/010105.html. If there was an airtight and watertight lid on the bucket, a hole drilled through the lid, the hose put through that hole and sealed watertight aroud the perimiter of the hose and the siphon started, the water would continue to siphon and pressurize the air in the bucket. Would the downward force evetually so great that it could blow the lid off the bucket? or would it create a small amount of air pressure and discontine siphoning?

In other words, would the pressure be equal to the amount of water in the long end of the hose between intake (A) and end (C) or the water in the tank between D and E exert pressure through the hose until the lid would pop off?

In real-world terms (what I am thinking of doing), I want to build a canister filter for a large (55 gallon) fish tank using a 5 gallon bucket. I want to run two hoses to the fish tank using one as an intake and one as an output hose. If I were to place a submersable pump into a 5 gallon bucket to pump water up to the tank, and the power went out in my home, would the water pressure eventually build (trying to stuff 10 gallons of water in a 5 gallon bucket) until the bucket would leak at the weak spot? Or would there be little extra pressure exerted inside the bucket?

I'm assuming that I would use standard fittings and fill the gaps with standard silicone, the whole unit would be air and watertight, and the bucket lid would be snapped on as opposed to strapped on or externally secured and entire unit would be able to withstand minimal extra pressure.

What are your thoughts?
Thanks

[naita]

Originally Posted by questionable

I have a question about Cecils column on siphoning water http://www.straightdope.com/columns/010105.html. If there was an airtight and watertight lid on the bucket, a hole drilled through the lid, the hose put through that hole and sealed watertight aroud the perimiter of the hose and the siphon started, the water would continue to siphon and pressurize the air in the bucket. Would the downward force evetually so great that it could blow the lid off the bucket? or would it create a small amount of air pressure and discontine siphoning?

In other words, would the pressure be equal to the amount of water in the long end of the hose between intake (A) and end (C) or the water in the tank between D and E exert pressure through the hose until the lid would pop off?

In real-world terms (what I am thinking of doing), I want to build a canister filter for a large (55 gallon) fish tank using a 5 gallon bucket. I want to run two hoses to the fish tank using one as an intake and one as an output hose. If I were to place a submersable pump into a 5 gallon bucket to pump water up to the tank, and the power went out in my home, would the water pressure eventually build (trying to stuff 10 gallons of water in a 5 gallon bucket) until the bucket would leak at the weak spot? Or would there be little extra pressure exerted inside the bucket?

I'm assuming that I would use standard fittings and fill the gaps with standard silicone, the whole unit would be air and watertight, and the bucket lid would be snapped on as opposed to strapped on or externally secured and entire unit would be able to withstand minimal extra pressure.

What are your thoughts?
Thanks

It's been a while since I took physics, but since the process of siphoning is driven by gravity working on the column of water in the long part of the tube, I think the maximum pressure in your bucket would be equivalent to the pressure of water at the bottom of that column, minus a bit for the water pulling down in the other end of the tube and such.
So unless you've got a huge tube and the bucket lid pops off if you fill the tube up to a height equivalent to where the top of your siphon will be, you should be safe.

[Xema]

Originally Posted by questionable

would the pressure be equal to the amount of water in the long end of the hose between intake (A) and end (C) or the water in the tank between D and E exert pressure through the hose until the lid would pop off?

The pressure in your sealed bucket will be proportional to the height difference between the water levelin the source tank and the water level in the bucket. (This height difference is known as the "head".)

Since 33 ft of water exerts a pressure of one atmosphere (14.7 psi), you can figure on a bit less than one psi for each 2' of head. If your bucket is well enough sealed to stand this pressure, the water will simply stop siphoning. If not, the bucket will leak.

[questionable]

Thank you both.

Xema, would that be the top of the water level in the source tank and top water level in the bucket? Do you think it would make a difference if the siphon started at the top of the source tank just below the water line as opposed to the bottom of the source tank? (or would it not matter at all where in the source tank the siphon tube opening was placed?)

Also, when you say the figure one psi for each 2 feet of head, would it (or not) make a difference how large of internal diameter the siphon tube is (like Naita suggested)? Wouldn't a large siphon tube (say a garden hose) exert more pressure than something small (straw)?

Thanks again

[naita]

At this point I can't see how I can be right, but it seems counter intuitive that I should be wrong.

[Xema]

[quote=questionable]would that be the top of the water level in the source tank and top water level in the bucket?[quote]
That's right.

Do you think it would make a difference if the siphon started at the top of the source tank just below the water line as opposed to the bottom of the source tank? (or would it not matter at all where in the source tank the siphon tube opening was placed?)

As long as it's submerged (so air can't get in) it doesn't matter where the inlet is located.

would it (or not) make a difference how large of internal diameter the siphon tube is?

Pressure isn't influenced by the diameter of the tube*. Flow rate certainly would be - larger tube gives a higher flow rate at any pressure. The length of the tube also affects the flow rate - longer means less flow at a given pressure.

[Xema]

Messed up the coding. (I usually preview - I swear I do.)

[Xema]

And I forgot to explain the asterisk:

* Static pressure is unaffected by tube diameter. When there's flow, the fatter and shorter tube will show less pressure drop.

[Sapo]

I have seen some "self-priming" syphons. They have some turns on the neck and you dip them quickly a couple times to get it going. Can anyone explain exactly what turns are needed and how this works?

[SmackFu]

BTW, this is the 400,000 thread.

[kanicbird]

The answers have been in the static state, but that is not the total answer. Once the siphon gets water moving, that moving water has momentum, which must be absorbed/dissipatedly before a steady state can be reached. Now if the bucket is sealed well enough, there will be some oscillations. But what could happen is that water will be forced into the bucket, compressing the air and increasing the pressure, if there is not enough air in the bucket, the pressure could climb very high and rupture your bucket or siphon tube.

This principal is how a ram pump works to use only the energy from flowing water to move water higher then it's source flow.