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Old Mar 3rd 2015, 08:22 AM   #1
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Question Help me fix curious behavior of rainbarrel

I have a barrel that collects rainwater from the gutters on my house. At the bottom of the barrel is a manual 'faucet' with a round handle for turning the flow on and off. I leave this in the full 'on' position. On that spigot, I have installed a "Y" fitting. Each leg of the Y fitting has a separate on/off level.

One side of the "Y" goes to the irrigation hose which I turn on when the garden gets dry and there is water in the barrel. This irrigation hose goes down a slope and is always below the bottom of the rainbarrel.

The other side of the "Y" is always turned on and goes to the bottom of a clear 1/2" plastic tube that is mounted alongside the rainbarrel so I can see how much water is in the rainbarrel without opening it. The top of the tube is open.

Now comes the curious thing. With the rainbarrel full and the plastic tube showing full, I turn on the 'Irrigation' side of the "Y". The water in the plastic tube immediately drains all water and stays empty while the irrigation is on. When I turn off the irrigation, the plastic tube fills back up.
So I have 2 questions.
1. How do I get around this? I really want to know the water level while I am irrigating so I can use a certain amount and then turn off irrigation. I might want to use, say, 10% every day until empty. I was thinking of cutting a hole in the rainbarrel above the spigot for the level sense tube, but I don't understand why this would work any better unless it is the Bernoulli effect.
2. How can this be explained? I have tried to think this through but nothing makes sense. Is it the Bernoulli effect? Maybe, but I don't think the flow rate is strong enough to cause this effect. Since the tube is empty while irrigating, I should be able to cut it off with no water coming out. In fact I should be able to remove all connections to that side of the Y and still not have water coming out. Then I could look into the Y and see the water flowing without any coming out. Ack.
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Old Mar 3rd 2015, 12:31 PM   #2
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I think the cause may indeed be due to the Bernoulli's effect. The water coming from the rain barrel runs past the leg of the Y that goes to the sight tube, and the velocity of that water creates a small but significant negative pressure at the connection point, which causes the water in the sight tube to drain. Do you have an estimate for the velocity of the water flow? Keep in mind that if there is a constriction in the Y-connector at the point where the two legs meet this would cause the water flow to increase in velocity, which would cause the pressure drop due to Bernoulli to become greater.

The solution is to do as you suggest - connect the sight tube directly to the rain barrel so that water flow to the irrigation hose is separate.
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Old Mar 4th 2015, 05:35 AM   #3
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After thinking about this some more, I now believe my initial response is incorrect. I think the cause of the behavior of the sight tube has a simpler explanation:

Consider that with the irrigation valve closed and the other two valves open the pressure of all three legs of the Y connector is equal, whihcis what maintains the water in the sight tube at the same level as the water in the rain barrel. But when you open the irrigation valve the pressure in the Y connector goes to essentially zero above atmospheric, because the irrigation tube is open to the atmosphere. So now there is nothing to keep the water in the sight tube and it drains out. Why doesn't water from the rain barrel flow back into the sight tube to raise its level? Because that would mean water is flowing up hill whereas the irrigation tube - which is sloped down hill and is at zero pressure - offers a lower resistance path for he water to flow.

I think this provides a better explanation. The solution however remains the same - the sight tube needs its own connection into the rain barrel.
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Old Mar 4th 2015, 08:41 AM   #4
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The response time for the sight tube does not need to be fast.

If you put a flow restrictor betwen the sight tube and the barrel, the preferential flow route to the irrigation system will be from the barrel rather than from the sight tube.

In other words, reduce the internal diameter of the pipe between the Y connector and the sighting tube.

You will probably still get a misreading between the tube and the barrel, but then how accurate does it really need to be?
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Old Mar 4th 2015, 08:42 AM   #5
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Entrainment seems to answer my issues

This is an answer from the same question in another forum. I'm accepting this since it seems to explain the behavior and solve my problem.
My guess is combination of both "entrainment" and perhaps a slight "Bernoulli effect".
Wiki's first line on this, is jibberish to me; "Entrainment is the transport of fluid across an interface between two bodies of fluid by a shear induced turbulent flux."
But that's probably my fault.

In the navy, we had pumps called "eductors", which used a water stream as "the pump". The reason I think entrainment is predominant in your situation, can be shown by experiment:

Take a water hose, connected to your water main, put it in your rain barrel, turn the water on, full blast, holding the exit of the hose underwater, and point it out of your rain barrel. You will notice that your rain barrel will start to empty, implying that the water from the rain barrel, is somehow latching onto the flow of water out of the hose.

It's been about 35 years since I've studied this, but the following website seems to confirm what I remember about it:

Viscosity and Fluid Flow

Nonideal fluids have viscosity. This is a property which, when large in value, might be thought of as stickiness. An object moving through a viscous fluid tends to drag a surface layer of fluid with it.

Likewise, fluid flowing through a pipe will move more slowly because it adheres to the pipe surface. The boundary layer adhering to the pipe slows down the next layer in and so on, giving a velocity profile which changes from zero at the wall to a maximum speed in the center of the pipe.


Likewise, flowing water, due to this "stickiness", will drag the non-flowing water along with it.
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Old Mar 4th 2015, 11:55 AM   #6
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I think that ChipB was correct, in his second post.

Pressure is required to push the water up the clear tube,
When the irrigation tap is closed, this comes from the water in the barrel, until the levels equalise.
at this point the pressure at the Y junction due to the height of the water in the tube matches the pressure at that point due to the height of the water in barrel.

Turn on the irigation tap and the pressure at the Y junction drops dramaticaly.
The pressure is no longer enough to push the water up the pipe.
In fact your level indicator is now showing the level of water in the irrigation pipe,
If you raise the open end of the irrigation pipe above the level of the Y junction, but below the level of the water in the barrel, you will see the level in the clear pipe will match this.

Your options are either to cut a seperate hole in the barrel for the level indicator pipe,
Or put a flag on a float poking out the top of the barrel...
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Old Mar 4th 2015, 03:31 PM   #7
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I was following you up to this:
"Turn on the irigation tap and the pressure at the Y junction drops dramaticaly."
Isn't there still 4' of water above it? The pressure can't just drop significantly. If my level sense tube was small, I could just push it up through the Y into the barrel and the level would rise back to the full level. And if the pressure is low at the Y, then the irrigation hose wouldn't have the high flow rate that it does. I'm convinced that this issue is a problem in dynamics not statics so trying to analyze it with static pressures won't yield the solution.
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Old Mar 4th 2015, 03:38 PM   #8
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The pressure falls dramatically because the irrigation pipe is open to the atmosphere. Therefore its pressure is near zero. This is especially true if the rate of flow is slow so that the irrigation pipe is not completely filled with water. Think about this as well - if the pressure at the Y was not significantly less than in the rain barrel, water wouldn't flow out!
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