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I recently set up a small EZ-Flo fertigation system. This is a tank full of fertilizer solution that you attach to a pipe that you are using for a sprinkler or irrigation. As water flows down the pipe, small amounts of fertilizer are "injected" into the water from the tank. This is a passive process entirely driven by the pressure from the water source.

There is an adjustable valve in the pipe at the site at which it attaches to the tank. There are branches in the pipe on either side of this valve. These are an input to (before the valve) and output from (after the valve) the tank. Both the input and the output are thin, flexible hoses that connect to the tank, where they become dip tubes that travel to the bottom of the tank.

When the system is working, as water flows down the pipe, some of it is diverted down the input into the tank, where it mixes with fertilizer. Simultaneously, fertilizer solution is siphoned up the output, where it joins the pipe on the other side of the flow restrictor.

The above EZ-Flo link is a simple illustrative video of how the system functions. Mine is basically like the one shown, with the exception that the fertilizer tank is about 1 foot under the pipe.

I have observed that, if the pipe's valve is too open (i.e. not restricting flow), then no fertilizer seems to be injected-- there doesn't seem to be any outflow from the tank. It is hard to tell exactly what is happening, but the (transparent) outflow tube remains clear. When the valve is further closed, the outflow tube fills with the (colored) fertilizer solution, indicating that it is being siphoned up into the pipe. So it seems 1 of 3 things is happening when the valve is too open:

  1. The clear pipe water flows down to fill both the input and the output, rather than water flowing down the input and fertilizer up the output.
  2. Air remains in the output tube and the pipe water flows over it.
  3. Somehow a very diluted form of the fertilizer (so diluted it looks clear) is taken up the output. But I don't know how this would possibly occur, since the input water has to travel through the fertilizer solution.

Can someone explain what is happening?

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  • $\begingroup$ Do you mean that when you do not close the valve at all, it's the pipe water that's getting filled in the input and output tubes? $\endgroup$
    – lee
    Jan 11 at 5:47
  • $\begingroup$ "It seems that the pipe water flows down to fill both the input and the output, rather than water flowing down the input and fertilizer up the output." - Is it your feeling or do you actually see water flowing inside the tubes? $\endgroup$
    – lee
    Jan 11 at 5:51
  • $\begingroup$ So, did you check this before using the EZ-Flo fertigation system? If you had used the EZ-Flo fertigation system before experimenting with the ballvalve, then the output tube will have the coloured fertilizer in it. $\endgroup$
    – lee
    Jan 11 at 8:11
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This how the EZ-Flo fertigation system works-
When you close the ballvalve a little(or more than a little), the pressure of the water inside the pipe (before the valve) increases. So the water inside it tries to escape through the input tube into the tank. When it reaches the tank it pushes the fertilizer inside the tank for more space. And the fertilizer is pushed out through the output tube into the water pipe where, it gets mixed with the water flowing.

Now, when you don't close the valve, the water doesn't have any reason to flow down the input tube and up the output tube i.e., it has got a straight path through the pipe. So it travels straight without mixing with the fertilizer. One may think that the force of gravity might be enough for the water to travel down the input tube. But air occupies space. So the air inside the tube prevents the water from entering the tube. Moreover, even if the water somehow makes its way down the input tube, it will take some work for it to push the fertilizer up the output tube which can't be provided by the force of gravity.

"It seems that the pipe water flows down to fill both the input and the output, rather than water flowing down the input and fertilizer up the output."

No, it doesn't. The pipe water doesn't flow down to fill both the input and the output tubes. It's just your feeling. Maybe the water inside input tube and the fertilizer inside the output tube(if you've used them before) will still remain there. Or if you haven't used the EZ-Flo fertigation system before experimenting with the ballvalve, the air inside the tubes will remain there(which will look transparent).

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  • $\begingroup$ Thanks for your answer. See my revision to my question above, which addresses your comments. From my observation of the EZ-Flo, it is not true that "when you close the ballvalve a little" the process you describe occurs. The flow has to be restricted quite a lot to cause fertilizer solution to be visibly siphoned up the output. There is a threshold point at which the siphoning occurs. Also, I do not understand your point about the "water not having any reason to flow" with an open valve. There is a branch in the pipe at the input, shouldn't gravity be sufficient? $\endgroup$ Jan 11 at 7:23
  • $\begingroup$ About the ballvalve, I don't know how closed it should be to let the water flow through the input tube. As you've told, it will probably have some threshold point at which the siphoning occurs. $\endgroup$
    – lee
    Jan 11 at 8:04
  • $\begingroup$ Air occupies space, so it is hard for the water to push the air in the input tube into the fertilizertank and the fertilizer up the output tube. And I don't think that water inside the pipe comes too slowly to let the air inside the input tube escape through the hole itself. Even if it does come that slowly, it won't be able to push the fertilizer up the output tube. $\endgroup$
    – lee
    Jan 11 at 8:35
  • $\begingroup$ Yes, we both understand there is a threshold point. My question is about why there is this threshold-- what is going on in terms of physics? I'd like to know the math. $\endgroup$ Jan 11 at 9:07

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