# Fluid Dynamics - Auto filling a reservoir to a constant water-level

I have created a reservoir that is intended to automatically self-fill at exactly 1/4" depth. The idea is that the reservoir is hooked up to the airtight jug and dispenses the liquid until the opening of the tubing is fully submerged in the liquid. At that point air can no longer travel up the tubing to push more liquid out and the depth of the liquid remains constant, until some liquid is removed and the reservoir refills back to the depth. However when I tested this the theory did not work and water continued to come out. When testing the reservoir was below the jug. I question whether my logic is correct? Do I have an incorrect understanding of the physics, or could the problem stem somewhere else? Maybe some air was leaking into the jug causing more liquid to pour? Could the solution be maybe dependent on the height difference between the reservoir and jug?

On another note, as you can see in the photo the part I created is actually a "2 story" reservoir. The aim is to hook a splitter up to the jug and fill the two reservoirs from the same jug. I'm thinking this may cause some more complications.

If anyone has any advice on how to make this work it would be of great help. I am also open to other suggestions. I was thinking about potentially working in a float valve into this design.

• The water will flow until the water levels (height, absolute) equalize. Instead, look at how a toilet water tank controls its level. Do that. It can be in a parallel tank, because the levels will equalize. If you want something fancier to show off to your friends or customers, you can use an electro-optical sensor and an electronic valve. Aug 4, 2021 at 15:44
• Your setup has a flaw that is difficult to overcome. - creating an airtight environment after releasing water. It is possible but difficult to calculate the suction required to hold the lid airtight. Even a solution is found, it would not be practical.
– r13
Aug 4, 2021 at 19:20
• I think the OP was asking how to create the phenomenon that the water flow in a tube can be controlled by using the thumb tightly cover over the upper end of the tube.
– r13
Aug 4, 2021 at 19:26
• @PeteW In the OP's setup, water level equalization (by gravity) will not occur.
– r13
Aug 4, 2021 at 19:35
• Maybe some air was leaking into the jug causing more liquid to pour? ... more likely reason is that the jug is soft walled ... ambient pressure collapses the sides when water is withdrawn from the jug Aug 4, 2021 at 20:51