I see that alot of pics have "bags" ( hot water bottles etc ) that appear to gravity feed.
I have tried this & it doesnt seem to work very well.
I did use an open container, so I have used an air tight vessel to hold the liquid?
Here's a lesson in fluid physics. If you want more info, check your Halliday and Resnick text. You do have one, don't you?
1--The solution in the bags we use, unless we pressurize them somehow, all flow into us by gravity. A column of water exerts approximately one pound of pressure for every 27.68 inches of water column height (INCLUDING the water height in the bag).
2--All the air in the tube must be expelled for the water to flow smoothly.
3A--In an OPEN TOP bag, the height of the water column CHANGES (goes down) as the bag empties, hence, the pressure when the bag is full is higher than when it is just about empty. If your bag is 12" tall, the pressure will be about 0.43 psi less than when it was filled to the top. You can see the level of water go down in the bag.
3B--In a CLOSED TOP bag, if you completely fill it to the top, the height of the water column STAYS THE SAME HEIGHT as the bag empties because no air enters the bag as water flows out and the bag approaches being empty. Therefore, the pressure in a closed top bag STAYS THE SAME from the time you first open the clamp until it is empty.
This IS how it works. If you need more explanation, see your Halliday and Resnick.
I usually keep the height of the bottom of my bag between 3 and 5 feet above the floor and use the clamp to adjust the flow as necessary for comfort and the flow rate I want.