Explaining the Unexpected: Water Bottle Collapse

[scudrunner]

Understanding that the reason for keeping the water inside of an upside down water bottle is negative pressure and gravity, I decided to make a water feeder for my chickens. I had a shutt-off valve on the water bottle. Set it upside down at waiste high. Then I attached a long plastic hose to the shutt-off and the other end was connected to a smaller water feeder below using the same principle as before. Amazingly, the water did not stay in the upper water bottle. It, in fact, drained out and with the negative pressure the bottle began to collapse. I cannot understand why this happened since, I thought, that the same principle worked on both bottles. Can somebody explain this phenomenon to me? Dave

 

[KOSS]

It is the same principle at work. Use a glass bottle next time, or a tougher plastic for the smaller bottle so that it doesn't collapse. It's not really negative pressure you create, the pressure in the air gap created in the upper bottle as the initial volume of water flows out, is still positive, but it is lower than the outside atmospheric pressure, the pressure difference is what causes the bottle to compress and collapse.

 

[scudrunner]

Thank you Koss for responding. I am still not understanding why the same water bottle I had from my kitchen which is about 5 gallons would collapse since it does not collapse in my kitchen. It is the same size water bottle you see being delivered to houses on a commercial truck. I am thinking that the size of the "water line" which leads down from the larger water bottle to the much smaller water supply for the chickens is smaller. Or could it be the atmosphere pressure is too much greater dropping 4 feet to the ground level? I do have a glass water bottle so I will try your suggestion. Thank you, Dave

 

[JaredJames]

The higher the bottle is above the feeder (output), the lower the pressure inside the water bottle due to the suction created by the water in the output pipe.

Atmospheric pressure can be equal in both cases, but the lower internal pressure is what causes the collapse. The greater the pressure difference, the more likely the damage.

It's as simple as that.

 

[Q_Goest]

Hi scudrunner. Welcome to the board. If I take a soda straw, put it in a glass of water, put my finger on the top, and lift it out, the water stays inside the straw until I let my finger off, at which time it all drops straight out. The reason the water is able to stay up inside the staw is twofold. First, there's a vacuum created above the water in the straw. The pressure at the bottom of the straw then is sufficient to support the water. But the air can only support the water if the surface tension at the lower, exposed end, is strong enough to keep that surface together. If you had no surface tension, bubbles of air would flow up the straw while water would fall out. Imagine the water being like sand. Air can easily get through the sand so a vacuum can't form above the sand. Same thing would happen if there was no surface tension.

Of course, as soon as you take your finger off the straw, the vacuum disappears and the water can drop out. Surface tension alone isn't able to support much water, only about a drop of water can be supported by the meager strength of the surface tension. So if your opening is much larger than a drop of water, it isn't strong enough to hold any water inside your straw or bottle. That's what happens to a 5 gallon water bottle. The air simply bubbles up into the bottle and the water falls down. Surface tension isn't strong enough to keep the exposed surface together.

The way it works 'in the kitchen' with your water bottle stand is that the surface tension is replaced by a liquid trap. The surface tension isn't sufficient to prevent air from bubbling in because the mouth of the bottle is so large. So the idea is to provide a liquid trap instead to prevent air from getting in. That way, the water can stay inside the bottle.

A liquid trap is simply a level of water above the opening of the water bottle. If the water is held inside a bowl for example, and the opening of the bottle is below the level of the water, air can't get in, and a vacuum forms inside the bottle to prevent it from escaping.

You can do the same thing in your chicken coup by having a bowl that the bottle opening sits in. You can make this bowl a long trough if you'd like, so long as the water in the trough can fill up to the opening in the water bottle.