Vacuum applied to a system

  • Thread starter JohnGe
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  • #1
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First of all, I want to ask for your understanding for my not well English.
I want to discuss all the effects will take place if we begin applying vacuum to the following system http://www.inkline.gr/temp/Vacuum-ElasticTube.gif [Broken] .
Data:
The tube is elastic (squeezable)
The chamber is rigid
The tube contains tap water and it's closed at the right end
The final vacuum supposed to be about -0.9 atm (the water don't boil in this pressure at ambient temperature)

When I apply vacuum to the actual system happens a strange thing: the water brim over the open end. Why? After a bit an air bubble is formed inside the tube (the bubble is about 1/10 of the total tube volume). Why?
When I apply a vacuum about 0.75 atm the above effects are 10 times less.
 
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Answers and Replies

  • #2
691
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Tap water has a lot of trapped gas in it. Fill a glass with tap water and let it sit on a table over night. You should see a lot of bubbles form on the glass. Some of the bubbles are a result of exposure to the air while sitting on your table while others are causesed by gases trapped within the water escaping. That could be the cause of your gas bubble. To help eliminate this you can draw a vacuum on the water to remove all air bubbles before use. This sort of thing shows itself when working with silicone molding gels or with epoxies.

The size of the bubble is a function of the ideal gas law. Remember, an ideal gas at a volume of V at some pressure will become a volume of kV at some lesser pressure (k being some constant) and that is why the bubble grows as you reduce pressure.

[tex]V\uparrow=\frac{\overrightarrow{nRT}}{P\downarrow}[/tex]

Next the brimming: You say you are using a soft hose. The brimming is probably due to a) the gas bubble formation and expansions along with b) the collapse of your hose. Get a stiffer hose and remove the trapped gases from your tap water.

Good luck.
 
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  • #3
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Just one more question. To eliminate the collapse of the tube: according to my understanding, only the green part of the tube needs to be stiffer http://www.inkline.gr/temp/Vacuum-ElasticTube2.gif [Broken] , am I right?
 
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  • #4
691
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Yes, you are correct.
 

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