# Vacuum pump question

1. Sep 7, 2012

### alibaba2

hi to all,
i have been trying to find some similar project solution on the internet but so far have found nothing, so i am posting here.

here is the setup ( see attached picture) :
in "fig 1" a metal chamber, perfectly sealed contains inside it a balloon filled with air (red color). the gaps between the balloon and the inner chamber walls contain water(blue color) . a pipe is attached to the chamber and the balloon (again a perfect seal) and the pipe leads to a vacuum pump.

in fig 2, the air from the balloon is sucked by the vacuum pump leaving the metal chamber in a state of vacuum.since there is a perfect seal no air or water can enter the chamber.

has anybody done such an experiment? if so what vacuum pump is needed to achieve this experiment? specification? please kindly explain how much is the suction force (bar or torr) needed to excavate the air and create a vacuum in the metal chamber.
thank you

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2. Sep 7, 2012

### Nessdude14

If you start with water in the tank, you won't have a vacuum in the tank after you suck the air out of the balloon. The water molecules will eventually vaporize and create pressure in the tank.

Also, when you decrease the pressure inside of the balloon, it will only shrink if there is pressure outside of the balloon, pushing in on it. If you try to create a vacuum in the tank using a balloon, you'll find that the balloon won't shrink to the size you indicate in the illustration; rather, it will stay in the completely inflated state when you suck the air out of the balloon.

3. Sep 7, 2012

### alibaba2

thanks Nessdude,
i am looking a creating at least 80% vacuum (rough vacuum). the comment regarding the shape of the balloon was very helpful.

if i understood right even if all the air is sucked from the balloon it will maintain its initial inflated shape although it is "empty"?
and that will be due to the absence of any pressure from the outside to make it shrink ?

any idea as to what pump (make, model) will do the job?

thanks

4. Sep 7, 2012

### RocketSci5KN

Just use a simple water aspirator - works fine with air or liquids. Ultimate vacuum achievable is the water vapor pressure at whatever temperature the water flowing through the aspirator is at - lowest pressure slightly above 0 degrees C. You'll have a 95%+ vacuum even with room temp water flow.

5. Sep 7, 2012

### Nessdude14

To create a vacuum in the metal chamber you described, you would connect the vacuum pump directly to the empty chamber (with no balloon inside). The gas inside the tank will be pumped out and the pressure will drop. To create "80% vacuum" (by that I assume you mean less than 1/5 atmospheric pressure) you need a vacuum pump capable of creating 20 kPA pressure or lower, and a tank capable of withstanding over 80 kPA. I'm not very familiar with specific vacuum pumps, but I do know that you shouldn't have a problem finding pumps that are capable of creating these low pressures. According to wikipedia, a piston pump is the cheapest option to achieve this range of pressure, but rotary vane pumps are more common and can achieve lower pressures (closer to ideal vacuum).

6. Sep 9, 2012

### alibaba2

what if the balloon is made of elastic material that will cause it to contract by itself if there is no air inside?the elastic material will create the outside pressure needed
will this solve the problem and lead to the expected result as shown in Fig 2?

thanks

7. Sep 9, 2012

### Nessdude14

It would only be able to completely contract if it can equalize the pressure inside and outside of the balloon (effectively makes the balloon pointless). Otherwise, when the balloon contracts, it must expand the existing gas in the tank, further lowering the tank's pressure, and when the tank's pressure decreases, the balloon will want to expand again.

If you're simply trying to create a vacuum in a metal tank, the balloon is unnecessary. You simply need to connect a vacuum pump directly to the tank. This will create a vacuum (or at least a very low pressure environment.)

8. Sep 9, 2012

### mrspeedybob

Available at any automotive parts store or anyplace which sell automotive tools. This will easily reach 25" of vacuum (1/5 atmosphere) and cost $30 -$40