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I seem to be having an issue with my little vacuum chamber I am building where I can only reach a base pressure of 6,000mTorr which is pretty high and am trying to figure out what is wrong. I have a new, clean viton gasket; a brass base plate, I have tried thick plastic and glass chambers. All the ports are sealed with either teflon tape and/or dow corning vacuum grease. I am using just water valves with the internal ball and rubber seals to vent or pump the chamber.

Any suggestions on what is wrong?

enter image description here

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    $\begingroup$ So where do you think the leak is? Or has the pump just reached its limit? $\endgroup$ – Solar Mike Jul 3 '18 at 5:45
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    $\begingroup$ I'm voting to close this question as off-topic because this is unlikely to be of any use to any other visitor to Eng.SE $\endgroup$ – Carl Witthoft Jul 3 '18 at 17:56
  • $\begingroup$ Doesn't your last sentence point to the problem? Common water valves are not likely rated for vacuum. $\endgroup$ – hazzey Jul 3 '18 at 18:46
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6 Torr isn't too bad really and that vacuum pump looks a bit small and I am guessing is not a high quality unit? Also, as said in the comments, that water valve may be leaking as well as it isn't rated for vacuum but instead for pressure.

Regardless, here is a simple leak detecting technique that can be of help. When we have a vacuum problem in the lab we start with a squirt bottle of acetone and lightly apply around the connection points. If there is a leak where the acetone is applied it will quickly enter the chamber, vaporize, and increase pressure on your gauge letting you know where the leak is.

Hope this helps, and good luck with the project!

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eatscrayons suggestion for leak detection is great. However, instead of acetone, I would use isopropyl alcohol (or even better, a helium gas source since He is such a small molecule).

It is certainly a small roughing pump so I would not be surprised if you're hitting a floor of 6 Torr. Time to start looking into larger roughing pumps -- those which use oil tend to be able to achieve harder vacuum levels!

If you're trying to reach pressures <50 mTorr (dubbed "high vaccum") you will almost certainly need to introduce a cryo-pump or a turbo-molecular pump.

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-I have 2 of the cheap chinese vacuum pump from the brand "Value" , they look very much like yours. They often go up to 3 Torr when the oil is fresh then 6 Torr when the oil is contaminated with water

-1st step is to test the max vacuum with simply a jauge connected directly to the pump, top see the max you can get without potential leaks.

-then check the oil, if it's cloudy, there is water in it, it has to be changed with proper vacuum oil

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