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Topic: Vacuum system construction/operation  (Read 3671 times)

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Offline bedwar4

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Vacuum system construction/operation
« on: July 12, 2013, 11:16:42 AM »
Hello. New to board. I appreciate any input. Here is problem:

I am building a cryogenic extraction system for stable water isotope lab. I do not have much experience with vacuum systems, but the system is small and simple, all of the components are new, and only a rough vacuum (100 mtorr or so) is needed. The system is set up as follows: Edwards RV5 two stage pump (KF flange/hose barb adapter) - thick walled clear tubing -speedivac diaphragm isolation valve (KF flange to hose barb connections)- tubing - cold trap (hose barb fittings(glass) - thick walled tubing - small glass manifold - 1) 531 thermocouple gauge (tubing to hose barb/flange to npt adapter) 2) reaction vessella via built in Teflon valves. All connections seem tight etc. To start, I have not connected the reaction vessels and left the valves closed. When I pumped down, regardless of whether it was for a few minutes to a couple hours, once I isolated the pump from system, the system side rapidly rises in pressure. I don't have access to a helium leak detector, so I used a solvent near the joints, and saw no difference in gauge reading. When the pump is on, the pressure gets down to about 60 mtorr, which is our target range. Once I isolate it, it takes a couple minutes to rise to 1000 mtorr, and continues to rise. Suspecting a leak, I carefully redid all of the connections, etc, and same result. So, I removed everything but the isolation valve and one piece of tubing going to the gauge. Same result. I thought it must be the npt fitting not sealing well and redid that a few times. Same result. Finally, I removed the fitting, and due to the diameter of the gage and the thick walled tubing, just worked the tube over the gage with no fitting. Same result. So, all I have now is the pump via hose barb flange to one piece of 2' tubing to the isolation valve (KF flange/hose barb fittings) to another 2' piece of tubing, to the gauge stuck in the end. The connection all seem tight enough for our application. IT is a struggle to get the tubing onto the hose barb fittings, and the gauge is crammed into the end of the tubing pretty well, BUT, I get the same result. I pump down to 50 or 60 mtorr, and as soon as I isolate the pump, the pressure rises rapidly. I realize that some outgassing is going to happen, but this seems excessive. Also, I don't see where a leak that large could be coming from now. Even after the gage reading moves all the way back towards atmosphere, there still seems like a vacuum in the tubing if I disconnect it. The question is: is there something too obvious I am missing here?

Offline curiouscat

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Re: Vacuum system construction/operation
« Reply #1 on: July 16, 2013, 02:23:08 AM »
Can you bake the system? Or leave it on vacuum for 24 hrs or so? See if outgassing will stop.

Quote
it takes a couple minutes to rise to 1000 mtorr, and continues to rise

Continues to rise to? What pressure does it equilibriate at, if at all.

Offline curiouscat

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Re: Vacuum system construction/operation
« Reply #2 on: July 16, 2013, 02:24:38 AM »
What's your "thick walled clear tubing" made of? Maybe you have some material that's porous to gases especially at vacuum?

Can you pressurize your system? How stable is it under pressure.

Offline H2O2JHD

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Re: Vacuum system construction/operation
« Reply #3 on: August 30, 2013, 05:06:38 PM »
A picture of your setup would be nice. Try installing a glass bulb to your manifold.

Offline magician4

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Re: Vacuum system construction/operation
« Reply #4 on: September 05, 2013, 10:13:07 PM »
Quote
Even after the gage reading moves all the way back towards atmosphere, there still seems like a vacuum in the tubing if I disconnect it.
just a nutty idea, but might it be that your manometer is faulty?

that fast a rise in readout pressure, with obviously vacuum still present in the vessel, this makes it for a likely candidate, esp. if the leak was somewhere in the reed pen  (if we'd talk standard manometer type, for example)


regards

Ingo
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Offline Enthalpy

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Re: Vacuum system construction/operation
« Reply #5 on: November 02, 2013, 09:52:34 AM »
What is the temperature?

For instance water over +45°C has a vapour pressure exceeding 60mtorr, so if you heat source is warmer, the pressure will rise as soon as the pump cannot maintain the liquid that cool through evaporation.
http://www.engineeringtoolbox.com/water-vapor-saturation-pressure-d_599.html

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