[bedwar4]

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?

[curiouscat]

Can you bake the system? Or leave it on vacuum for 24 hrs or so? See if outgassing will stop.

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.

[curiouscat]

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.

[H2O2JHD]

A picture of your setup would be nice. Try installing a glass bulb to your manifold.

[magician4]

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

[Enthalpy]

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