30 04 2009

It appears the chamber is not holding a vacuum. The pump and gauge check out (see previous post). But with the chamber attached, I can’t even get down to 5e-2 torr (the top of the gauge’s range) after running for over an hour. So Houston, we have a problem with the chamber.

F*ck me.

Also f*ck WordPress for blocking posts with the word f*ck. What are you? my f*cking nanny?



3 responses

30 04 2009

If the pump takes 45 minutes to get a vacuum on the small volume of the closed piece of pipe I hardly see how it could create a decent vacuum once connected to the huge (relatively to the pipe) chamber.
It looks like this pump is greatly underdimensioned for this work or there is some major leakage on the pump structure.
Also, try to keep the air inside the chamber as much dry as possible, vacuum pumps and humid air are not friends :-)


30 04 2009
A. McEvoy

Well, unfortunately it isn’t as easy as taking 150 L divided by your flow rate to get the pumping time. I believe that 40 L/s is at 1 atm or some baseline pressure, and it doesn’t take into account that the pressure changes. The flow rate curve as a function of pressure will go way down at lower pressure, simply because there isn’t 40 L/s of air available statistically. However, at some point you will reach equilibrium between outflow and inflow through your pump system, and the pressure should stabilize.

The real test of whether you are leaking or not is to turn on the turbo (once you get down to mTorr range), pump down a ways and then turn everything off and see what happens to the pressure. If you go back up to atmospheric pressure then you have a serious leak. If you go up a moderate amount but still have some vacuum then you might have some out-gassing issues to deal with, and if your pressure stays very low then you are in great shape.

I also agree with Giorgio in that your pump should have reached its lowest operating pressure (~1E-9 Torr?) within a matter of minutes on that tiny volume. Something is either not right with your guage calibration or your pump system is leaking somewhere. God forbid your turbo seals are shot and it needs a rebuild…that takes a long time and more money. Also, if you cannot get below the gauge’s upper limit after a full hour then you have chamber problems as well. By the way the turbo should not be operating at such high pressure…

Tighten all of your seals too, as I would have to say that conflats must be tightened considerably more than finger tight. You don’t have to go all the way down, but you need a good bite into the copper ring.

1 05 2009

Upon asking a friend working in the field, he advised me that to get a vacuum in the range of what you are looking for and for the chamber volume that you have, a turbomolecular pump of 300/400 l/sec is needed, coupled with a correctly dimensioned primary (backing) pump.

Check all the venting connections and every point that can be a possible leak in the vacuum line.


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