In Reply to: Re: help with vacuum leak detection posted by Bill S on 01/12/02 at 1:18 AM:
good Evening Bill,
First, let me say that you have an impressive piece of glassware there. The photos were most helpful. I am hopeful that we can do something about the leak as the system is somewhat simple, but it may be time consuming and frustrating as all vacuum systems can be.
First off can you be sure that there are no pockets in the mechanism that could trap air and release it slowly so as to appear that there is a leak? This is commonly called a "virtual leak" if you are not familiar with the nominclature. As an instance, is the main pendulum shaft solid or hollow? If hollow, are there ample holes to allow air to escape. Or is the pendulum weight solid or a sealed container with loose weights? I think you get the drift of what to look for in this regard.
Next, assuming that there is actually a real leak, I would concentrate on the bottom plate as the likely source since the rest of the chamber is so simple. ( of course I have been stung many times by just such logic). You were somewhat vague in exactly how the feedthrus on the lower plate were sealed. It seems as if you are depending on pressure differential to hold these tighty in place. If so it is not the best idea. The pressure on a 1/2 inch hole is really not that much and spread over a 1 1/2 inch square surface could be insufficient to give a good seal. If you could machine threads into the nipples and pull these up tight with a nut on the inside it would be better. If you could cut "O" ring grooves into the bodies of the feedthrus that would be better still. ( You probably don't need me to remind you not to tighten up metal objects directly unto the glass, but I will). You could also make the gaskets somewhat smaller to increase the pressure holding them.
Some additional thoughts. The stem of the valve doesn't look to me to be all that great for vacuum. I would look at it very carefully to be sure it can do the job. If you have any doubts, check with the Swagelock people and get a vacuum rated valve from them. Also, to make things really simple I would mount the lower plate directly on the belljar and repeat the rate of pressue rise test again to see if you can rule out the clock mechanism as a source of the problem.
There really is no good instrumentation that I know of that is inexpensive and usable at the pressure that you are using. Some ultrasonic devices have been sold but are not very reliable and tricky to use and not particularly inexpensive. As a last resort, I might try some alcohol dyed a distinctive color and flood the various feedtrus and joints for an extended period of time and see if some "track" would show up.
That is about all I can think of for now. Good luck and please get back to me to let me know how you made out. If I can be of any further help, please contact me. If I think of anything else I will get back to you.