Pinch-off tubes can be vacuum tight for years for a small volume dewar such as those used in IR detectors. My guess is that the volume is around 30 cc, assuming 1 inch OD and 0.5 inch ID for cold finger and about 3 inch long.
I have a chamber that houses a getter pump and nothing else.
The chamber is closed with a gate valve and a pinch-off tube.
I plan to pump it down, activate the getter, seal the gate valve, and then look for leaks using an RGA.
When satisfied, I'll close the pinch-off, and the whole assembly (which is now not monitorable) will have to live for a year or two before being launched into orbit.
At a later time, the gate valve will open.
I need to be certain that the getter does not saturate during the years of storage.
I don't have a way to monitor the pressure after I pinch-off, which is a big headache for me.
I don't have any numbers to hand, but can offer some small reassurance that the pinch off system works (I'm assuming it's an OFHC copper tube of extreme cleanliness). This same system is used to seal off electron tubes of various types before being put into storage. There are many applications requiring that these tubes be able to be storage for over 2 years, and be functional at the end of it. Also, metallurgical examination of the pinch, using cross sections examined in SEM show that the pinch off becomes just as homogenous as the rest of the tube, ie the leak across the pinch is the same as the leak through the solid metal. If I manage to find any actual numerical data I will forward it.
ok - so I undrstand the the issue is not leak rate, but success (or failure) rates.
With high volume applications, if there's a certain chance of the procedure not sealing, it translates directly into yield numbers.
For this design, if the procedure fails, the experiment won't work, so it becomes a mission assurance issue.
The only way I know of verifying the seal quality is from the inside, which means another feedthrough.
Anyone familiar with a way to verify the seal from the outside?
