×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

leak rates for pinch-off tubes

leak rates for pinch-off tubes

leak rates for pinch-off tubes

(OP)
I'm looking for numbers on leak rates using pinch-off tubes.
Thanks,
 Ben.

RE: leak rates for pinch-off tubes

For what application?

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.

TTFN

RE: leak rates for pinch-off tubes

(OP)
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.

RE: leak rates for pinch-off tubes

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.

RE: leak rates for pinch-off tubes

The only caveat is that if you've never done it to find someone who has and/or make lots of practice runs.

Given your description, a good pinch-off should certaintly be no worse than a gate valve.  

TTFN

RE: leak rates for pinch-off tubes

(OP)
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?

RE: leak rates for pinch-off tubes

Is building an ionization gauge into the experiment out of the question?

TTFN

RE: leak rates for pinch-off tubes

(OP)
Has to be non-magnetic.
A hot-cathode gauge can be added, though will probably break on launch. (but will work during storage)

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources