Hi Rajan,
1. Just to clarify, the "Helium dewar" is actually hanging from thin
metal tubes welded to the top helium stacks. The o-rings are only designed
to hold vacuum, but are designed for near-room temperature duty. I assume
you meant this, but just in case..
2. I'd be very surprised if the material wasn't viton, but the best I
can do for evidence is that an Oxford engineer said so once, and everyone
calls them that. Also, in most vacuum applications I've seen, viton o-rings
are chosen for their chemical and mechanical durability (viton is not the
least permeable). Maybe someone has seen manufacturer's actual order sheets.
3. The holy book for o-rings is the Parker catalog at
http://www.parker.com/literature/ORD%205700%20Parker_O-Ring_Handbook.pdf
Page 17 lists the properties of FKM o-rings, the non-proprietary
name for viton.
They list safe usage temperature down to -26C, and below that for
"special formulations".
Another site is
http://www.marcorubber.com/viton.htm
They list alternative viton formulations, with higher and lower use
temperatures.
Most users don't like the idea of going below 0 C, though, and neither do
most magnet manufacturers. That seems to be a conservatively safe limit for
ALL elastomers (See Parker, p. 20).
I myself have seen ice caps on the upper vacuum flange of a magnet,
formed by frozen condensation during improper VT operation, with no
(apparent) deleterious effect. But since I didn't check the helium blowoff,
that may be more of a reflection of the high efficiency of cryopumping than
proof that the seal remained intact.
During magnet decommissioning, after deenergization, N2 gas is bled
into the vacuum chamber to break the vacuum. That operation takes on the
order of an hour, even with that deliberately very bad leak, before
cryopumping finally fails and the vacuum goes.
So if you're worried about a "challenger effect" due to a leak for
some reason, use your He boiloff meter as your early warning system. (And
don't get fooled by increasing blowoff caused by incoming low pressure
fronts!)
Best Regards - Gerry
Gerard C. Chingas, Ph.D.
Research Instrumentation Manager
Chemistry, COAS, MS 1520
Boise State University, Boise ID 83725
PH: 208-426-4913, FAX: 208-426-1311
gerrychingas_at_boisestate.edu
-----Original Message-----
From: rajan paranji [mailto:paranji_at_chem.washington.edu]
Sent: Tuesday, February 23, 2010 12:45 PM
To: AMMRL
Subject: AMMRL: question about magnet O-rings
Dear All
* Can you share your information on what kind of material the
O-rings that are used in NMR supercon magnets are made of (viton
or similar, for example). I am particularly interested to know the
type of material that is used for the seals that separate the room
temperature bore from the Helium dewar.
* Do you also know what is the lowest temperature these O-rings
can sustain before something similar to what led to the 'shuttle
challenger disaster' can happen ?
Thank you for your kindness and patience.
Best Wishes
Rajan
--
*_______________________________
Rajan K Paranji, Ph.D.
*NMR Facility Manager
Department of Chemistry
Room 65, Bagley Hall
University of Washington
*Seattle, WA 98195*
*ph: 206 685 2581 pager:206 680 3779
fax: 206 685 8665
email: paranji_at_chem.washington.edu
___________________________________*
Received on Wed Feb 24 2010 - 09:50:20 MST