Dear Ammrl,
Here is the summary about the EPR cryostat.
Thanks to Alex Marchione, Roger Clausen, Torbjörn Astlind, Jane Strouse,
Yong-Wah Kim, Jerry Dallas, Phil Dennison and Joseph Massey.
The inner profile (determined by poking inside with a thin rod) is
cylindrical with one step at the depth of ~25.0 cm and dead end of the
narrow part at 38 cm depth.
So only the thinnest part of the transfer line gets past the step. Teflon
gasket at the base of TR line's thin part has to be seated at that step,
where it forms the barrier between cryogen inlet and exhaust.
I've put up a sketch at
http://nmrwiki.org/wiki/index.php?title=ESR900_continuous_flow_cryostat
Please feel free to add anything.
Here is the summary of problems that can lead to He bypassing the cryostat
and going directly to the exhaust:
1) missing or damaged teflon O-ring
2) O-ring stuck inside the entry port (can be taken out with a ~30 cm long
wire hook)
3) bent tip of the transfer line that causes O-ring to be not properly
centered (if the tip is bent it may be very carefully straightened while
checking the angle with the straight angle ruler)
4) very slight misalignment of the transfer line inside the entry port (this
can be remedied by slightly wiggling the tube)
5) lock nut on the TR-line being positioned too close to the end of the
line. This will cause insufficient depth of insertion.
6) in addition, leaks anywhere in the cryogen line inside the cryostat will
cause failure of the sample cooling
In my case it was "4" (at first I moved the lock nut so that it is exactly
at 25 cm from the far end of the teflon gasket). I was testing the system on
liquid nitrogen. At first there was no flow of cold gas through the sample,
when I saw that the exhaust line started freezing but the sample was still
at ~300K I have loosened the knurled nut securing the TR line to the
cryostat, loosened the lock nut then pushed TR-line down, it gave in a
millimeter or so. That fixed the problem.
The nut securing TR line must be placed a bit further from the teflon ring
then the estimated measurement of depth to the first step inside the
cryostat entry port to allow tightening of the O-ring.
Thank you!
Evgeny.
--
Evgeny Fadeev, Ph.D.
Director, BioMolecular Spectroscopy Facility
1212 Natural Sciences 1
University of California Irvine
Irvine, CA 92697
telephone: 949-824-5842
www.physics.uci.edu/~biomolenmr <http://www.physics.uci.edu/%7Ebiomolenmr>
nmrwiki.org
Received on Tue Sep 23 2008 - 12:57:55 MST