Hi Spinlanders,
In the AMMR-newsgroup there was a summary of Brian Myers concerning Liquid
He purity:
"… the impurities in the LHe are likely frozen and will probably remain on
the bottom of the dewar".
We believed this for long time, but we had to learn at the BMRZ of
Frankfurt University that this isn't true.
LHe can be contaminated by ppm's of hydrogen. This seems to be no problem
for cooling normal supercons. (We don't know yet if there is any objection
to using it for "pumped" NMR magnets!)
The hydrogen contamination was found to disturb in a non-NMR experiment,
when LHe had to pass a very fine slit inside the transfer-line (for
regulating a stream of He gas at few degrees above 4K).
We observed plugging within less than 1 hour. Even fixing the position of
the transfer line 30 cm above the bottom of the dewar didn't help: plugging
seemed to be independent of the height of the transfer-dewar tip.
In LHe the hydrogen impurity might form very fine solid particles which are
not settling down on the bottom but are swirrled up by the boiling liquid
helium.
When we used LHe with a lower content of hydrogen, the experiments run
over many hours whithout having this problem.
Using the same quality of LHe, with the 3 pumped NMR-magnets at our
facility we didn't have any problem in the last 9 years.
Greetings from Frankfurt
Helmut Hanssum and Gottfried Zimmermann
Dr. Johannes Gottfried Zimmermann
Goethe-Universitaet Frankfurt
Institut f. Organ. Chemie und Chem. Biologie
Zentrum f. Biomolekulare Magnetische Resonanz (BMRZ)
Marie-Curie-Str. 11
D-60439 Frankfurt
Tel: +49 69 798 29122
Fax: +49 69 798 29515
gozim_at_nmr.uni-frankfurt.de
Received on Wed Dec 01 2004 - 09:38:27 MST