What I have done is slip ca. 2 feet of foam pipe insulation (readily
available in various sizes ca. 1/2" id, 5/8" id etc. from my local
hardware store) over the tygon tubing. The insulating sheath is
pushed all the way down over the exhaust port to the point where the
port meets the top of the cryostat. An inch or two of the tygon
tubing sticks out the other end and I plug it up with a nalgene quick
disconnect with a disposable pipet bulb slipped over its end. I slit
the bulb to allow N2 to exhaust while maintaining a possitive
pressure in the cryostat. When filling N2, I remove only the quick
disconnect and its bulb. There is no ice formation during normal
operation, and very little frost formation during filling. Some
condensation may form on the insulation during a particularly humid
South Carolina summer day, but it does not run down the cryostat or
cause any other incovenient or unsightly complications. I have used
a piece of string to bend the tubing and insulation into a somewhat
flared inverted "U" so that exhaust is vented down but away from the
cryostat during filling.
The presense of N2, and normal boiloff, is easily acertained by
checking for abnormal presence or absence of moisture and by checking
for a low flow on the face if one holds one's face up close to the end
of one of the pipet bulbs. I have never liked to fill N2 with an
accumulation of ice on the exhaust ports. The butt welds associated
with those ports is one of the most likely places for a vacumm
failure in a weld. The contraction of a large block of water ice as
it cool to near liquid N2 temperature would seem to me to place an
inordinate strain on these welds.
If anyone is successful in locating a source of the aluminum finned
extrusion, please let me know.
Ron
Dr. A.R. Garber Phone: (803)-777-2088
NMR Services FAX: (803)-777-9521
Dept. of Chemistry email:Garber@chem.scarolina.edu
University of S. Carolina
Columbia, SC 29208