Low Temp Clogs

George DeTitta detitta at hwi.buffalo.edu
Wed Oct 11 13:24:28 EST 1995

It isn't your imagination that crystals of ice and/or CO2 are coming
out of the transfer tube.  When commercial dewars are sent back to
the plant for filling they are typically opened to the atmosphere.
As they maintain their temperature for some time wet air funneling
back inside the dewars deposits ice crystals on the inside.  When you
start filling with those supplies there will be a slow escape of very
fine crystals into the transfer tube and on to your crystal.  As we
have also optimized the laminar flow, and come in on the crystal at 45 degrees
to the omega axis (over the tube design) we know that it's not a
question of turbulence blowing back ice into the transfer line orifice.
The only way around this problem is to physically trap the ice crystals in
a filter of some kind, such as fine mesh.  There are many strategies for
avoiding ice buildup on crystals (many touched upon in this newsgroup
already) but this is a general problem that has its solution in frequent
purging (if you do short term experiments (1 week) or in filtering (if you
do longer term experiments (two months)).



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