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Randy

The ice blocks you are experiencing are being caused by the melting and 
refreezing of ice particles in the heat exchanger of the Cryostream 
Coldhead (blue tin).  These ice particles come from your liquid nitrogen 
supply.

Be sure to clear out your dewar every month or so as the concentration of 
ice particles will increase.  The amount of ice does not depend on what 
dewar you are using.

All liquid nitrogen has some ice in it, picked up during manufacture, 
transport and storage.  The ice particles are sucked out of the dewar 
with the liquid nitrogen by Cryostream, and into the first half of the 
heat exchanger in the blue coldhead.  The ice (with the nitrogen) passes 
out the coldhead along the SUC path of your nylon tubing.  While passing 
through the pump and the controller these ice particles will melt.  As 
the water particles reenter the coldhead along the second path of the 
heat exchanger they will refreeze very quickly (as the coldhead is at 
about 80K,) and stick to the wall of heat exchanger  After some time a 
block will occur.  The bore of the capillary tube inside Cryostream is 
extremely narrow and freezing particles of moisture will stick to the 
wall of this capillary.  As you have discovered, Cryostream will 
automatically monitor the the build up of this ice block and will warn 
you.  This should give you time to finish a data collection.  This sort 
of ice blockage is a common problem for which the solution is well known.  
In these situations, we normally advise the use of one of our Line 
Driers.

A Line Drier will filter the particles of water from the nitrogen gas 
line and will extend continuous run time from a few weeks, as you have 
discovered, to many months.  Once these Line Driers are saturated, they 
can be regenerated and reused.

Please read the instructions you got with your Line Driers, Tech Note 
9403a.  this explains all about ice blocks and how to use your Line 
Drier.

Ice blocks are not caused by humid weather.  Humidity will only affect 
the cold gas stream in atmoshpere not the inside of Cryostream or the 
nitrogen in the dewar. Any kind of mesh over the end of the leg will do 
very little to prevent ice blocks.  We are talking about very small 
particles of ice.

Remember that once a ice block occurs it will take about 5 or 6 hours to 
melt.  This is because the blue tin is vacuum insulated.  If you are not 
using a Line Drier to filter out the moisture from the nitrogen and you 
get a block every few weeks, the quick way to remove it is to blow dry 
air from your dry air unit down the flow path AT UNDER 10 LITERS PER 
MINUTE until the system warms up and the ice block disappears.  
Alternatively, if you have go the time to wait, (ie overnight) just 
switch Cryostream on and the moisture will blow out of the nozzle.

Best regards
Richard L Glazer
Oxford Cryosystems

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