[3dem] Column vacuum problem with a Tecnai F20

[Shi, Dan (NIH/NCI) [C]]

Dear John,
You're right it's very difficult to find out the minor leak source. I
saw the engineer has done some very good test to sort out the problem.
It's not clear to me the IGP1 went to 5-6 after blanked the cryo-box was
at room temperature or cooled down or both cases. The leak seems to be
caused by cooling down the cryo-box, so either insertion rod of the
cryo-box or the cooling rod could generate the problem. If the IGP1
could be hold at 6 at both room and cold temperatures after the cryo-box
blanked, it might indicate the leak is from the cryo-box insertion rod.
As other people have pointed out the O-rings along the rod play major
role to seal the vacuum. If the engineer has replaced all the o-rings
and still not solving the problem, another possible cause might be the
thermal insulation between the cryo-box/cooling rod and the insertion
rod is not good enough which may cause the rod shrink a little. That's a
pure guess. Good luck,

Dan Shi, PhD
LCB, NIH/NCI
Bethesda, MD 20892
USA 

-----Original Message-----
From: John Rubinstein, U of T [mailto:john.rubinstein at utoronto.ca] 
Sent: Tuesday, July 22, 2008 12:53 PM
To: 3dem at ncmir.ucsd.edu
Subject: [3dem] Column vacuum problem with a Tecnai F20

Dear Colleagues,

We have been having a strange vacuum problem with our FEI Tecnai F20 G2 
.  Our engineer says that he has not come across reports of this problem

through FEI, but I was wondering whether anyone in the 3DEM community 
has seen the problem before and knows what the cause is.

Our column vacuum deteriorates when liquid nitrogen is added to the cold

trap.  At room temperature the column vacuum (IGP1) sits at around 13.  
Upon addition of liquid nitrogen, IGP1 deteriorates to ~20.  We have 
aready had some work done on the microscope and learned some interesting

things.  We had the component that couples the external copper braid to 
the cryobox replaced but that did not solve the problem.  Subsequently, 
as a test, our engineer  removed the cryobox from the column and blanked

off the cryobox bellows.  Without these components,  IGP1 improved to 
~5-6.  However, replacing the cryobox bellows with a new one did not 
solve the problem. 

As you will know, vacuum leaks can take a long time to sort out, so any 
advice that helps us solve this one faster would be greatly 
appreciated.  Also, could people who use a F20 (with or without film) 
please send me some typical values for their column IGPs (IGP1 and IGP4 
in our instrument).

Best wishes and many thanks,
John

-- 
John Rubinstein
Molecular Structure and Function Program
The Hospital for Sick Children
555 University Avenue, Rm. 3330
Toronto, ON
Canada
M5G 1X8
Tel: (+001) 416-813-7255
Fax: (+001) 416-813-5022
www.sickkids.ca/rubinstein


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