Strange FEG IGP problem

[Bill Tivol]

Dear Lists,
	We have been experiencing an unusual problem on the F30H.  At 
intervals of a few weeks the vacuum measured by the IGP at the top of 
the FEG (IGP3 for those who know the scope) deteriorates and the FEG is 
shut off by the computer.  Examination of a record of the vacuum shows 
that IGP3 has a vacuum too good to show above baseline for the first 
few days--the reading is constant at 10^-12 Pa--then the pressure rises 
into the range of ~10^-7 Pa, often will briefly get slightly better, 
then will rather quickly get worse and the FEG will shut off.  The 
boards containing the power supply and the electronics for measuring 
the vacuum have been replaced, but that did not solve the problem.  
Both the FEG and IGP are pretty simple devices, and it is hard to think 
of what could be going wrong.  A vacuum leak would not show the 
observed time course of vacuum readings, and the SF6 that surrounds the 
outside of the FEG apparatus would give different symptoms.
	Has anyone seen anything similar?  TIA for any help.
Yours,
Bill Tivol, PhD
EM Scientist and Manager
Cryo-Electron Microscopy Facility
Broad Center, Mail Code 114-96
California Institute of Technology
Pasadena CA 91125
(626) 395-8833
tivol at caltech.edu