Grease

[Paul Jorden]

Andrew,
To continue the grease discussion:

> I was rivetted by you statement that:
> 
>> Finally, In case you don't know, we have now learnt that silicone 'high vac.
>> grease' is bad news in a cryostat. It is clear that it can become volatile
>> and deposits itself on surfaces. (It's vapour pressure was quoted as ~ 10-6
>> mbar; this is clearly not low enough!) We no longer use it. We use only a
>> small trace of 10-12 mbar grease (non-silicone) now on o-rings only.
> 
> I have been using Apiezon L, M and N with impunity (thermal coupling
> O-rings, etc.) I have, of late, had contamination problems with
> my thin chips.
> 
> Tell me more ....
> 
We used to use Dow Corning High Vacuum grease; this is silicone, with a quoted 
vapour pressure of < 10-7 mbar I believe.
We used it as a thin layer to improve thermal contact under our Tek/SITe 1K CCDs 
and the cold block.
We definitely got contamination within the cryostat from this source- it's a 
long story, but mass spectrometer analysis confirmed it. It is 'well known' that 
this vacuum grease is actually bad in this respect!

Apiezon 'M' (a mineral-based hydrocarbon grease) is quoted as having a vapour 
pressure of < 10-7 mbar also.

We have now switched to using Fluorinated Fomblin grease FM090 from Balzers; 
this has a vapour pressure < 10-12 mbar. It costs us #200 per 50 gm, but we 
don't use much of it! (Only the minimum on 'o' rings now, never elsewhere..)

For improved thermal contact we now use a thin layer of indium where necessary.

Cheers, Paul Jorden
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