[time-nuts] insulation/packing material for OXO?

[Dr Bruce Griffiths]

Christopher Hoover wrote:
> Folks -
>
>  
>
> I've got several OXO's suffering from disintegrating insulation between the
> can and core.   The insulation in these OXO's is the greenish grayish foamy
> stuff.  It turns to dust over time.
>
>  
>
> The two units I opened most recently also had mechanical damage (broken
> crystal, broken solder connection).  I hypothesize this is a result of the
> disintegrating foam allowing the core to bang around inside the can.   The
> OAC 10MHz OXO from my 3856C actually rattled.
>
>  
>
> I've since fixed the broken solder connection and gotten the oven and
> oscillator to work.  Now I need to repack the core into its can.
>
>  
>
> What kind of material is this greenish-grayish foam?   Is it easy to get?
> Easy to work with?
>
>  
>
> If not, is there another material that works well and is easy to source?
> It seems like HP used a Styrofoam of sorts on the 10811.
>
>  
>
> -ch
>
>  
>
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>   
One of the biggest problems in selecting a foam material is to ensure 
that it doesn't melt or soften near the oven operating temperature.
Thus one needs to know the operating temperature of the oven and the 
maximum oven temperature during warmup, overshooting the set point is 
not uncommon.
For example polystyrene distorts at temperatures in the range of 74-88C.
Styrofoam is  essentially foamed polystyrene and may have a maximum 
service temperature as low as 66C.
Polyurethane foams have a higher maximum service temperature of around 100C.
Phenolic foams have even higher service temperatures, up to 155C.
All of these materials are readily available.

More inert foams with relatively high maximum service temperatures, such 
as foamglass would be attractive except for the corrosive sulphur 
dioxide (at least it smells like that) used to fill the the bubbles 
blown in the black glass matrix.

You could always use silica aerogel but it is extremely delicate.
It has been used to insulate peltier heat pump cooled marine refrigerators.

Bruce