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'[EE] potting and low temperatures'
2006\05\17@201205 by Henry Hallam

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Hi all,
Does anyone have experience with potting a device that has to survive a
very low temperature? I am building a GPS tracker for a high altitude
balloon where the temperature will get down to -50C or colder.  I'm
considering potting it for better mechanical security but is this likely
to cause problems with different rates of thermal contraction?  Are
there any particular potting compounds that are recommended? I haven't
done much potting before and have always used ordinary "araldite" epoxy.

Cheers

Henry

2006\05\18@035318 by Alan B. Pearce

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>Does anyone have experience with potting a device that
>has to survive a very low temperature? I am building a
>GPS tracker for a high altitude balloon where the
>temperature will get down to -50C or colder.

Yikes. Spacecraft instruments we build use that for survival temperature
tests, but not operating. Is there not some form of survival heater
available powered from a solar cell?

The other thing I would do is have the unit encased in some form of thermal
insulation so its own heat is kept local. A suitable plastic or metal box
with a blob of pink or blue foam carved out to take the receiver should do.
I would think you would get away with a passive aerial, and a short bit of
coax to the receiver, so you shouldn't need to worry about a pre-amp getting
cold.

2006\05\18@053622 by Nathan Duehr

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Alan B. Pearce wrote:
>> Does anyone have experience with potting a device that
>> has to survive a very low temperature? I am building a
>> GPS tracker for a high altitude balloon where the
>> temperature will get down to -50C or colder.
>>    
>
> Yikes. Spacecraft instruments we build use that for survival temperature
> tests, but not operating. Is there not some form of survival heater
> available powered from a solar cell?
>
> The other thing I would do is have the unit encased in some form of thermal
> insulation so its own heat is kept local. A suitable plastic or metal box
> with a blob of pink or blue foam carved out to take the receiver should do.
> I would think you would get away with a passive aerial, and a short bit of
> coax to the receiver, so you shouldn't need to worry about a pre-amp getting
> cold.
>
>  
I somehow deleted the original post, but the original poster may want to
talk to these folks about their thermal techniques...

http://www.eoss.org

They've got over 100 flights under their belt, and since they're local,
I have been able to listen in during some on-air "tech talk" on my
club's repeater system and another local club's repeater when they've
discussed some of their successes and failures.

It's quite fascinating -- if I wasn't super busy with the rest of my ham
radio hobby activities, it'd be a lot of fun to help these guys work on
their balloon projects.

Nate

2006\05\18@140440 by Peter

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On Thu, 18 May 2006, Henry Hallam wrote:

> Hi all,
> Does anyone have experience with potting a device that has to survive a very
> low temperature? I am building a GPS tracker for a high altitude balloon
> where the temperature will get down to -50C or colder.  I'm considering
> potting it for better mechanical security but is this likely to cause
> problems with different rates of thermal contraction?  Are there any
> particular potting compounds that are recommended? I haven't done much
> potting before and have always used ordinary "araldite" epoxy.

Try to obtain some milspec polyurethane potting foam (should be good for
+125/-55deg C operating). The idea is to obtain a potting compound that
is not rigid and weaker than whatever is in it so it does not tear it
apart due to thermal contraction and mechanical shock.

Peter

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