Re: PIC at dry ice temperature


"martin griffith" <martingriffith@xxxxxxxxxxxxx> wrote in message
news:gpq6h1dh1gf7pocb0dldvgv1q59i65o31f@xxxxxxxxxx
> On 25 Aug 2005 11:19:19 -0700, in comp.arch.embedded
> scott.manton@xxxxxxxxx wrote:
>
>>I'm looking at building a simple temperature monitoring device for
>>items stored and shipped at dry ice temperature which is -75*C
>>(-167*F). The specs for most PICs state a lower operating limit of
>>-40*C. Is there any PIC (or other uproc) that can handle such low
>>temperatures? Or is limit imposed by the semiconductor itself?
>>
>>This brings me to batteries. Are the BR type batteries the best choice
>>for low temperature applications? It's seems they are also only rated
>>for down to -40*C.
> keep the electronics warm in a dewar flask, with a bit of self
> heating, maybe
>
> I did a quick search and found this
> http://www.finemech.com/kgw_isotherm/cylindrical.shtml
>
> dont bother googling vacuum or thermos, try dewar
>
>
> martin

I agree that extreme isolation with a small amount of heating is probably
required here. There are quite a few battery chemistries rated at -40 C most
are lithium based. Li/SO2, Li/MnO2 and for a low cost try Energizers new
Lithium E2 (Li/FeS2). I don't know the make-up of the Dewar flasks but in
sure they are excellent insulators. Also look into something call AeroGel it
has absolutely amazing insulating properties. As suggested before,
micro-wire your thermometer onto the outside of the container and have a
small heater keeping the batteries and electronics around -40 or greater.
You could do this efficiently by having a thermometer near the batteries and
toggle or PWM the heater with the microcontroller.

Good luck

Thomas


.

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