Re: PIC at dry ice temperature

On Tue, 30 Aug 2005 15:28:27 GMT, "Thomas Magma"
<somewhere@xxxxxxxxxxxxxxxxxx> wrote:

>
>"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.

A Dewar flask is simply a glass vacuum bottle, like the brand-name
Thermos bottle. Lab-style Dewars are generally open-top, but for this
application you'd probably use one with a stopper.

Polystyrene has been suggested, and is a good insulator (R value up to
5 for extruded) but polyurethane foam is better (6.88) and probably
more readily available.

> Also look into something call AeroGel it
>has absolutely amazing insulating properties.

Yes, indeed. R values up to 50, in some configurations.

> 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
>
--
Al Balmer
Balmer Consulting
removebalmerconsultingthis@xxxxxxx
.

Relevant Pages

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