Re: PIC at dry ice temperature

Thanks for the replies everyone. As for adding more detail to my
inquiry, I currently have a simple PCB containing PIC, CR lithium
battery, thermistor and crystal. This PCB keeps time and records
temperature threshold crossings(time and duration). I've used it down
to -40 without problems, but when asked to ship it in dry ice it did
not work. The ram would clear for whatever reason.

The idea of using an insulated container is interesting to me. My PCB
is 3cm by 4cm. If I encase it in styrofoam 1cm thick and assuming I
could control the internal temperature to above -40*C to keep all the
parts within spec, and the ambient temp is -78.5*C for a temp diff of
38.5*C, k value of 0.01 for styrofoam and total surface area of 24cm^2
( counting both sides). I get a total power disipation of about 0.1
watts. I used the formula H=k*A*dT/L, A-area, L-thickness, dT-temp
diff, and k-thermal conductivity. The power consumption of my
product is easily 1000 time less than that, so I would essentially need
to disipate 0.1W through a resistor in order to keep the temp up. But
I don't know enough about batteries to know how much heat they
dissipate as they deliver the current. This side effect of batteries
would obviously help me here. Can anyone give me an approximation for
this?

Also does any one have any design tips for the enclosure? For instance
is stryofoam the best? I'm not too mechanical...

Since I currently use a 50mAh battery I would need to oversize this by
at least a factor of 10 to get a reasonable amount of logging time.
The other alternative, to place the thermistor on leads and place it
in the shipment, while the electronics remain outside. This is
obviously the best scenario, but I'm not sure if I can do it.

.

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