PIC at dry ice temperature

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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.


Re: PIC at dry ice temperature
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I strongly suspect so.  Which would leave you with only one truly
viable solution: "don't do that, then".  I.e. don't operate your micro
at environment temperature.  Put it in a thermally isolated housing,
together with its battery, where the two can keep each other warm, and
expose only the temperature sensor itself to the more extreme
temperatures out there.

--
Hans-Bernhard Broeker ( snipped-for-privacy@physik.rwth-aachen.de)
Even if all the snow were burnt, ashes would remain.

Re: PIC at dry ice temperature

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I think that there are some gels that change characteristics at these
temperatures. However, I suppose you are wanting an electronic solution.

Is there anyway you can keep the electronics separated from the cold areas
(perhaps by remoting the sensors). There are no electronics I know of that
perform that low down in temperature. Where such environments are
encountered either keeping electronics and batteries remote from the items
to be measured or enclosing in a thermally insulated heated box is the
norm. Adding a heater will up the battery size and cost but I don't think
you will need much in a suitably insulated enclosure.

As ever with these sort of enquiries, more details are required for a
suitably targetted answer.

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Re: PIC at dry ice temperature
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Just because your sensor must measure -75, doesn't mean your electronic
have to live at that temperature as well. Only the sensor must be capabl
of surviving at that temperature.

        
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Re: PIC at dry ice temperature

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     Semiconductors and a lot of other electronic components are rated
in temperature range bands such as commercial, industrial, and
military.  These temperature bands have both upper and lower limits to
them.  As you progress through the different temperature ratings, the
range expands on both ends.  I have often found that a component is
keep in a less rigorous temperture band due to limitations on the high
end even though it may be able to function at lower temperatures than
the data sheet lists.  In the case of semiconductors, I have found it
useful to get storage tempertures for them.  If they can be stored at
-75 degrees, they can probably operate there even if the industrial or
military temperature range listed for operation appears to exclude
such usage.  Usually the limiting factor for low temperature operation
for a microprocessor will be the point where metal in the chip package
contracts to the point that contacts are pulled apart.
     You may also need to look at other components in your design.
Temperatures that low can be a problem for power supply capacitors.

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.


Re: PIC at dry ice temperature

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I wouldn't be surprised if the battery was frozen
and didn't deliver any current at all.


Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

Re: PIC at dry ice temperature

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 I've used it down
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     I would agree that the problem here is most likely in the
battery.  Temporary power loss would clear the RAM.
     You might go over the manufacturers specs on the battery.  You
might also look at supercapacitors instead of batteries if you find
one that can operate at those temperatures.  I have read that they can
be used at low temperatures, but I do not recall specific specs.

Re: PIC at dry ice temperature

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Battery internal resistance is usually fairly low when they are fresh and
increases as they discharge. Power dissipation is the usual IIR formula.
 
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Styrofoam is quite good. Closed cell expanding foams are also quite good.
Vacuum is probably the best though (but may be difficult for you). I think
if you go with a Styrofoam within a plastic box you should be OK. You will
probably need a resistor to provide the necessary heat to keep the
temperature up. The resistor dissipation can be controlled so that you keep
it to the minimum. I have presumed that the proximity of the box (from your
response) is not likely to be out of the dry ice environment but that you
do have some room for a thermally insulated enclosure.
 
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Its the price to pay. When you remote the sensor (as you would even when
the PIC and battery is in a box) remember to use the thinnest possible
wiring to prevent too much heat transferring along the wires. Four wire
measurement circuits are best for this type of sensing (feed a small
current down one pair to the sensor and read the voltage back along another
pair.

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Re: PIC at dry ice temperature
snipped-for-privacy@gmail.com writes:
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What is the operating environment?  Are you dealing with a
shipping package (cardboard box or whatever) containing
the product being cooled with dry ice?  Or is this a
shipping container travelling by road/rail/ship?

Re: PIC at dry ice temperature
On 25 Aug 2005 11:19:19 -0700, in comp.arch.embedded

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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

Re: PIC at dry ice temperature

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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



Re: PIC at dry ice temperature
On Tue, 30 Aug 2005 15:28:27 GMT, "Thomas Magma"

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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.

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

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Al Balmer
Balmer Consulting
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