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.
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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 (broeker@physik.rwth-aachen.de) Even if all the snow were burnt, ashes would remain.
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.
-- ******************************************************************** Paul E. Bennett .................... Forth based HIDECS Consultancy ..... Mob: +44 (0)7811-639972 Tel: +44 (0)1235-811095 Going Forth Safely ....EBA. http://www.electric-boat-association.org.uk/********************************************************************
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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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.
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.
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
keep the electronics warm in a dewar flask, with a bit of self heating, maybe
I did a quick search and found this
dont bother googling vacuum or thermos, try dewar
martin
Battery internal resistance is usually fairly low when they are fresh and increases as they discharge. Power dissipation is the usual IIR formula.
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.
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.
-- ******************************************************************** Paul E. Bennett .................... Forth based HIDECS Consultancy ..... Mob: +44 (0)7811-639972 Tel: +44 (0)1235-811095 Going Forth Safely ....EBA. http://www.electric-boat-association.org.uk/********************************************************************
I've used it down
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.
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
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.Yes, indeed. R values up to 50, in some configurations.
-- Al Balmer Balmer Consulting removebalmerconsultingthis@att.net
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?
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