Re: PIC at dry ice temperature
- From: "Paul E. Bennett" <peb@xxxxxxxxxxxxxxxxxx>
- Date: Mon, 29 Aug 2005 21:19:06 +0100
scott.manton@xxxxxxxxx wrote:
> 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?
Battery internal resistance is usually fairly low when they are fresh and
increases as they discharge. Power dissipation is the usual IIR formula.
> Also does any one have any design tips for the enclosure? For instance
> is stryofoam the best? I'm not too mechanical...
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.
> 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.
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 ....................<email://peb@xxxxxxxxxxxxxxxxxx>
Forth based HIDECS Consultancy .....<http://www.amleth.demon.co.uk/>
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-811095
Going Forth Safely ....EBA. http://www.electric-boat-association.org.uk/
********************************************************************
.
- References:
- PIC at dry ice temperature
- From: scott . manton
- Re: PIC at dry ice temperature
- From: Gary Reichlinger
- Re: PIC at dry ice temperature
- From: scott . manton
- PIC at dry ice temperature
- Prev by Date: Re: Flash Prom Size
- Next by Date: Re: MSP4530 and gdbproxy.
- Previous by thread: Re: PIC at dry ice temperature
- Next by thread: Re: PIC at dry ice temperature
- Index(es):
Relevant Pages
|
