Post by thread:Cold Weather Pics

I built a BASIC Stamp circuit (which uses a PIC chip) for a weather balloon flight. It hits sixty below at altitude, so I decided to test the circuit (and calibrate an on-board thermister) by putting it in a cooler and covering it with dry ice snow. I had a Radio Shack indoor/outdoor thermometer and put the outdoor probe up against the Stamp. The whole affair was sandwitched between two bags of anti-freeze for to give it some thermal intertia and keep the temperature from dropping too fast. (I had to record the readings.) The Radio Shack thermometer quit registering temps at -55F. The stamp kept working for another half hour. I estimate it got down to about -65F. It finally quit when the 9 volt battery froze and it's voltage dropped below 5 volts! I took the Stamp out and it was incased in a solid block of ice from water vapor condensation. I plugged in a new 9 volt battery and it started working again, still encased in ice!

Dave Mullenix wrote: > > I built a BASIC Stamp circuit (which uses a PIC chip) for a weather balloon > flight. It hits sixty below at altitude, so I decided to test the circuit > (and calibrate an on-board thermister) by putting it in a cooler and > covering it with dry ice snow. I had a Radio Shack indoor/outdoor > thermometer and put the outdoor probe up against the Stamp. The whole > affair was sandwitched between two bags of anti-freeze for to give it some > thermal intertia and keep the temperature from dropping too fast. (I had to > record the readings.) > > The Radio Shack thermometer quit registering temps at -55F. The stamp kept > working for another half hour. I estimate it got down to about -65F. It > finally quit when the 9 volt battery froze and it's voltage dropped below 5 > volts! > > I took the Stamp out and it was incased in a solid block of ice from water > vapor condensation. I plugged in a new 9 volt battery and it started > working again, still encased in ice! Bravo! However, here are a few cautionary words: 1. Maintaining power and lowering the temperature is not the worst case experiment for 2 reasons: a) an oscillator already running may continue to oscillate at temperatures considerably its lowest starting temperature, and b) self-heating may be maintaining parts above ambient temperatures that they would reach if you removed power. MORAL: if your device will be required to start up in very low temps, test it that way. 2. Ice is much less conductive than liquid water. 3. What happens when you get a circuit board wet depends on many things. In particular, the ionic content of surface contaminants will depend on how well the board was cleaned. At any rate, after a few hours of operation, the water is bound to contain lots of ions from metals on the board. Also, whether the resulting leakage currents will affect circuit operation noticeably depends on many factors. -- Paul Mathews, consulting engineer AEngineering Co. spam_OUToptoengTakeThisOuTwhidbey.com non-contact sensing and optoelectronics specialists