[Gdeaver]

You may want to consider a Maxim one wire temperature sensor like the DS18B20. In the multi-drop configuration only 3 wires are needed. 3.0 -5 volt, ground, and signal. Only one processor pin is needed. The one wire protocol is complicated but libraries are available for many processors. 750 ms for a sample but you do not need speed. Accuracy can be improved by calibration. The to-92 package lends it's self to directly soldering wires to the leads and insulating and then pot the unit in a metal tube. Use high quality wire. Be sure to insulate and water proof well. The micro controller is not going to keep time well in an out door temperature environment. May want to add a real time clock with a real good TCO. If it had a timer interrupt, it could wake the processor from sleep and do the sampling then go back to sleep. Maxim does have one wire real time clocks that with the proper TCXO could work. Some where on their web site there is an app note on real time clocks and oscillators for wide temperature extremes. Check the operating temperature of all electronics used. Many do not work well at low temps. Atmel serial data flash works well for data logging in harsh conditions. Digi does make xbee modules That have very long ranges with high gain antennas. Once again the are allot of libraries for the xbee stuff. Don't for get to turn the power off to the sensors to save power. What battery works well at -20F? With this data you could answer the question of 'how deep do I have to bury the horizontal geothermal heat exchange pipes in this area. Mine are at 14'.