Measuring Cryogenic Temperatures I need an inexpensive instrument to measure cryogenic temperatures (down to -200C).
I can build a thermistor-based thermometer using an Arduino that is accurate to under 1 degree for 0 to 100C.
First of all, can ordinary NTC thermistors be used at cryogenic temperatures?
Second, if I were to try to use this approach, I would need ways to calibrate the device.
I have found a chipset from Analog Devices that would let me use a thermocouple down to that range.
I would take other ideas on how to build one, but then I need some equipment to calibrate whatever I build.  Is there anything I can buy for < $100, I don't need huge accuracy, but I need something.
 A: You ether can take usual K-type thermocouple (like in multimeters), and calibrate it - it supposed to work fine in LN2. I personally tried that with DMM and it worked (though showed large error). Multimeters use simplified conversion formulas and could give huge errors at cryogenic temperatures - so remember that you ether need table-based conversion, or non-linear formula. For example, you can use this data: http://www.omega.com/temperature/z/pdf/z204-206.pdf or in Google "K-type table"
Another possibility is to take PT100/PT1000 platinum resistance probe/sensor. They are not expensive, some even under 5$. You might get decent (I guess <5C) accuracy with it even without calibration, but measuring resistance might be slightly trickier, than voltage. 
A: If this case is still open, here are a couple of options. Both PRT-100 and thermocouples are feasible solutions. If using a thermocouple, E type has the highest sensitivity at cryogenic temperatures (even down till 40 K, LN2 is 77 K as you know). However as pointed out above, thermocouples generate very low voltage and this output signal is prone to EM interference. Hence it needs proper signal conditioning. The advantage is thermocouples are cheap.
On the other hand, there are PRT-100 which work perfectly accurate down till about 50 K. They in fact follow a standard IEC-751 calibration. There are PRTs specifically developed for cryogenic temperature measurements and so calibrations may not be necessary. These PRTs are a little more expensive than thermocouples, but would still fit in your budget of $100. The output can be measured on any DMM (2/4 wire).
The cryogenic thermocouples and PRTs can be bought from Lakeshore Cryotronics, Inc. which is an industry standard for cryogenic sensors and controllers (I buy all my thermometers from there).
