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 < $100, I don't need huge accuracy, but I need something.
Best Answer
You ether can take usual K-type thermocouple (like in multimeters), and calibrate it - it supposed to work fine in LN2. I personally tried that with DMM and it worked (though showed large error). Multimeters use simplified conversion formulas and could give huge errors at cryogenic temperatures - so remember that you ether need table-based conversion, or non-linear formula. For example, you can use this data: http://www.omega.com/temperature/z/pdf/z204-206.pdf or in Google "K-type table"
Another possibility is to take PT100/PT1000 platinum resistance probe/sensor. They are not expensive, some even under 5$. You might get decent (I guess <5C) accuracy with it even without calibration, but measuring resistance might be slightly trickier, than voltage.