Deous:
I have tried DS18B20, LM34, MAX31855.... they don't work - I get random readings no matter what.
Tried different boards Uno, Mega, STM, ESP32.
Everytime I get different bad readings. Maybe all those circuits are bad or defective....
All I want to do is measure temperature with low degree values - like going to -100C. Range let's say -200~200C
What is the best temperature sensor and thermocouple for that? What would you recommend?
"they don't work" and "random readings" and "different bad readings" is not very helpful.
Be specific.
The DS18B20 is a very stable and easy to use sensor, with high (0.0625°C) resolution, and factory calibrated to 0.5C.
But like all semiconductor based sensors, it has a limited temp range (-55 to +125C).
The LM34, and other analogue sensors (LM35, TMP36), have a similar temp range,
but could have a much lower resolution if read with Arduino's 10-bit A/D.
The wrong code could also make readings of those analogue sensors instable,
and factory calibration of analogue sensors in general is meaningless. You must calibrate yourself.
NTC (thermistors) could have a wider temp range than the above sensors, but resolution varies over that range.
Higher in the middle, and lower at the extremes. You need to calculate that to see if that's a problem or not.
And Arduino's default 10-bit A/D could also be a problem.
PTC sensors (PT100 platinum RTD) are probably the best choice for a temp range of -200C to +200C.
But they need a special preamp+A/D to be useful (MAX31865).
Tell us why you need a 400C span, and what resolution you expect.
Leo..