Measuring thermocouples is a different process from measuring PT100 sensors. With a thermocouple you need a sensitive amplifier to amplify the small voltage between your "cold junction" and your "hot junction", you also need either an Ice point or an artificial ice point, usually a block with the cold junction in it that can be measured with an independant sensor. Of course your ultimate precision will depend on how well you can measure the cold junction, as well as all the other error sources.

PT 100 sensors are resitance sensors whose resistance needs to be measured to a high precision. The resistance of a PT 100 sensor varies by .1% or so per degree of change at 25C the resistance is 100 Ohm, at 150C it is 157.4.

Both TC and PT100 suffer from non-linearity which must be well compensated to get any kind of precision. We built a PT100 system good to 0.05 C, so it is possible to get excellent results.

Analog devices do a basic analogue lineariser/ amplifier

Burr Brown make some good instrument opamps to do the signal conversion.

Steve