Yes you're right,its RTD.
I have a module IO-PT400 and RTD. I put the RTD and calibration thermocouple from IFM in hot water and observe the difference in °C as you see in attachment file.
The blue line is RTD and the red line is IFM. Look first graph
Second graph is Razlika (difference) .
I'd like to make some linearization for RTD or correction :
If IFM shows 85,50 and the same time the RTD shows 85,1 I'd like to see in my display RTD=85,5 instead of 85,1.
In generally I'd like to calibrate my RTD to show real temperature.