For those interested, I tried an Adafruit MAX31865 with the PT100 I was using. Did a ice calibration test. Ended up with ~ 0.1 degrees C. This is a LOT better than the clones (often purple pcb's in stead of blue) I have been using. While testing the same sensor, two of the clones were off by 0.9 and 1.5 degrees. The Adafruit board is not very cheap, between 15 and 25 euro's, but it appears to be better quality. Probably precision of components like resistors and such.