Datasheet

Upload to your Arduino and open up the serial console at 115200 baud to see a print out of the sensors data. The

MAX31865 doesn't actually return the resistance it measures. Instead it returns the

ratio

between the resistance

measured and the Rref reference resistor.

For the PT100 version of the breakout, this is a 430 ohm 0.1% resistor (marking is 4300 !!!)

For the PT1000 version of the breakout, this is a 4300 ohm 0.1% resistor (marking is 4301 !!!)

You can use that ratio to calculate the resistance and then determine the temperature