MAX31865 is for
resistance thermometers (RTDs), most commonly of either Pt100 or Pt1000 type. I prefer the three-wire configuration, as they seem to be easier to find and cheaper than four-wire ones.
The break-out boards you can find are either clones of the
Adafruit one, or just reimplementations of the suggested application circuit in
MAX31865 datasheet (PDF). It is a very simple break-out board, requiring only passives to interface to a 3-wire or 4-wire sensor and a microcontroller using 3.3V voltage; the Adafruit one has a 3.3V regulator so it can be used with 5V microcontrollers (like original Arduinos).
Essentially, MAX31865 is a resistance-to-digital converter, which yields a 15-bit digital value
V via SPI (mode 1 or 3); the resistance measured being
R=
Rref×
V/32768, where
Rref is the reference resistor being compared against. Adafruit uses a 430 Ohm 0.1% reference resistor, but I've seen other values on the BOBs I bought off eBay. For optimum results, the reference resistor should have
about four times the resistance of the sensor at 0°C (which is 100 Ohm for Pt100, and 1000 Ohm for Pt1000 sensors).
That resistance is then converted to temperature based on the sensor type. From -100°C to +100°C, Pt100 sensors have a pretty linear response, so if you don't mind an absolute error of less than 2°C, the temperature is just the resistance multiplied by a constant plus/minus another constant. For better accuracy, the
Callendar-Van Dusen equation is used, either directly, or via a look-up table. The datasheet describes how, and includes example tables as well.
Adafruit provides an
Arduino library for their break-out board, which is very easy to use. You obviously also need a microcontroller. I recommend getting several break-out boards and sensors (they aren't expensive), so you can rule out device/sensor issues when developing your gadget: comparing the results between different sensors and MAX31865 boards is very informative. Even replacing the reference resistors with some of your own (they're well marked on the BOBs I have, but a bit close to the MAX31865 chip, for hand-soldering) with precisely measured resistance or known 0.1% from a reputable vendor, can be very useful.
I haven't done any calibration testing on mine, because I don't have access to calibrated thermometers. I guess I could test at 0°C and 100°C with distilled water (I live close enough to sea level so atmosperic pressure doesn't affect the boiling point much) with water-ice slush and boiling water, respectively, but haven't found a reason to do so yet.
Realistically, you should be able to achieve 0.5°C accuracy and 0.05°C precision with these, but that of course depends on the break-out board, sensor, and the resistance-to-temperature model you use.