Precision

The voltage generated by a K-Type Thermocouple has a non-linear response to temperature change. That is to say, when you plot the voltage against temperature, you do not get a straight line. However, you do get an almost straight line. Because of this, a simple slope equation can often be used to convert from thermocouple microvolts to degrees Celsius. This is what you are reading as the probe temperature from the MAX31855K part. The slope used internally by the MAX31855K for a K-Type thermocouple is 41.276 µV/°C.

For greater precision over a larger range of temperatures or a better conversion over a particularly non-linear area of the thermocouple response curve, we can apply the NIST ITS90 polynomial for a K-Type thermocouple, to convert microvolts to degrees Celsius. We simply need to convert both Hot-Junction and Cold-Junction temperature outputs of MAX31855K back to microvolts and apply the ITS90 polynomial conversion.

Both of these methods are available with this library.