SparkFun Qwiic Thermocouple Hookup Guide
Alex the Giant, Ell C 
A Brief Theory of Operation
Thermocouples are fairly ubiquitous, being used in everything from industrial kilns and diesel engines to pilot light sensors and thermostats in your home or office. Let's take a brief moment to go over how they work.
Roughly a couple hundred years ago, a man named Thomas Seebeck discovered the principal that thermocouples use. He noticed that if you take two wires made of dissimilar metals, connect them at the two ends, and make a temperature gradient between one end and the other, a voltage potential formed and current flowed. One junction is held in the environment where the temperature of interest exists. This is known as the hot junction. The other junction is referred to as the cold junction.
There are many types of thermocouples, which mainly differ by the types of metals used in the two wires. The most common general purpose thermocouple is type K. They are made out of chromel and alumel. These two alloys produce a potential of approximately 41.276 µV/°C, and voltage out can be calculated using the equation below.
K-type thermocouples can read between from −200 °C to +1350 °C (−330 °F to +2460 °F) and are fairly stable.