A thermocouple is a temperature-measuring device consisting of two dissimilar conductors that contact each other at one or more spots. The point of interest, also known as the hot junction, is one of these spots. The Thermal Data Logger connected to one end is another, also know as the cold junction. The thermocouple produces a voltage when the temperature at the hot junction differs from the reference temperature at the cold junction.

The thermocouple voltage difference between the known cold junction temperature and a lookup table reference temperature can be calculated (from a temperature-to-voltage lookup table), and the appropriate correction is applied as an offset from measured voltage before looking up the measured temperature in the voltage-to-temperature lookup table. This is known as “cold junction compensation” (CJC).