This is a fun question because it involves science! We also have other content here at Visaya where we explain thermocouples in detail. However, let me give you a quick answer to this question. If you want to know more, then you can check out those posts.
The thermoelectric effect and heat to electricity conversion
A thermocouple has in it two different metals joined at one end. For instance, a type-K uses chromel and alumel. When you heat the sensor, the heat creates a difference in temperature between the joints, converting the heat to electricity. We call this the thermoelectric effect, and this voltage is proportional to the heating temperature.
Every type of thermocouple has a reference table, so make sure you use the right table. For a type-K, you find its table, look up the voltage generated, and voila! Your temperature!
You can read about thermocouple calibration, cold junctions, and more here:
#WishIknew – When do I use a thermocouple compensating cable? – When you use a thermocouple, you can connect it to your control system or transmitter in many ways. The simplest way uses an extension cable or compensating cable.
#WishIknew – How do you calibrate a thermocouple? – You have several ways that apply to all thermocouple types, but the right way for you depends on your conditions. However, you always need a calibration furnace. So let’s go through the options.