A thermocouple is a pair of two dissimilar metal wires connected together (welded or just twisted) at one end. The connection between these two dissimilar metals creates a reaction in which an electromotive force ( emf) is generated, a voltage source.

The generated voltage (emf) is proportional to the temperature where it is exposed to. A zero (0) mV corresponds to a Zero Temperature for a K Type thermocouples at Zero reference junction and increases with increasing temperature.

Since thermocouple wires create a voltage change every time it senses or detects the temperature difference, it is also called a thermocouple sensor.

Thermocouple wires have 2 junctions, these are:

1. Hot Junction  – this is the tip where the actual temperature measurement is sensed, the closed end. The part where we exposed to the temperature we want to measure.
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2. Reference Junction (cold junction) – this is the part where we connect into the display or temperature controller, the open end. It is called a reference junction because this is the reference point used to attain a correct reading. Any temperature reading in this junction will be subtracted to the temperature reading from the hot junction. If there is no cold-junction compensation added in the circuit, you may think that it is reading incorrectly.
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   One solution for this is to create a cold-junction or an ice bath where the reference temperature will be exposed to zero temperature. Therefore any reading from the hot junction will be exactly the same as is because the effect of the reference junction will be compensated by the zero reference value.

   
   

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Most of the digital thermometer that we use today has a cold-junction compensation. Because of this, we do not need to worry regarding the effect of the reference junction in the temperature output because it is already compensated. You only need to worry about it if you are using a multimeter directly measuring a mV output. See no 3 below.

Types of Thermocouple Wires

So many types of thermocouple wires are being manufactured mainly because each has its unique qualities, capability, and ranges depending on the suitability of the environment where they are used and exposed to. Below are some examples of commonly used thermocouple types with their ranges.

If you observe,  K Type thermocouple wire has the widest range, therefore, it is the most used type of thermocouple wires.
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Each type of thermocouple has its own unique characteristics which include:

1. Diameter size or wire thickness
   

2. Type of coating or insulation material

3. Classifications – class 1 to class 3 thermocouples
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This is why it is important to determine first your requirements before using any of them such as your user range, tolerance, and of course your budget.

Why Calibrate Thermocouple Wires?

To verify accuracy because of:

• Aging or decay

• Contamination by the working environment- make them brittle and shortens the life

• Oxidation in the environment when unprotected.

• Mechanical stress or breakage