A thermocouple is a sensor used to measure temperature in a range of different processes and consists of two wire legs made from different metals joined together at their two ends to form two junctions. The hot, or measuring junction is connected to the body whose temperature is going to be measured. The cold junction or reference junction is connected to a body of known temperature. When the measuring junction is placed on something hot a voltage or potential difference between this and the reference junction occurs. This voltage can then be converted into a temperature measurement using thermocouple reference tables. This process is also known as the Seebeck Effect.

An insulated or ungrounded junction is where the thermocouple wires are welded together and fully covered by but insulated from the outer protective sheath, often by the use of mineral insulation. The advantage of this is that there is a reduction in electrical interference and a stable reading.

Mineral insulation is used to insulate the thermocouple wires from one another and from the metal probe sheath that surrounds them. The sheath is filled with a highly compacted magnesium oxide powder that prevents the conductors from contacting one another other than at the fixed junction. The high density of the mineral powder enables the thermocouple to have a rapid response as it promotes a fast heat transfer between the conductor and the sheath. Mineral insulation also helps to protect the thermocouple wire from environmental damage including corrosion, prevents electrical interference and allows the cable to be flexible whilst maintaining its mechanical strength.