They connect the point of sensing to the point of cold junction compensation, or the end where the measurement takes place.
Thermocouples measure temperature by sending an electrical current down each wire when the opposite parts, the CJC end and sensing end are at different temperatures. The voltage from the current flow is measured to determine the exact temperature difference between the two sensing points. The wires are integral parts of the thermocouple assembly, along with tubes, insulators and sheaths that provide protection. They are often joined to electronic readouts to provide temperature monitoring and quick measurement. There are two main grades of thermocouple wire—thermocouple and extension grades.
Thermocouple grade wire is actually used to make the sensing point, while the extension grade wires extend the signal from a probe back to the reading instrument. Thermocouple wires are used in every type of thermocouple instrument, including E, J, N, T, B, S, R, W and C, all of which use different metal alloys for their wires.
Like all wire, thermocouple wire is fabricated by extruding metal preforms. The extrusion process involves forcing heated and malleable metal through successively smaller dies until the desired diameter and profile is achieved. Each type of thermocouple requires a different metal or alloy for each thermocouple wire. For example, type K thermocouples, which are the most popular and widely used kind, are made of two different alloys—one wire is a chromium nickel alloy, while the other is an aluminum nickel alloy.
Every thermocouple requires wires of dissimilar metals because the sensing ends are joined together and are not able to measure or detect a temperature difference if they are the same metal material. Some thermocouples, which are able to pick up sensitive differences and must be accurate, use SLE (special limits of error) wire, which has a slightly better accuracy than standard thermocouple wire. In order to tell the different types of wire apart, the ANSI developed color coding to distinguish the two wires. Red signals negative lead in the insulated thermocouple wire, brown represents the center jacket of the wire, and white coated wires are usually used in high temperature thermocouples. Their maximum length is usually under 100 feet, that is, if they have 20 AWG or thicker wire.