Thermocouple grade wire always has a BROWN jacket and is used to manufacture thermocouple probes. Thermocouple grade wire is normally used for the sensing junction and inside the stem sheath. This is because the thermocouple grade wire meets the ASTM requirements which has a higher accuracy specification than extension grade wire. Extension grade wire is a less expensive, lower grade wire. It is used to extend from the thermocouple probe to the control system or digital display. Extension grade wire is more economical due to the length requirements. Also, it is not in the process itself and does not play as critical a role nor does it experience the temperature extremes & temperature cycling.