Thermocouple extension wire is a single pair wire that cannot be made into a thermocouple, but is used to carry the signal form a thermocouple to the recorder, controller, or instrument reading the signal. Extension grade wire usually has lower ambient temperature specifications. It is used to carry signal representing the higher temperature seen by the sensing location, but extension wire cannot be generally cannot be exposed to those higher temperatures. Extension wire cannot be used to make a thermocouple, but thermocouple wire can be used as extension wire. Insulation is typically PVC, but other option are available.
For more information about Duro-Sense thermocouple wire, visit their website here.
Duro-Sense is currently having their annual wire sale (while supplies last). Here is a list of inventory that is on sale now. Call 310-533-6877 with questions.