Fe-CuNi thermocouple (type J) is also named Fe- Constantan thermocouple. The positive leg Fe(JP) is made by pure Iron, and the main chemical composition of negative leg Constantan (JN) is Cu: Ni≈ 55%: 45%. Although they are all called Constantan, JN does not give the same EMF output curve as EN and TN, it cannot be instead by other Constantan.
The operating temperature range of type J thermocouple is -210~1200℃, but it is usually used in the temperature range 0~750℃ due to embrittlement of Iron.
Type J thermocouple has good resistance to oxidation and high EMF output against Platinum 67, with good temperature accuracy, sensitivity and stability. It is recommended for vacuum, oxidizing, reducing or inert atmospheres, but it can not be used in sulphurous atmosphere above 500℃.