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Fe- CuNi thermocouple (type J) is also named Iron-Constantan thermocouple.. It is used in vacuum, oxidizing, inert or reducing atmospheres. Iron element oxidizes rapidly at temperatures exceeding 538'C, and therefore heavier gauge wire is recommended for longer life at these temperatures.

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 500ºC.

Detailed Parameter of J type thermocouple wire

Chemical Composition

Conductor Name Polarity Code Ni /% Fe /% Cu/%
Fe Positive JP -- 100 --
Constantan Negative JN 55 -- 45

Working Temperature

Diameter/mm Long time Working temperature /ºC Short period Working temperature /ºC
0.3, 0.5 300 400
0.8, 1.0, 1.2 400 500
1.6, 2.0 500 600
2.5, 3.2 600 750


  1. Heating - Gas burners for ovens
  2. Cooling - Freezers
  3. Engine protection - Temperatures and surface temperatures
  4. High temperature control - Iron casting