tc type jThermocouple Type J: Type J (Iron–Constantan) is a common general purpose sensor as it is one of the most linear thermocouples.

Type J thermocouples have an iron positive leg and a constantan negative leg. They are inexpensive, and a wide variety of probe configurations are available in its 0 – 750°C (32 – 1382°F) range.

Thermocouple type J is recommended in vacuum, inert, and reducing atmospheres. It may be used exposed or unexposed where there is a deficiency of free oxygen. For cleanliness and longer life, a protecting tube is recommended.

Type J is an extremely popular thermocouple, but has some disadvantages. They need to be calibrated carefully before usage, output signals are very small, and they may have a problem with noise. They are prone to stress, strain, and corrosion, particularly as they age. Below are some issues to be aware of when determining whether or not to use type J.


As the JP (iron) leg of the wire will oxidize rapidly at temperatures over 540°C (1000°F), it is recommended that larger gauge wires be used to compensate. Iron wire may rust or become brittle at sub-zero temperatures. If unprotected, iron wire may be attacked by ammonia, nitrogen, and hydrogen atmospheres. Therefore the use of the stainless steel metal sheath, MgO style of construction, is an option that can help to overcome some of these problems. The MgO style for type J is much preferred over the beaded bare wire style thermocouple. 

Maximum recommended operating temperature is 760°C (1400°F). Type J should not be used in sulfurous atmospheres above 540°C. Additionally, J types should not be used above 760°C as an abrupt magnetic transformation will cause permanent decalibration. Type J wire is sometimes confused with regular copper red and white wire. If in doubt, check it with a magnet. The (white) iron wire is highly magnetic; copper wire in not.

For assistance in choosing the proper sensor for your process, please contact us and our Temperature and Process Engineers will be happy to assist you!


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