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What are type J thermocouples?

What is a thermocouple?

Thermocouples are used in many industrial, scientific, applications. They can be found in nearly all industrial markets: Power Generation, Mining, Oil/Gas, Pharmaceutical, Biotech, Cement, Paper, Glass, and many more. Thermocouples are also used in everyday appliances like stoves, furnaces, kilns, and pizza ovens. Thermocouples are typically selected because of their low cost, high-temperature limits, wide temperature ranges, and durable nature. When it comes to choosing the right one, it is all about the research and your manufacture’s knowledge. The correct temperature of any element is crucial for the manufacturing of all temperature related products.

Thermocouples are designed to measure temperature; they are an electrical device consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage which is interpreted to measure temperature. Commercial thermocouples are inexpensive, interchangeable, are supplied with standard connectors, and can measure a wide range of temperatures. In contrast to most other methods of temperature measurement, thermocouples are self-powered and require no external form of excitation

Isl ImageWhat is a type J thermocouple?

Thermocouple 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, 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 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.

Pyrosales engineers are available to consult on various stages of any project, from the development and planning of a new project to modifying or upgrading an existing facility. Our staff is here to provide you with assistance and recommendations to ensure the best outcome and value. We are happy to visit our clients on-site or conduct meetings from one of our offices.