Type J thermocouple is a very common and general purpose thermocouple. It has smaller temperature range and a shorter lifespan at higher temperatures. It consist of positive leg made of an Iron wire and negative leg made of an Constantan (Copper-Nickel) alloy wire. Due to the Curie Point of the iron at 770°C Type J has a limited temperature range of -40°C to 750°C. It should not be used at high temperatures in an oxidizing atmosphere as iron undergoes a molecular change and permanently loses its standard voltage output versus temperature. It does not recover when the iron is cooled. Type J has sensitivity of approx 50 microvolts/ degree C. The expenses and reliability of Type J is same as Type K. For proper working of J Type thermocouple reduction atmosphere is desired and use at low temperature is also not recommended.

In J Type thermocouple linearity varies by -70°C over its full range from -210°C to 1200°C. It has a very straight section from 100°C to 500°C which deviates at about -0.5°C. The lower & higher ranges can be extended with a loss in linearity.

Among all types of thermocouple Type J is the cheapest thermocouple

J Type thermocouple gives 1mV output for 18 degree C

Useful in reducing atmospheres

If J Type is protected by compacted mineral insulation and appropriate outer sheath, it is useable from 0 to 816°C, (32 to 1500°F). It is not susceptible to aging in the 371 to 538°C, (700 to 1000°F) temperature range

In J Type thermocouple positive leg is composed of Iron (Fe) and negative leg is composed of mixture of Copper 55% and Nickel wire 45% which is known as constantan alloy.

Thermocouple grade wire, -346 to 1,400F (-210 to750°C)

Extension wire, 32 to 392F (0 to200°C)

Standard: +/- 2.2°Cor +/-0.75%

Special Limits of Error: +/- 1.1C or4%

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