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What is a Type J Thermocouple?

A thermocouple puts out a voltage signal related to the temperature difference between to points. It has no dedicated sensing element but instead consists of two joint wires made from different metals. In case of the type J thermocouple, one wire is made of iron and the other wire is made of constantan.

Characteristics of Type J Thermocouples

Thermocouples are the simplest sensors in the world – what is easier than just putting two wires together? A thermocouple is physically very robust and can measure in a wide temperature range. It can also be built in miniature diameters as small as 0.25 mm. As opposed to RTD like Pt100 and Pt1000, wire length and wire diameter do not cause measurement errors with thermocouples.

Generally speaking, a thermocouple’s signal is much weaker than that of an RTD. This makes it more sensitive to electrical interference. Also, if you require a high resolution, it may not be the best choice. A specific delicacy of the J Type thermocouple is that its leads are prone to oxidation.

The hot junction of a thermocouple is where the two wires are welded together

Mineral Insulated J-Type Thermocouples


Type J Thermocouple



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Type J in the Plastics Industry

Type J sensors are very widespread in the plastics industry. With our melt temperature probes, you can precisely measure the melt temperature in the extruder barrel. Our purpose-designed accessories (bayonet fittings, compression springs, tip adapters, etc.) enable you to mount the sensor in various ways.

For injection molding applications, we have developed dedicated hot runner thermocouples that feature a robust cable transition that withstands high pull strengths (up to 15 kg) and temperatures of up to 500 °C. 

Suggestions for Thermocouple Type J

For most applications, a K-type thermocouple may be a better choice. Its leads cannot corrode and it covers a much wider temperature range (up to 1,150 °C). The rationale for the remaining relevance of the type J is mainly its slightly cheaper price.

Please note that as with every thermocouple, the cold junction needs to be compensated. Additionally, you need compensated cables and connectors with a thermocouple. 



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