The Fe-CuNi thermocouple, also known as the Type J thermocouple, is widely recognized for its robust performance in moderate temperature measurements. Composed of a positive iron leg (Fe) and a negative leg made from Constantan (specifically JN alloy), the Type J thermocouple exhibits unique properties that make it suitable for various industrial applications. Importantly, though various Constantan-based alloys exist (e.g., EN, TN), the JN alloy provides a specific Electromotive Force (EMF) output curve that is not interchangeable with other Constantan alloys, ensuring the Type J’s distinct measurement accuracy.
Composition and Structure
The Type J thermocouple is constructed from iron (Fe) on the positive side, which maintains its pure form, and a JN alloy for the negative side. This JN alloy is predominantly a mix of copper and nickel with a Cu: Ni ratio of 55% to 45%. Below is a breakdown of its chemical composition:
Type | Ni (%) | Cu (%) | Mn (%) | Si (%) | Fe (%) |
Fe JP | — | — | — | — | 100 |
CuNi JN | 45 | 55 | — | — | — |
Operating Temperature Range
The recommended operating temperature range for the Fe-CuNi Type J thermocouple is -210°C to 1200°C. However, to avoid potential embrittlement of the iron, its practical usage is often limited to the range of 0°C to 750°C. This range is suitable for most industrial applications and ensures prolonged thermocouple durability.
Electromotive Force (EMF) Output
The EMF output of the Type J thermocouple when measured against platinum at a reference temperature of 0°C varies across different temperatures, as shown below:
Temperature (°C) | EMF (mV) Range |
100 | 5.207 - 5.331 |
200 | 10.717 - 10.841 |
300 | 16.244 - 16.410 |
400 | 21.760 - 21.936 |
600 | 32.956 - 33.236 |
This data confirms the consistent performance and reliability of the Type J thermocouple across a broad temperature range.
Physical Properties
The physical attributes of each leg in the Type J thermocouple contribute significantly to its performance. These include:
Property | JP | JN |
Density (g/cm³ at 20°C) | 7.8 | 8.9 |
Melting Point (°C) | 1407 | 1220 |
Tensile Strength (MPa) | ≥240 | ≥390 |
Elongation (%) | ≥20 | ≥25 |
Electrical Resistivity (μΩ·m) | 0.12 | 0.49 |
These physical properties highlight the durability and functionality of the Type J thermocouple, particularly under conditions that demand reliability and accuracy in temperature measurement.
Standards and Compatibility
The Type J thermocouple conforms to the IEC60584-1 standard, ensuring global consistency in quality and performance across various industrial applications.
In conclusion, the Fe-CuNi (Type J) thermocouple’s specific EMF characteristics, durability, and temperature range make it an ideal choice for moderate temperature applications, especially within the 0-750°C range. With its unique composition and reliable output, it remains a trusted instrument in industrial and scientific fields alike.