Thermocouple iron is used for the positive leg of thermocouples type J and L. The alloy is also used as the positive leg of compensating cables type KCA (WX).
The alloy is transformed magnetically at 770°C (1420°F) and a crystallographic transformation occurs at 900°C (1650°F). Both transformations affect the thermoelectric properties of the alloy.
Iron can not be exposed to sulphur-containing atmospheres and to temperatures below zero. Thermocouple iron wire is supplied copper-coated to prevent oxidation.
Thermocouple type J and L are frequently used because of their high emf output and their low cost. Compensating cables of type KCA are used together with thermocouples of type K.
Chemical composition
|
C % |
Si % |
Mn % |
Fe % |
| Nominal composition |
0.03 |
0.05 |
0.30 |
Bal. |
Mechanical properties
| Wire size |
Yield strength |
Tensile strength |
Elongation |
Hardness |
| Ø |
Rp0.2 |
Rm |
A |
|
| mm |
MPa |
MPa |
% |
Hv |
| 2.0 |
- |
450 |
15 |
100 |
Physical properties
| Density g/cm3 |
7.86 |
| Electrical resistivity at 20°C Ω mm2/m |
0.13 |
| Temperature coefficient of resistance between 20°C and 100°C x 10-6/K |
5000 |
Coefficient of thermal expansion
| Temperature °C |
Thermal Expansion x 10-6/K |
| 20 - 100 |
11.7 |
Thermal conductivity
| Temperature °C |
100 |
| W m-1 K-1 |
66.2 |
Specific heat capacity
| Temperature °C |
20 |
| kJ kg-1 K-1 |
0.445 |
| Melting point °C |
1535 |
| Magnetic properties |
The material is magnetic |
Recommended maximum continuous operating temperature
| Wire size Ø |
3.26 |
1.63 |
1.00 |
0.50 |
0.25 |
| Bare wire °C |
760 |
760 |
720 |
650 |
560 |
| Protected wire °C |
760 |
760 |
760 |
760 |
670 |
Note that the indicated temperatures should be considered as guide values
Thermoelectric properties
Iron is used as positive leg in thermocouples of type J, L and in compensating type KCA. The emf values vs the negative thermocouple leg are listed in the data sheets for the respective thermocouples.
