When measuring temperature, it is important to know how long the heat resistance of the protective tube can last, but the thermocouple element may deteriorate depending on the atmosphere.
At high temperatures (500°C or higher), the change in thermoelectromotive force becomes large and disconnection often occur.
Caution is required in an oxidizing atmosphere as WRe thermocouples will disconnection at constant temperature.
In a reducing atmosphere (such as a carburizing furnace), the thermoelectromotive force of a type K thermocouple decreases.
Effect of atmospheric gas on thermocouple
| Thermocouple Types |
hydrogen |
Carbon monoxide |
sulfur |
Metallic gas (Ni) |
remarks |
| R |
weak
Becomes brittle
Decrease in electromotive force
|
weak
May disconnection |
weak
Becomes brittle |
Weak against Fe and Ni
Decrease in electromotive force |
Weak against silicon |
| K |
Weak, increased resistance
Becomes brittle
Decrease in electromotive force |
weak
Decrease in electromotive force |
Chromel is weak
May disconnection |
Weak against Fe
Decrease in electromotive force |
|
| E |
Chromel is weak |
Chromel is weak |
Chromel is weak |
weak |
|
| J |
strong |
Somewhat strong
Stable for carbon |
Iron is weak |
Somewhat strong |
There is a problem with the iron used for thermocouples rusting. |
| T |
No impact due to low operating temperature |
Copper, the material used for thermocouples, is easily oxidized. |
