A FeCuNi thermocouple (element) is an electrical device consisting of two dissimilar electrical conductors that form an electrical connection. A FeCuNi generates a temperature-dependent voltage that can be evaluated to measure the temperature.     

Commercially available FeCuNi sensors are inexpensive, interchangeable, come with standard connectors and can measure a wide range of temperatures. Unlike most other methods of measuring temperature, thermocouples are self-sustaining and require no external excitation. The main limitation with thermocouples is accuracy; system errors of less than one degree Celsius (°C) can be difficult to achieve.

The FeCuNi type has a narrower range (-40 °C to +750 °C), but a higher sensitivity of around 50 µV/°C. The Curie point of iron (770 °C) causes a smooth transition in the characteristic curve, which determines the upper temperature limit.