Thermocouples

Thermocouples are temperature sensors that generate an electrical voltage proportional to the temperature difference between two dissimilar metals joined at one end. This effect, known as the Seebeck effect, produces a small voltage at the measuring (hot) junction relative to a reference junction elsewhere in the circuit. The voltage is typically in the millivolt range per 100 degrees Celsius, and it is interpreted by a meter or control system after applying cold-junction compensation.

A thermocouple consists of two wires of different metals forming two junctions: the hot junction at the measurement point and the reference junction where the circuit is completed. The choice of metal pair defines the thermocouple type, with common examples including Type K (Chromel–Alumel), Type J (Iron–Constantan), Type T (Copper–Constantan), Type E (Chromel–Constantan), and Type N (Nicrosil–Nisil). Thermocouples are available in various form factors, from bare wires to insulated probes and mineral-insulated cables, often with protective sheaths of stainless steel, Inconel, or ceramic.

The output voltage is a function of temperature difference and the thermocouple type. Accurate readings require reference junction compensation, which can be performed by maintaining a known reference temperature (such as an ice bath in the laboratory) or by electronic compensation in the measurement instrument. Thermocouples are rugged, inexpensive, and capable of operating over wide temperature ranges, but they can drift with time, exhibit nonlinearity, and provide lower accuracy than resistance temperature detectors (RTDs). Proper installation, material compatibility, and regular calibration are essential for reliable performance.

Common applications include industrial process control, furnaces and kilns, exhaust and gas-turbinemonitoring, automotive and aerospace engines, welding, and environmental monitoring.