termocouples

Thermocouples, also spelled termocouples, are temperature sensors that generate a small voltage when two dissimilar metals are joined at a measuring junction. This voltage, produced by the Seebeck effect, is proportional to the temperature difference between the sensing junction and a reference junction at a known temperature.

A thermocouple is formed from two dissimilar metal wires welded at the measurement end and connected to a measuring instrument at the reference end. The instrument applies cold-junction compensation to convert the measured microvolt signal into an absolute temperature, using standardized tables or software because the voltage-to-temperature relationship is nonlinear and alloy-specific.

Common types include K (chromel–alumel), J (iron–constantan), T (copper–constantan), E (chromel–constantan), N (Nicrosil–Nisil), and the high-temperature platinum–rhodium types S, R, and B. Each type has different sensitivity and usable temperature range, with ranges spanning from well below 0°C to over 1300°C depending on alloy and sheath.

Thermocouples have advantages such as a wide operating range, fast response, simple construction, and ruggedness. They also have limitations, including drift with time and exposure to high temperatures, nonlinear response requiring calibration, and reliance on accurate cold-junction compensation. They are widely used in industrial process control, aviation and automotive engines, kilns, and laboratory research.