How thermocouples really work

Although today's instrumentation makes using thermocouples easy, some of the literature that explains how thermocouples work is confusing, vague, or wrong. The confusion stems from how some technical notes describe the Seebeck Effect. These technical notes—including several from reputable instrument makers—can lead you to believe that a voltage develops at the junction where the two dissimilar metals in a thermocouple meet. Not so.

A temperature difference (Thot–Tcold) across a thermocouple's two metal wires produces a related output voltage, Vout.

The figure shows how the voltages develop. Two wires meet at the junction, whose temperature is T_hot. Voltage V_A forms across the wire made of metal A, and V_B forms across metal B (assume that the open ends of the wires are at the same temperature, T_cold). Therefore, the voltage across the open ends will be V_out =V_B –V_A. If both wires are made from the same material or if both wires are at the same temperature, V_out will be zero.

For a listing of several papers that get it right, see below. The papers also discuss cold-junction compensation and the effects of connecting thermocouple wires to instrument terminals.