Thermocouple

When two dissimilar metals are joined and a temperature gradient exists along them, charge carriers diffuse at different rates in each metal, producing a net voltage across the open ends. This is the Seebeck effect. The voltage depends on the metal pair and the temperature difference between the hot and cold junctions, not on the wire length or junction size.

A thermocouple measures temperature differences, not absolute temperatures. The reading is the voltage between the hot junction and where the wires connect to the measuring instrument. The instrument must know the temperature of that connection point (the cold junction) and add it back. Modern systems integrate a separate temperature sensor at the terminal block for cold-junction compensation.

Each pair has a unique Seebeck coefficient, range, and accuracy. Type K (chromel/alumel): general-purpose, -200 to 1,260 °C. Type J (iron/constantan): vacuum, reducing atmospheres. Type T (copper/constantan): cryogenic. Types R, S, B (platinum-rhodium): very high temperature, expensive. Standard color codes and wire identification are critical to avoid mismatched extension wire errors.

RTDs (platinum resistance) are more accurate (±0.1 °C) but slower, narrower range, and need excitation current. Thermistors are most sensitive at room temperatures, very nonlinear, and limited range. Thermocouples cover the widest range, respond fastest, are simplest, but have lowest accuracy. Use thermocouples for furnaces, RTDs for process control, thermistors for medical and consumer.

Depends on junction size and sheath. An exposed-bead 0.13 mm K-type responds in tens of milliseconds. A grounded-junction sheathed probe (3 mm) takes a few seconds. An ungrounded probe is electrically isolated but slower. For furnace control, multi-second response is fine; for combustion diagnostics, exposed beads are needed.

(1) Cold junction temperature errors. (2) Mismatched extension wire (using copper instead of matching alloy). (3) Thermal gradients along the wire near the junction. (4) Aging and oxidation, which shifts calibration. (5) Electrical noise picked up by the millivolt-level signal. (6) Mechanical strain on the wires, which alters Seebeck coefficient.

Many thermocouples in series, with alternating junctions exposed to source and reference temperatures. The voltages add, multiplying sensitivity. Thermopiles are used in radiation pyrometers, gas-fired pilot light flame sensors, and IR sensors for non-contact temperature measurement (forehead thermometers, motion detectors). They produce enough voltage to drive small valves directly without amplification.