Thermocouple

A thermocouple consists of two dissimilar conductors joined at a measurement junction. A temperature difference between that junction and the reference junction generates a small voltage. The relationship between voltage and temperature is nonlinear and depends on thermocouple type, so practical instruments use standard tables or calibration polynomials.

Common types include K, J, T, E, N, R, S, and B. They differ in temperature range, sensitivity, oxidation resistance, stability, cost, and compatibility with the environment. The sheath, insulation, junction style, response time, and mounting method often matter as much as the thermocouple alloy.

Engineering use

Thermocouples are useful where ruggedness and wide temperature range are more important than very high absolute accuracy. They can be grounded, ungrounded, or exposed. Grounded junctions respond quickly but can introduce ground loops. Ungrounded junctions improve electrical isolation but respond more slowly. Exposed junctions are fast but vulnerable to contamination and mechanical damage.

Cold-junction compensation is mandatory because the measured voltage corresponds to a temperature difference. Extension wire must match the thermocouple type or use proper compensated alloys; ordinary copper connections in a thermal gradient can create unwanted thermoelectric voltages.

Common mistakes

A common mistake is assuming a thermocouple directly measures the hottest point near it. It measures the junction temperature, which can differ from the target because of conduction along the wire, radiation, poor contact, response lag, or gas-flow effects. Another is mixing thermocouple types or using the wrong extension wire. A strong thermocouple review states type, junction style, sheath material, mounting method, reference-junction compensation, calibration class, response time, electrical isolation, and expected thermal gradients.