Thermocouple (Type K)

A thermocouple is one of the most widely used temperature sensors in industry. It works on the Seebeck effect: when two dissimilar metals are joined at both ends and one junction is at a different temperature than the other, a small voltage is produced. This voltage — typically a few millivolts at industrial temperatures — is measured and converted to a temperature reading.

Type K thermocouples use chromel and alumel as the two metals. They are the most common type because of their wide temperature range (-200 °C to +1260 °C) and relatively linear output curve (approximately 41 µV/°C at mid-range). Other types (J, T, E, N, R, S, B) trade off range, accuracy, and cost differently.

Cold junction compensation (CJC) is critical. The thermocouple voltage represents the temperature difference between the hot measurement junction and the cold reference junction — usually where the thermocouple wires connect to the PLC module terminals. The module measures ambient temperature at the terminal block and automatically adds the correction.

Thermocouple extension wire must use the same metal pair as the thermocouple itself. Using copper wire to extend a thermocouple introduces a new thermocouple junction at the connection point, generating a spurious error voltage.

PLCs read thermocouples through dedicated thermocouple input modules (e.g. Siemens SM331 TC, Allen-Bradley 1756-IT6I) that include the instrumentation amplifier, cold junction sensor, and linearisation lookup table. The module outputs a scaled integer representing temperature in tenths of a degree.