Thermocouple (Type K)
Two dissimilar metal wires joined at a measurement junction produce a millivolt signal proportional to temperature.
01 / Recognize it
What this sensor looks like
Learn the housing, active face, mounting, and connector before you meet it on a machine.
Hardware recognition
Know what to look for
Use the silhouette, active face, and connection style to identify the device before checking its part number and datasheet.
- Body and mounting
- A metal sheath or probe terminates in a small connection head or flexible lead.
- Active face
- The measuring junction is at or near the probe tip.
- Cable and terminals
- Extension wire colors depend on thermocouple type and regional standard.
02 / Understand the principle
Watch cause become a PLC signal
Follow the physical event through the sensing element and into the exact controller value.
Signal story / live loop
Thermocouple (Type K): cause to controller
Now showingPhysical event
Hot junction heats up → Seebeck voltage rises → %IW66 = 700 °C
03 / Test and commission it
Commission it on the bench
Move the process, adjust the setpoint, invert the logic and inject faults. Watch the PLC value respond immediately.
Commissioning bench
Thermocouple (Type K)
PLC channel
%IW66
RAW 9677
Engineering value
388 °C
Thermocouple input
Channel healthy
Signal is inside the expected operating range
Commissioning note: Use the correct extension alloy and input type; copper cable creates another junction.
Field guide
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.
Use this when…
- Measuring process temperatures from -200 °C to +1260 °C
- Monitoring furnace, oven, or kiln temperature for PID control
- Detecting overtemperature on motors, bearings, or transformers
Where you will see it
Heat treatment
Type K thermocouples inside quench furnaces feed the PLC PID block that controls gas burner output, holding soak temperature within ±2 °C.
Plastics injection moulding
Multiple thermocouples along an injection barrel barrel zones allow independent zone control of melt temperature.
Next skill
Connect it to PLC logic
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