Thermocouple

A thermocouple is a differential temperature sensor or thermoelectric device measuring temperatures. They have 2 wires of different metal alloys, linked at the end.

[2] At the thermocouple's other end, the wires connect toa  thermocouple capable device like a meter (meter, controller, transmitter) that stays at constant known temperature, the cold junction (CJ).

Operation

[2] Changing the HJ's temperature createa s a small mV signal, which occurs since if metal is heated, the metals' electrons move away from the heat source to the cold junction. And since the 2 wires are made of dissimilar metals, the electrons move at various rates. The wires whose electrons move around more will have negative charge at CJ and the wire with slower ones is the positive lead wire.

As a result, a formula can be derived that converts the Vout into a temperature value, the Seebeck effect equation, depicting the thermocouple voltage–temperature relationship:

V = SΔT

• V = thermocouple Vout (often in mV)

• S = Seebeck coefficient (depends on the 2 metals)

• ΔT = temperature difference between HJ and CJ

ΔT is defined as: ΔT = THJ​ − TCJ

So thermocouples don't directly measure absolute temperature, but rather the difference between HJ and CJ, hence being "differential" sensors. Real thermocouples's relationship isn't perfectly linear, so they use polynomial equations or lookup tables standardized by organizations like NIST.

E.g., a common thermocouple conversion equation is: T = a0 + a1​V + a2​V2 + a3​V3 + ...

• V = measured thermocouple voltage

• a0, a1​, a2​, a3... = calibration constants for thermocouple type (K, J, T, etc.)

K-type thermocouples are called general purpose thermocouples due to being cheap, so they're in various industrial and commercial usages where temperature must be actively monitored.

References

• [1] Wikipedia

• [2] Thermocouples: What They Are And How They Work - DwyerOmega (YouTube)

My materials

• [D1] ELE8931 Week 2 theory (Canva)