What is a thermocouple and how does it measure temperature?

• A. A device that measures heat flux
• B. A temperature sensor that generates a voltage proportional to temperature difference between the junctions of two dissimilar metals
• C. A resistor whose resistance changes with temperature
• D. A sticky label that changes color with temperature

Answer: (B)

Thermocouples work on the Seebeck effect: when two dissimilar metals are joined, a small voltage is produced that depends on the temperature difference between the two junctions. A thermocouple is a temperature sensor that generates this voltage as the hot junction (the process temperature) and a reference junction have different temperatures. The output voltage is proportional to the temperature difference, so by knowing the reference temperature (or using reference compensation) you can convert the voltage into an accurate temperature reading. The signal is small, so precise electronics and calibration are needed, and different metal pairs (types) give different temperature ranges and sensitivities. This combination of a voltage output and robust sensing makes it ideal for many industrial temperature-measurement tasks.

The other descriptions describe different things: a heat-flux sensor measures energy flow rather than temperature via a voltage from a junction of metals, an RTD or thermistor changes resistance with temperature instead of generating a voltage from a junction, and a temperature-indicating label changes color rather than providing a live electrical signal.