Question

Which of the following statements is/are correct for a thermocouple?

• A. It is based on the Seebeck Effect.
• B. The change in resistance is measured.
• C. The voltage difference between the two junctions is linearly proportional to the temperature difference between the junctions.
• D. It is based on the concept of Joule Heating.

Hint:

Recall what physically generates the signal in a thermocouple: is it a resistance change, a self generated EMF from two dissimilar metal junctions, or resistive heating?

Answer 1

The Correct Option is A

A thermocouple is a temperature sensor made from two dissimilar metal wires joined at one end, the measuring junction, with the free ends kept at a reference junction. Let's check each statement against how a thermocouple actually works.

1. It is based on the Seebeck Effect.: True. When two different metals are joined and the two junctions sit at different temperatures, a small voltage (EMF) appears across the open ends. This thermoelectric effect is called the Seebeck effect, and it is exactly the principle a thermocouple uses to generate its output voltage.
2. The change in resistance is measured.: False. Measuring a resistance change with temperature is how a resistance temperature detector (RTD) or a thermistor works, not a thermocouple. A thermocouple is a self generating sensor: it produces a voltage directly, and no resistance measurement or excitation current is needed.
3. The voltage difference between the two junctions is linearly proportional to the temperature difference between the junctions.: False. The Seebeck EMF of a real thermocouple does not follow a simple straight line against temperature difference. It follows a polynomial (higher order) relation over its working range, which is why standard thermocouples such as type J, K, and T are read out using multi term polynomial reference tables, not a single proportionality constant. Treating the EMF as strictly linear is only a rough approximation over a narrow band, not a general property of a thermocouple.
4. It is based on the concept of Joule Heating.: False. Joule heating is the \(I^2R\) heat produced when current flows through a resistance, used in heaters and fuses. A thermocouple does not rely on this at all; its signal comes from directly converting a temperature difference into a voltage through the Seebeck effect, with no heating current involved.

Only the first statement holds: a thermocouple works by the Seebeck effect. The resistance based description fits an RTD or thermistor, the strictly linear voltage-temperature claim does not hold for a real thermocouple, and Joule heating is an unrelated resistive heating effect.

Let's summarize:

• A thermocouple generates an EMF through the Seebeck effect from a temperature difference between two junctions of dissimilar metals.
• Resistance based sensing belongs to RTDs and thermistors, and a thermocouple's EMF versus temperature relation is polynomial, not linear, over its full range.

So the correct statement is (A) only.