What is the working principle of a thermocouple?

I. Core Principle: The Essence of the Seebeck Effect

1. Thermal Diffusion Characteristics of Electrons

   All conductors contain free electrons, and the intensity of electron thermal motion is positively correlated with temperature: the higher the temperature, the more vigorous the electron motion and the stronger the diffusion capacity. When two conductors of different materials (e.g., nickel-chromium alloy + nickel-silicon alloy for K-type thermocouples) are joined, the difference in their electron density and work function causes "unbalanced diffusion" of electrons at the contact surface. For example, if electrons in Conductor A diffuse faster than those in Conductor B, Conductor A loses electrons and becomes positively charged, while Conductor B gains electrons and becomes negatively charged, forming a tiny "contact potential" (also known as the Peltier potential).
2. Potential Difference Caused by Temperature Gradient

   When there is a temperature difference (T ≠ T₀) between the two junctions of the loop (hot junction T and cold junction T₀), the contact potentials at the two junctions differ:

• Hot junction (T): Higher temperature leads to more vigorous electron diffusion and a larger contact potential;
• Cold junction (T₀): Lower temperature results in milder electron diffusion and a smaller contact potential.

• Sₐᵦ is the "Seebeck coefficient" of the two conductors (an intrinsic material property, varying with thermocouple types such as K, S, and J);
• T is the temperature of the hot junction;
• T₀ is the temperature of the cold junction.

3. Current Formation in the Closed Loop (Measurement Core)
   Since the potential differences at the two junctions cannot cancel each other out (due to the temperature difference), a continuous thermo-EMF is formed in the closed loop. When connected to a measuring instrument (e.g., thermocouple thermometer, PLC), this millivolt-level signal (typically tens of microvolts per °C) can be detected. As long as the cold junction temperature (T₀) and the conductors' Seebeck coefficient (Sₐᵦ) are known, the hot junction temperature (T) can be inversely calculated from the thermo-EMF (E) — this is the core logic of thermocouple temperature measurement.

II. Key Supplements: 3 Easily Confused Points

1. "Two Different Conductors" Is a Prerequisite

   If a loop is formed using two conductors of the same material, no thermo-EMF will be generated even if there is a temperature difference between the two ends (since the contact potentials at the two junctions are exactly equal and cancel each other out). Therefore, thermocouples must be paired with "two different materials" (e.g., platinum-rhodium 10%/platinum, nickel-chromium/copper-nickel).
2. Importance of Cold Junction Temperature

   The thermo-EMF (E) is a "function of temperature difference" rather than a single-valued function of the hot junction temperature (T). Fluctuations in the cold junction temperature (T₀), such as changes in ambient temperature, will directly cause thermo-EMF errors. In practical applications, cold junction compensation is mandatory (e.g., placing the cold junction in a constant-temperature environment, using electronic compensation circuits, or software correction). Its essence is to fix T₀ (usually set to 0°C) to ensure the measurement result depends solely on the hot junction temperature (T).
3. Magnitude of Thermo-EMF

   The thermo-EMF of a thermocouple is extremely weak: for example, a K-type thermocouple generates approximately 41 μV (microvolts) per °C in the range of 0~1000°C, with a total thermo-EMF of about 41 mV (millivolts) at 1000°C. Therefore, electromagnetic interference must be avoided during measurement, and the secondary instrument must have high sensitivity.

III. Popular Analogy: Understanding the Seebeck Effect with "Water Flow"

• Hot junction (high temperature): The water source is at a high level, so the water flow (electrons) has strong diffusion momentum (large contact potential);
• Cold junction (low temperature): The water source is at a low level, so the water flow (electrons) has weak diffusion momentum (small contact potential);
• Thermo-EMF in the loop: Equivalent to the "water level difference between the two sources" — the larger the water level difference (temperature difference), the stronger the water flow (current/EMF);
• Conductors of different materials: Equivalent to "water pipes of different diameters" (different electron diffusion difficulties), which determine the "correspondence between water level difference and water flow" (i.e., the Seebeck coefficient).

Summary