Thermocouples produce a voltage related to temperature difference based on the Seebeck effect. Common materials used include chromel-alumel for Type K and iron-constantan for Type J. Thermocouples have advantages such as wide temperature range, long transmission distances, low cost, and fast response time. Limitations include needing cold junction compensation and signal amplification. Applications include temperature monitoring in steel making and heating appliances.

1. THERMOCOUPLE ∙ Principle of Operation ∙ Materials Used ∙ Advantages ∙ Applications ∙ Comparison with RTD ∙ Limitations By AnandBongir GirjashankarMishra

2. A thermocouple is a junction between two different metals that produces a voltage related to a temperature difference.

3. Principle of Operation Thermocouples are based on the principle that two wires made of dissimilar materials connected at either end will generate a potential between the two ends that is a function of the materials and temperature difference between the two ends (also called the Seebeck Effect).

4. Seebeck Effect

6. Materials Used Type K: Chromel – Alumel • Range: −200 °C to +1350 °C • Sensi: 41 µV/°C Type J: Iron – Constantan • −40 to +750 °C • 55 µV/°C Type E: Chromel – Constantan • 401 to 900° C • 68 µV/°C Type N: Nicrosil – Nisil • >1200 °C • 39 µV/°C

7. Advantages It is rugged in construction Covers a wide temperature range Using extension leads and compensating cables, long transmission distances for temperature measurement possible. This is most suitable for temperature measurement of industrial furnaces Comparatively cheaper in cost Calibration can be easily checked Offers good reproducibility High speed of response Satisfactory measurement accuracy

8. Limitations For accurate temperature measurements, cold junction compensation is necessary The emf induced versus temperature characteristics is somewhat nonlinear Stray voltage pickup is possible In many applications, amplification of signal is required

9. Applications Type B, S, R and K thermocouples are used extensively in the steel and iron industries to monitor temperatures and chemistry throughout the steel making process. Gas-fed heating appliances such as ovens & water heaters. In the testing of prototype electrical and mechanical apparatus

10. Comparison with RTD