A thermocouple is a widely used temperature-measuring device that converts temperature differences into an electric voltage. It is simple, reliable, and suitable for industrial and high-temperature applications.
Table of Contents
Definition
A thermocouple is defined as a device consisting of two dissimilar metal wires joined at one end (hot junction), which produces an electromotive force (EMF) proportional to the temperature difference between the hot and cold junctions.
Construction
- Two dissimilar metal wires (e.g., Copper–Constantan, Iron–Constantan)
- Hot junction: Exposed to the measured temperature
- Cold junction: Reference junction
- Insulation and protective sheath
Working Principle (Seebeck Effect)
When two dissimilar metals form a closed circuit and their junctions are at different temperatures,
an EMF is generated.
Mathematical Expression:
\[
E = S \cdot (T_h – T_c)
\]
- \(E\) = EMF generated (Volts)
- \(S\) = Seebeck coefficient
- \(T_h\) = Hot junction temperature
- \(T_c\) = Cold junction temperature
Types of Thermocouples
| Type | Metals Used | Temperature Range (°C) |
|---|---|---|
| K | Chromel–Alumel | -200 to 1260 |
| J | Iron–Constantan | 0 to 760 |
| T | Copper–Constantan | -200 to 370 |
Advantages
- Simple and rugged construction
- Wide temperature range
- Fast response
- No external power required
Disadvantages
- Low output voltage (mV range)
- Non-linear characteristics
- Requires cold junction compensation
Applications
- Industrial temperature measurement
- Furnaces and boilers
- Gas turbines and engines
- Process control systems