Thermocouples: Types and Temperature Ranges
1. Introduction
A thermocouple is a temperature-measuring device based on the Seebeck effect, where an electromotive force (emf) is generated when two dissimilar metals are joined and subjected to a temperature difference.
Seebeck Effect
$$ E \propto (T_{hot} - T_{cold}) $$
2. Construction and Working
- Two dissimilar metal wires
- Hot junction (measurement junction)
- Cold (reference) junction
When temperature difference exists, a voltage is generated and calibrated to measure temperature.
3. Classification of Thermocouples
Thermocouples are classified into standard types (ANSI/IEC):
- Base Metal Thermocouples
- Noble Metal Thermocouples
4. Types of Thermocouples and Their Temperature Ranges
4.1 Base Metal Thermocouples
Type K (Chromel–Alumel)
- Range: −200°C to 1260°C
- Most widely used
Type J (Iron–Constantan)
- Range: −40°C to 750°C
- High sensitivity
Type T (Copper–Constantan)
- Range: −200°C to 350°C
- Best for low temperatures
Type E (Chromel–Constantan)
- Range: −200°C to 900°C
- High output voltage
Type N (Nicrosil–Nisil)
- Range: −200°C to 1300°C
- Better stability than Type K
4.2 Noble Metal Thermocouples
Type R (Platinum–Rhodium)
- Range: 0°C to 1600°C
- High accuracy
Type S (Platinum–Rhodium)
- Range: 0°C to 1600°C
- Calibration standard
Type B (Platinum–Rhodium)
- Range: 0°C to 1700°C
- Very high temperature applications
5. Summary Table
| Type | Materials | Temperature Range (°C) | Key Feature |
|---|---|---|---|
| K | Chromel–Alumel | −200 to 1260 | Most common |
| J | Iron–Constantan | −40 to 750 | High sensitivity |
| T | Copper–Constantan | −200 to 350 | Low temp stability |
| E | Chromel–Constantan | −200 to 900 | High output |
| N | Nicrosil–Nisil | −200 to 1300 | Stable |
| R | Pt–PtRh | 0 to 1600 | High accuracy |
| S | Pt–PtRh | 0 to 1600 | Standard |
| B | PtRh–PtRh | 0 to 1700 | Very high temp |
6. Advantages
- Wide temperature range
- Fast response
- Rugged construction
- No external power required
7. Limitations
- Lower accuracy than RTDs
- Requires reference junction compensation
- Nonlinear output
8. Summary
- Type K → Most widely used
- Type T → Best for low temperature
- Type B/R/S → Noble metals, high temperature
- Based on Seebeck effect
- Output in millivolts (mV)