The Cu-CuNi thermocouple, commonly referred to as the Type T thermocouple, is a robust and reliable temperature measurement device. With one leg made of pure copper (TP) and the other leg consisting of Constantan (TN), which is an alloy of 55% copper and 45% nickel, it offers a unique combination of properties suitable for various applications.
Composition and Properties
Positive Leg (TP)
Material: Pure Copper
Chemical Composition: 100% Cu
Physical Properties:
Density: 9 g/cm³ at 20°C
Melting Point: 1084°C
Tensile Strength: ≥190 MPa (soft annealed at 20°C)
Elongation: ≥20%
Resistivity: 0.018 Ω·mm²/m at 20°C
Negative Leg (TN)
Material: Constantan (Cu-Ni alloy)
Chemical Composition: 55% Cu, 45% Ni
Physical Properties:
Density: 8.9 g/cm³ at 20°C
Melting Point: 1220°C
Tensile Strength: ≥390 MPa (soft annealed at 20°C)
Elongation: ≥25%
Resistivity: 0.5 Ω·mm²/m at 20°C
EMF Characteristics
Type T thermocouples generate electromotive force (EMF) when exposed to temperature differences. The following table highlights the EMF values generated versus platinum at specific temperatures:
Temperature (°C) | TP (mV) | TN (mV) | Cu-CuNi (mV) |
100 | 4.229–4.325 | 0.763–0.783 | 3.484–3.524 |
200 | 9.238–9.334 | 1.826–1.846 | 7.412–7.488 |
300 | 14.792–14.932 | 3.137–3.161 | 11.655–11.771 |
Operating Temperature Range
The Type T thermocouple has an operational range of -200°C to 350°C. Its copper component (TP) is prone to oxidation at higher temperatures, making it unsuitable for applications beyond this range. This thermocouple is best suited for low to moderate temperature environments where accuracy and stability are critical.
Interchangeability of Constantan
Constantan, the negative leg material, varies slightly in composition across different standards:
TN and EN can interchange due to similar EMF characteristics.
JN, despite being called Constantan, differs significantly and cannot be interchanged with TN or EN.
IN is not interchangeable with TN or EN, emphasizing the need for compatibility checks in specific applications.
Advantages and Applications
Advantages
High Accuracy: Excellent for precise temperature measurement, especially at low temperatures.
Stability: Reliable performance over prolonged usage within the specified range.
Robustness: Adequate tensile strength and elongation for mechanical durability.
Applications
Type T thermocouples are widely used in industries such as:
Cryogenics: Reliable for ultra-low temperature measurements.
Food Processing: Accurate temperature control during cooking or freezing.
Laboratory Testing: Ideal for controlled experimental setups requiring precise readings.
Conclusion
The Cu-CuNi Type T thermocouple is a versatile and dependable solution for temperature sensing in a variety of applications. Its unique combination of pure copper and Constantan provides stable and accurate performance, particularly in low to moderate temperature environments. However, careful consideration of operating limits and compatibility is essential to ensure optimal functionality.