The Cu-CuNi thermocouple, commonly known as the Type T thermocouple, is a widely used temperature sensor characterized by its reliable performance across various applications. This thermocouple consists of two different conductors: the positive leg made from pure copper (Cu, also referred to as Cu TP) and the negative leg composed of Constantan (Cu TN), which is an alloy primarily consisting of 55% copper and 45% nickel.
Composition and Interchangeability
The chemical compositions of the two types of legs are crucial for understanding the thermocouple's functionality:
Copper (Cu TP): 100% Copper
Constantan (Cu TN): 55% Copper, 45% Nickel
It is essential to note that while both Constantan TN and EN are similar, they do not produce identical electromotive force (EMF) output curves compared to the JN version. Moreover, although TN and EN can be used interchangeably, they are not compatible with IN.
Operating Temperature Range
Type T thermocouples are designed to operate effectively within a temperature range of -200°C to 350°C. This limitation is primarily due to the low resistance to oxidation exhibited by copper. Exceeding this temperature range could lead to inaccuracies in measurements or damage to the thermocouple.
EMF Output
The EMF output of the Cu-CuNi thermocouple varies with temperature. Below are the EMF values for both Cu TP and Cu TN at key temperature points:
Type | EMF vs. Pt.64 (mV) | 100°C | 200°C | 300°C |
Cu TP | 4.229–4.325 | 9.238–9.334 | 14.792–14.932 | |
Cu TN | 0.763–0.783 | 1.826–1.846 | 3.137–3.161 | |
Cu-CuNi | 3.484–3.524 | 7.412–7.488 | 11.655–11.771 |
These values demonstrate the distinct voltage outputs generated by each conductor at specific temperatures, highlighting the importance of using the correct type in practical applications.
Physical Properties
The physical characteristics of the Type T thermocouple legs further emphasize their suitability for various applications:
Property | Cu TP | Cu TN |
Density at 20°C | 9 g/cm³ | 8.9 g/cm³ |
Melting Point | 1084°C | 1220°C |
Tensile Strength (Soft, Annealed at 20°C) | ≥190 MPa | ≥390 MPa |
Elongation (Soft, Annealed at 20°C) | ≥20% | ≥25% |
Resistivity at 20°C | 0.018 Ω·m | 0.5 Ω·m |
These properties indicate that while copper is lighter and has a lower melting point, Constantan exhibits higher tensile strength and elongation, making it suitable for demanding applications.
Conclusion
In summary, the Cu-CuNi thermocouple (Type T) is an effective and reliable choice for temperature measurement, particularly in low-temperature environments. Understanding its composition, operating limits, and physical properties is essential for ensuring accurate readings and selecting the appropriate thermocouple for specific applications. With its unique combination of materials and characteristics, the Type T thermocouple continues to be a valuable tool in various industries.