Thermocouples
A thermocouple basically consists of any two dissimilar metal wires mechanically joined at one end, called the hot junction; the other end – or cold junction – is at a known reference temperature (usually 0°C), which is lower than the temperature of the hot junction.
When the hot junction is heated, this generates an electromotive force that is normally proportional to the temperature. The temperature/voltage relationship is repeatable.
Benefits
Potentially any pair of conductors can be used as a thermocouple, which is why alloys meeting the following requirements have been selected and standardised:
- high emission per °C
- good stability and repeatability
- good mechanical properties
- low cost
- metallurgical purity
- temperature/fem characteristic as linear as possible
Here are the most common industrial thermocouples
- Type B : (Pt 30% Rh / Pt 6% Rh)
- Type E : (Chromel / Constantan)
- Type J : (Iron / Constantan)
- Type K : (Chromel / Alumel)
- Type S : (Pt 10% Rh / Platinum)
- Type R : ( Pt 13% Rh / Platinum)
- Type T : (Copper / Constantan)
- Type N : (Nicrosil / Nisil)
Some thermocouples manufactured for special uses:
- Type C: (Tungsten-5 % rhenium / tungsten-26 % rhenium)
- Nickel−18 % Molybdenum versus −50 °C to 1410 °C Nickel−0.8 % Cobalt
Precision classes
| Type T Temperature range Tolerance Temperature range Tolerance | – 40°C to +125°C ± 0.5°C + 125°C to +350°C ± 0.004 (t) | – 40°C to +135°C ± 1°C + 133°C to E11 350°C ± 0.0075 (t) |
| Type E Temperature range Tolerance Temperature range Tolerance | – 40°C to +375°C ± 1.5°C + 3755°C to +800°C ± 0.004 (t) | – 40°C to +333°C ± 2.5°C + 333°C to +900°C ± 0.0075 (t) |
| Type J Temperature range Tolerance Temperature range Tolerance | – 40°C to +375°C ± 1.5°C + 375°C to +750°C ± 0.004 (t) | – 40°C to +333°C ± 2.5°C + 333°C to +750°C ± 0.0075 (t) |
| Type K, Type N Temperature range Tolerance Temperature range Tolerance | – 40°C to +375°C ± 1.5°C + 375°C to +1000°C ± 0.004 (t) | – 40°C to +333°C ± 2.5°C + 333°C to +1200°C ± 0.0075 (t) |
| Type R, Type S Temperature range Tolerance Temperature range Tolerance | 0°C to +1100°C ± 1°C + 1100°C to +1400°C ± 0.004 (t) | 0°C to +600°C ± 1.5°C + 600°C to +1600°C ± 0.0025 (t) |
| Type B Temperature range Tolerance Temperature range Tolerance | — | — — + 600°C to +1700°C ± 0.0025 (t) |
| Type C Temperature range Tolerance Temperature range Tolerance | — | 426°C to 2 315°C — — ± 0.01 (t) |
| TYPES OF THERMOCOUPLES | J,K,T,E,N,W,V,S,R,B |
| Sheath Material | AISI 316, 304, 321, ecc… |
| MGO Sheath Diameter | 0,25mm ÷ 12,5mm |
| Number of Elements | Single, double or triple |
| Operating temperature | – 200°C ÷+ 1800°C |
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