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