When the conductors and the thermocouple's wires are of the same nature, it is called extension cable, and when the conductors are of a different nature to that of the thermocouple's wires, a compensation cable.
The pyrometer is based on two effects:
PELTIER effect: if we establish an electrical contact between two wires of dissimilar metals, an electromotive force (EMF) is created at their junction point.
THOMSON effect: if there is a different temperature at each of the two ends of a conductor wire of uniform composition, an EMF is created.
The algebraic sum of the EMF of the above two effects creates a resultant EMF, called the SEEBECK effect, which is what it is measured.
The ideal thing would be for the cable concerned to be of the same material as the thermocouple (extension cable) but since the thermocouple is usually made of costly materials, another law of thermoelectricity is used to allow incorporation of a third metal into the circuit without causing the EMF to vary, as long as such a metal maintains the same temperature throughout its length.
Other cables are used with the particularity of giving in the same temperature zone of use, the same temperature-EMF (compensation cables) as the thermocouple.
The thermocouple cable are formed by the combination of 2 different materials and are identified with a letter (J, K, S, T), each of them is used for a temperature range.
|Thermocouple type||Materials||Normal range|
|J||Hierro - Constantan||-190ºC to 760ºC|
|T||Cobre - Constantan
Cobre - (Cobre-Níquel)
|-200ºC to 370ºC|
|K||Cromel - Alumel
(Cromo-Níquel) - Aluminio-Níquel)
|-190ºC to 1.260ºC|
|E||Cromel - Constantan||-100ºC to 1.260ºC|
|S||(90% Platino + 10% Rodio) - Platino||0ºC to 1.480ºC|
|R||(87% Platino + 13% Rodio) - Platino||0ºC to 1.480ºC|
The cables, the positive (+) and negative (-) wire are differentiated, since this is very important in their connection, the different standards what they do is an identification by colours. according to two standards are used.
The cable can insulate with different types for different temperature ranges. The types of insulation we made is:
KAPTON – Similar of FEP, but with the advantage that it handles a range from -267 a 350ºC.
FIBERGLASS – Very good for temperature 400ºC.
BRAID FIBERGLASS - We find that it is one of the most common insulators and the difference with the previous one is that the mesh protects the cable so that it does not fray easily, as well as helps protect it from shocks, which helps to extend its useful life.
PVC - Temperatures up to 105ºC, is economical, supports acid solutions, humid environments, they are normally anti-flame
They are to measure the temperature generated by a heat source and detected by a remote meter. thermocouple cable connecting both equipment