THERMOCOUPLES
A thermocouple is a device that is able to convert thermal energy into electric energy.
At present, thermocouples offer a wide range of industrial applications, because virtually all industrial processes require a strict temperature control. The use of thermocouples helps automating the temperature control thanks to the possibility of implementing programs that execute special actions depending on the temperature of the industrial process at a given moment.
A thermocouple consists of two electric conductors, usually two wires of different materials that are connected to each other at one end (the “hot” end). The other ends (the “cold” ends) are connected to a measuring instrument.
Most thermocouples are delivered ready for use, which means in a protective assembly made to avoid their suffering damages due to mechanical forces or chemical attacks.
The free extremities of the compensating cables, or often the thermocouple’s conductors themselves, are directly connected to the measuring instrument with copper conductors. The thermocouples’ protective assemblies shall be adapted to their particular operating conditions. Thermocouples of noble metal are always protected by an isolating ceramic, even if the protective unit has a protective metal sheath.TYPES OF THERMOCOUPLES
TYPE K
It is the most commonly used thermocouple because of its wide measuring range. It shows a good resistance in oxidising atmospheres but it is not recommended for sulphurous or reducing atmospheres. It is the most suitable for nuclear uses.
Working temperature: -200ºC to 1250ºC
TYPE J
To be used in oxidising, reducing or inert atmosphere. Not advised in sulphurous atmosphere above 500°C. Due to the presence of an iron conductor, its use under the freezing point is not recommended.
Working temperature: 0ºC to 760ºC
TYPE S, R y B
Recommended for high-temperature use in oxidising or inert atmosphere.
Working temperature: 0ºC to 1700ºC
TYPE T
To be used in oxidising, reducing or inert atmosphere as well as under vacuum. Particularly suitable for the measurement of low or negative temperature as well as in atmospheres with a high humidity percentage. The presence of a copper conductor restricts its high –temperature use.
Working temperature: -200ºC to 350ºC
TYPE E
Among usual thermocouples, it has the greatest electromotive force per degree. It is suitable for oxidising atmospheres. It is not advised to use it in an environment with alternate oxidation, reduction and vacuum.
Working temperature: -200ºC to 870ºC