CABLES FOR THERMOMETRY - THERMOCOUPLES
One of the methods currently used for temperature measurement in industry is the use of the “thermocouple”.
A thermocouple is a circuit consisting of 2 metal conductors with certain thermoelectric properties soldered together at one end technically referred to as the “hot solder”, while the free ends of the thermocouple are referred to as the “cold solder”.
The presence of a temperature difference at the hot soldering point of the thermocouple generates an electromotive force at the cold soldering ends that varies depending on the temperature and the combination of metals used.
The use of these physical properties for the precise measurement of temperature therefore requires:
- the conductor structure is perfectly homogeneous, so a careful selection of conductor elements is necessary;
- the temperature of the free end of the pair (cold soldering) is maintained at a constant temperature so that the induced electromotive force is solely dependent on the hot solder (contact point of the 2 elements).
Verification curves for the temperature/induced electromotive force ratio have been established for the different pairs.
The measuring devices are designed to directly accept cold solder (the two free ends of the thermocouple).
The control room is in most cases far away from the measuring point, so this procedure is very costly. For this purpose, the cable called: by extension thermocouple was designed, the elements composing it offer thermoelectric properties almost identical to those of the thermocouple itself in order to perform the temperature measurement with the tolerance class set by the international standard IEC 584.3 in the 2 defined ranges, i.e.: 0 to + 100 C° or - 15 C° to + 200 C°, see table.
What to choose
A careful and selected choice of thermocouple conductors, with respect for the junction temperature according to
required by IEC 584.3 can guarantee measurement accuracy and fidelity within the tolerances set by the standard.