The thermocouple compensation wire is a pair of metal wires with different chemical compositions, which have the same temperature-thermoelectric potential relationship with the thermocouple it is connected to within a certain temperature range. The thermocouple and the secondary instrument are connected by a compensation wire. If the polarity is connected correctly, it is equivalent to extending the hot electrode, so that the cold end of the thermocouple is extended to a lower temperature (the ideal temperature is 0°C) and stable Occasions, in order to perform cold junction temperature compensation, so as to achieve the purpose of accurate temperature measurement. The price of the compensation wire is much cheaper than the corresponding hot electrode.
For r and s-type thermocouples, related scholars have proposed an ideal temperature compensation system, whose compensation error is less than 2°C under the condition of 500°C. The specific method is to use a compensation cable with 3 wires.
The negative electrode is similar to braided multi-stranded stainless steel wire, and the positive electrode is ni, 20%cr, 10%fe alloy wire. The principle of material selection for this three-wire compensation cable system is:
① The thermoelectric characteristics of a'to c'and b'to c'should be consistent with the thermoelectric characteristics of the pt/rh thermocouple system with compensation wire within a certain temperature range.
②The resistance and ratio of a'and b'should be equal to the ratio of a'/c' and b'/c' to the pt/ph thermocouple potential difference. The resistance ratio of a'and b'braided multi-stranded wire can be adjusted by changing the relative number of each wire.
The actual effect fully proves that the temperature range of the three-wire compensation is obviously increased. The basic principle is:
In the temperature range of tn~τ0, the materials a′ and c′ and b′ and c′ respectively generate corresponding thermoelectric potentials. At the same time, a′ and b′ constitute an auxiliary thermocouple, and the thermoelectric potential generated by it is ea 'B' (tn, t0), this thermoelectric potential is similar to the Thomson potential of a thermoelectrode. The thermoelectric potentials of the above parts all increase with the increase of the temperature difference between tn and t0, and their integrated value is the same as that of the main thermocouple The electric potential value maintains a strict corresponding relationship, which can play a precise temperature compensation function in a relatively large temperature range.