A thermocouple consists of two electrical conductors of different materials connected at one end called hot junction. The two free ends build a compensation point. The thermocouple can be extended by using an extension cable. The extension or compensating cables are connected to a measuring instrument, e.g. a galvanometer or electronic measuring unit. The thermoelectric voltage appearing at the reference junction depends on the thermocouple wire material and on the temperature difference between the measuring junction and the reference junction. For temperature measurement, the temperature of the reference junction must be kept constant (e.g. 25 °C) or must be well known, to make a appriopriate correction in mV. Extension cables are manufactured of the same material as the corresponding thermocouple, e.g. Cu-CuNi, Fe-CuNi. Compensating cables are manufactured of special materials. Up to 200 °C compensating cables supply the same thermoelectric voltage as the thermocouples to which they are connected. The thermoelectric voltages of the thermocouples are laid down in so-called basic value series. e.g. PtRh30%-PtRh6% Type B Fe-CuNi Type J NiCr-NiAl Type K PtRh87/13%-Pt Type R PtRh90/10%-Pt Type S and others in DIN IEC 584-1 and Fe-CuNi Type L Cu-CuNi Type U These thermocouples are not more available for use in new plants (thermoelectric voltage according to DIN 43710). The compensating cable for a thermocouple must be made of a material to suit the particular type of thermocouple, so compensating leads are colour-coded. For standardized compensating cable the regulations contained in DIN EN 60584 apply. Maximum temperatures indicated by manufacturer have to be considered. Most thermocouples are supplied ready for operation, that is in a protective mounting to prevent damage to the thermocouple by mechanical forces or chemical attack.
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