Thermocouples Extension Leads

About the thermocouples extension leads

In order to make a thermocouple conform to some precisely defined e.m.f. - temperature characteristic described by standard tables, it is necessary that all metals used are refined to a high degree of pureness and all alloys are manufactured to an exact specification. This makes the materials used expensive, and consequently thermocouples are typically only a few centimeters long. It is clearly impractical to connect a voltage measuring instrument to measure the thermocouple output in close proximity to the environment whose temperature is being measured, and therefore extension leads up to several meters long are normally connected between the thermocouple and the measuring instrument. There are now three junctions in the system and consequently three voltage sources, E1, E2 and E3, which add to give a net e.m.f. output En given by:

En = E1 + E2 + E3

 

The e.m.f. of interest for calculating the unknown temperature is E1. However, only En, is measured and therefore E2 and E3 must be calculated to obtain E1, which requires that the temperature at the junction between the thermocouples and extension wires be measured. This difficulty can be avoided if the extension wires are manufactured from materials which make the magnitudes of E2 and E3 approximately zero. This is most easily achieved by choosing the extension leads of the same basic materials as the thermocouple but manufactured to a lower specification which significantly reduces their cost. Such a solution is still prohibitively expensive in the case of noble-metal thermocouples, and it is necessary in this case to search for base-metal extension leads which have a similar thermoelectric behavior to the noble-metal thermocouple. In this form, the extension leads are known as compensating leads. A typical example of this is the use of nickel/copper-copper extension leads connected to a platinum/rhodium-platinum thermocouple.


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