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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Temperature Measurements
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