6. From thermodynamic reasoning, Lord Kelvin showed that the interrelation of the
Peltier and Seebeck effects should lead to a linear relationship between
temperature difference and EMF. However, experiments showed that the relationship
is not linear.
He demonstrated that there must be an EMF developed in a single
homogenous (copper, silver, or aluminum) conductor whose ends are at different
temperatures.
This is the Thompson effect.
It is associated with the thermal
conduction of heat from the hot end of the conductor to the cold end.
7. If a third metal is introduced into the circuit, it may have both of its ends
connected to one of the original metals.
If the new connections are at the same
temperature, equal and opposite EMFs will be developed, and there will be no change
from the electrical condition of the original circuit.
If the new intermediate
metal joins the original pair, it is still true that the net EMF is not affected if
its ends are of the same temperature.
The proof of this statement involves
reasoning based on the second law of thermodynamics, the Law of Intermediate
Metals.
8. The Law of Intermediate Metals states that the algebraic sum of the
thermoelectric EMFs (voltages) of a circuit composed of any number of dissimilar
metals is zero if the circuit is at a uniform temperature throughout. There must
be a difference in temperature between the two junctions in order for there to be a
DC voltage output from the thermocouple.
Learning Event 2:
DESCRIBE THERMOCOUPLE MATERIALS AND TYPES
1. There are only a few combinations of metals that are commonly used for
thermocouples.
The requirements include: (1) a high coefficient of thermal EMF
with temperature, (2) a continuously increasing relationship of EMF to temperature
over a long range, (3) freedom from phase changes or other internal phenomena
giving rise to discontinuities in the temperature-EMF relation, (4) resistance to
oxidation, corrosion or contamination, (5) homogeneity and reproducibility to fit
an established temperature-EMF relation.
The following paragraphs describe
combinations commonly used.
a. The platinum-rhodium thermocouple invented by Le Chatelier is extremely
stable and free from internal changes. It is used as the primary device for the
reproduction of the International Temperature Scale from the freezing point of
antimony (about 630C) to the freezing point of gold at 1063C. Its disadvantages
are the high cost and the relatively low thermoelectric power.
b. The copper-constantan thermocouple has a well-established temperature-EMF
relationship. Its cost is low and it is convenient to use long leads of identical
material, which have relatively low resistance. In comparison to iron, copper is
resistant to rust at ordinary temperatures.
c. The iron-constantan thermocouple is probably the most widely used,
principally because of its low cost and its useful temperature limit is
considerably higher than that of copper, up to 1400F.
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