1. THeRmOCOUpleS THeORy
8
Law of Intermediate Temperatures
In most industrial installations, it is not practical to maintain the
reference junction of a thermocouple at a constant temperature.
So, some means must be provided to bring the EMF developed
at the reference junction to a value equal to that which would be
generated with a reference junction maintained at a standard
temperature, usually 0 °C (32 °F).
The ‘Law of Intermediate Temperatures’ provides a mean for
relating the EMF generated by a thermocouple under ordinary
conditions, to a standardized constant temperature. In effect,
the Law states that: the sum of the EMF’s generated by two ther-
mocouples (one with its junction at 0 °C (32 °F) and some re-
ference temperature and the other with its junction at the same
reference temperature and the measured temperature) is equi-
valent to the EMF produced by a single thermocouple with its
junction at 0 °C (32 °F) and the measured temperature (fg. 6).
Law of Intermediate Metals
When thermocouples are used, it is usually necessary to introdu-
ce additional metals into a circuit. This happens when an instru-
ment is used to measure the EMF and the junction is soldered
or welded.
It would seem that the introduction of other metals would mo-
dify the EMF developed by the thermocouple and destroy its ca-
libration. However the ‘Law of Intermediate Metals’ states that:
the introduction of a third metal into a circuit will have no effect
upon the generated EMF, so long as the junctions of the third
metal with the other two metals are at the same temperature.
If two dissimilar metals A & B with their junctions at T1 & T2 and
a third metal C are joined on one leg (if C is kept at a uniform
temperature along its entire length), the total EMF in the circuit
will be unaffected (fg. 5).
Fig. 5 Law of Intermediate Metals
C
T 3
B (–)
B (–)
E = EMF
A (+)
T 1
T 2