Page 283 - Instrumentation Reference Book 3E

Measurement techniques: thermocouples 267

is zero either if the two junctions are at the same
temperature or if the average of the temperature 1.4
of the two junctions is equal to the neutral tem-
perature. Figure 14.28 shows the graph of the 1.2
e.m.f. of a zinc-iron thermocouple with tempera- p 1 .O
ture. 9
g 0.8
0
14.5.1.5 Thermoelectric iizversioii 5 0.6
This reversal of the thermoelectric e.m.f. is "ther- 0.4
moelectric inversion." 0.2
Figure 14.29 shows the thermoelectric lines for
several common materials. It will be seen that the
lines for iron and copper cross at a temperature of 0
275 "C. If the temperature of the cold junction of
and
iron and copper is 270 "' the tempera-
Figure 14.28 Temperature/e.m.f. curve for zinc/iron
ture of the other junction is raised, the thermo- couple,
electric e.m.f. of the circuit (represented by a
trapezium) will increase until the temperature of
the hot junction reaches 275 "C (when the e.m.f. is
20

15

70

5
I
Y

-5

-10

-
- 5 1 -20 -100 0 100 200 300 400 500 600
OC
Figure 14.29 Thermoelectric diagramsfor several metals.