SM0486
insulator (6) and are clamped under the screws of the two terminals (3) in the
head. These terminals connect the lead wires to the thermocouple. The thermometer
(2) is also mounted on the head. The thermometer, covered by a protecting sheath,
extends into the head to measure the temperature at this point (reference
junction).
(5) We are interested in the principle used as a basis for the construction
of the thermocouple. From our discussion of the simple thermocouple of figure 11
you can reason that there is a similarity on principle and construction of the
thermocouples shown in figures 11 and 13.
Let's examine the corresponding
measurement circuits of the two thermocouples and see if we can establish a useful
comparison between the two.
(6) When you examine the measurement circuits A and B in figure 14, you see
that both circuits have hot junctions which are heated by an oven or other device
whose temperature is to be measured. Both measurement circuits use a galvanometer
as a readout device.
However, in the measurement circuit B, a precision
potentiometer and a galvanometer are used to measure and indicate the voltage
developed across the thermojunction. The voltage developed is proportional to the
difference in temperatures between points a and b in both measurement circuits.
The relationships of junction voltages developed by various junction temperatures
are shown graphically in figure 15.
Figure 14.
Thermocouple measurement circuits.
(7) Although the temperature scale indicated on the graph extends to
2000C., the platinum thermocouple is used for precise measurements between 0 and
1500. The temperature values and corresponding voltage values included in Figure
15 support the operating principles of the measurement circuits in Figure 14. We
are referring specifically to the B measurement circuit.
In this circuit the
reference junction is maintained at 0 by means of an ice bath (two ice bottles
could be used). Any
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