Page 151 - Mechanical Engineers' Handbook (Volume 2)

140 Temperature and Flow Transducers

In Fig. 5, it is presumed that T hot T candle . If the wire is uniform in calibration on both
sides of the hot spot, the potential hysteresis loop closes and no net emf is generated because
of the hot spot. The principal importance of this law is that it establishes the conditions
under which a thermocouple is a point sensor. If and only if wires A and B are uniform in
composition, the thermocouple output is determined only by the temperatures T ref and T hot
and is independent of the temperature distribution along the wire.
2. If a third homogeneous metal C is inserted into either A or B, as long as the two
new thermojunctions are at like temperature, the net emf of the circuit is unchanged
irrespective of the temperature of C away from the junctions.
Figure 6 shows a third material inserted into the A leg and then heated locally. It is
presumed that the temperatures at the two ends of C remain equal. If the material C is
homogeneous, the emf induced by the excursion in temperature from point 2 to point 3 is
canceled by that from point 3 to point 4, and no net signal is produced.
3. If metal C is inserted between A and B at one of the junctions, the temperature of
C at any point away from the AC and BC junctions is immaterial. So long as the
junctions AC and BC are both at the temperature T , the net emf is the same as if C
1
were not there.
Figure 7 illustrates this case. An intermediate material, C, is inserted between A and B
at the measuring junction. The diagram once more shows no change in the net emf if the
inserted material is homogeneous and does not undergo a net temperature change.
The situation involving intermediate materials at the junction is of great practical im-
portance because it addresses questions of manufacturing technique and how they affect
thermocouple calibration. For example, the third material (C), used to connected the two
materials A and B, might be the soft solder, silver solder, or braze material. The output of
the thermocouple is independent of that third material, provided that the third material is
homogeneous and begins and ends at the same temperature. In practice, these conditions are
usually satisfied because the joining material is isothermal. If the third material is isothermal,

Figure 5 Illustration of the law of interior tem-
peratures. (Reproduced from Ref. 2, with per-
mission.)