Page 150 - Mechanical Engineers' Handbook (Volume 2)

2 Thermocouples 139

Figure 4 Multiple measurements using a zone box and selector switch. (Reproduced from Ref. 2, with
permission.)

Commercially available zone-box and selector switch assemblies are sometimes made
with the assumption that the junction 6 will be placed in the reference bath and points 5
and 7 in the zone box. This requires reversing the polarity of the reference junction (i.e., the
wire between points 5 and 6 must then be constantan in this example, and the wire between
6 and 7 must then be iron).
If a wiring diagram is not available, a test for reference junction polarity should be
made. With the system connected, at any arbitrary temperature, the instrument reading should
go up if the temperature of the reference junction goes down, and conversely.

2.6 Laws of Thermoelectricity

Various authors have attempted to summarize the behavior of thermocouples through sets of
laws ranging from three to six in number. One of the more detailed sets is given by Doebelin. 6
Each law can easily be proven by recourse to an emf–temperature sketch. The first three
from Doebelin’s list are used as examples:
1. The thermal emf of a thermocouple with junctions at T hot and T is totally unaffected
ref
by temperature elsewhere in the circuit if the two metals used are each homogeneous.