Page 149 - Mechanical Engineers' Handbook (Volume 2)

138 Temperature and Flow Transducers

Figure 3 Temperature measurement using an icebath as the reference. (Reproduced from Ref. 2, with
permission.)

of the two pieces of copper wire are the same and (2) the temperature intervals across the
two copper wires are the same.

2.5 Zone-Box Circuits
When several thermocouples are used in a single test far from the measuring station, sig-
nificant economies can sometimes be achieved by using a common zone box and substituting
copper lead wires for much of the thermoelectric material. Such a circuit is shown in Fig.
4. The objective of the analysis is to determine the conditions under which the actual circuit
is thermoelectrically equivalent to the pattern circuit.
The function of the zone box is to provide a region of uniform temperature within which
connections can be made. The temperature of the zone box need not be constant, and it need
not be known—it need only be uniform.
The circuit consists of a reference bath, a selector switch, a readout instrument, and a
set of thermocouples extending from the zone box to their individual sensing points con-
nected together with copper wires. Providing the selector switch and the lead wires introduce
no spurious emf, the behavior of any one thermocouple for this circuit should be the same
as that of the pattern circuit shown. In the E–T diagram, the copper lead wires are shown
passing through the selector switch with no acknowledgment of its existence—the switch is
assumed to have a uniform temperature at the ambient temperature.