Campbell Scientific CR5000 Measurement and Control Module - Manual (Page 70)

Section 3. CR5000 Measurement Details
3-16
the box to the CR5000. Alternatively, the junction box can be used to couple
extension grade thermocouple wire to the thermocouples, and the CR5000
panel temperature used as the reference. Extension grade thermocouple wire
has a smaller temperature range than standard thermocouple wire, but meets
the same limits of error within that range. The only situation where it would be
necessary to use extension grade wire instead of a external measuring junction
is where the junction box temperature is outside the range of reference junction
compensation provided by the CR5000. This is only a factor when using type
K thermocouples, where the upper limit of the reference compensation
linearization is 100
o C and the upper limit of the extension grade wire is 200
o C. With the other types of thermocouples the reference compensation range
equals or is greater than the extension wire range. In any case, errors can arise
if temperature gradients exist within the junction box.
Figure 3.4-1 illustrates a typical junction box. Terminal strips will be a
different metal than the thermocouple wire. Thus, if a temperature gradient
exists between A and A' or B and B', the junction box will act as another
thermocouple in series, creating an error in the voltage measured by the
CR5000. This thermoelectric offset voltage is a factor whether or not the
junction box is used for the reference. This offset can be minimized by making
the thermal conduction between the two points large and the distance small.
The best solution in the case where extension grade wire is being connected to
thermocouple wire would be to use connectors which clamped the two wires in
contact with each other.
CR9000
H
L
A' A
B' B
Junction Box
TC
CR5000 Junction Box
FIGURE 3.4-1. Diagram of Junction Box
An external reference junction box must be constructed so that the entire
terminal area is very close to the same temperature. This is necessary so that a
valid reference temperature can be measured and to avoid a thermoelectric
offset voltage which will be induced if the terminals at which the thermocouple
leads are connected (points A and B in Figure 3.4-1) are at different
temperatures. The box should contain elements of high thermal conductivity,
which will act to rapidly equilibrate any thermal gradients to which the box is
subjected. It is not necessary to design a constant temperature box, it is
desirable that the box respond slowly to external temperature fluctuations.
Radiation shielding must be provided when a junction box is installed in the
field. Care must also be taken that a thermal gradient is not induced by
conduction through the incoming wires. The CR5000 can be used to measure
the temperature gradients within the junction box.