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ROC800-Series Instruction Manual
4-28
Input/Output Modules
Revised Jul-14
Compensation (CJC) correction factor is applied to compensate for
errors due to any voltage inducted at the wiring terminals by the
junction between the different metal of the TC wiring and the TC
module’s terminal blocks.
Note
:
The use of dissimilar metals is not supported. It does not provide
the correct results, as CJC is applied at the module level.
Thermocouples are self-powered and require no excitation current. The
TC module uses integrated short-circuit protected isolated power
supplies and completely isolates the field wiring side of the module
from the backplane.
Caution
If using the Type J above 750°C (1382°F), abrupt magnetic
transformation causes permanent de-calibration of the TC wires.
De-calibration
De-calibration can occur in thermocouple wires. De-calibration is
the process of unintentionally altering the makeup of the
thermocouple, usually caused by the diffusion of atmospheric
particles into the metal at the extremes of the operating temperature
range. Impurities and chemicals can cause de-calibration from the
insulation diffusing into the thermocouple wire. If operating at high
temperatures, check the specification of the probe insulation. It is
advised to use thermocouples with insulated junctions to protect
against oxidation and contamination.
Thermocouples use thin wire (typically 32 AWG) to minimize thermal
shunting and increase response times. Wire size used in the
thermocouple depends upon the application. Typically, when longer life
is required for the higher temperatures, select the larger size wires.
When sensitivity is the prime concern, use smaller size wiring. Thin
wire causes the thermocouple to have a high resistance that can cause
errors due to the input impedance of the measuring instrument. If
thermocouples with thin leads or long cables are required, keep the
thermocouple leads short and use a thermocouple extension wire to run
between the thermocouple and measuring instrument.
The thermocouple connects directly to the module’s removable terminal
block (see
Figure 4-34
). No special terminal or isothermal block is
required.