DL8000 Preset Instruction Manual
Revised February-2016
Modules
3-27
3.2.14 Thermocouple (TC) Input Module
Caution
The TC2 module is NOT supported in the Series 1 CPU.
The four-channel Thermocouple Input module monitors types B, C, E,
J, K, N, R, S, or T thermocouples, based on how you configure the
module with ROCLINK 800 Configuration software. The type of
thermocouple refers to the material used to make a bimetallic junction.
Dissimilar materials in the thermocouple junction generate different
millivolt levels as a function of the heat to which they are exposed.
The Thermocouple module measures the voltage of the thermocouple to
which it is connected. The TC voltage is measured and a Cold Junction
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.
