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ROC800-Series Instruction Manual
Revised Jul-14
Input/Output Modules
4-29
Figure 4-34. Thermocouple Input Module Wiring
Be sure to use the correct type of thermocouple wire to connect the
thermocouple to the ROC800. Minimize connections and make sure
connections are tight. If you use any dissimilar metals (such as copper
wire) to connect a thermocouple to the ROC800, you can create the
junction of dissimilar metals that can generate millivolt signals and
increase reading errors.
Ensure any plugs, sockets, or terminal blocks used to connect the
extension wire are made from the same metals as the thermocouples and
observe correct polarity.
The thermocouple probe must have sufficient length to minimize the
effect of conduction of heat from the hot end of the thermocouple.
Unless there is insufficient immersion, readings will be low. It is
suggested the thermocouple be immersed for a minimum distance
equivalent to four times the outside diameter of a protection tube or
well.
Use only ungrounded thermocouple constructions.
Grounded
thermocouples are susceptible to the creation of ground loops. In turn,
ground loops can cause interaction between thermocouple channels on
the thermocouple module.
Note
:
Use thermocouples as individual sensing devices. All modules
are isolated on the field side. Be aware that you can induce
ground loops by tying module-to-module commons together.
Noise Susceptibility
Millivolt signals are very small and are very susceptible to noise.
Noise from stray electrical and magnetic fields can generate voltage
signals higher than the millivolt levels generated from a
thermocouple. The TC modules can reject common mode noise
(signals that are the same on both wires), but rejection is not perfect,