Thermocouples - Automation Direct DL405 User Manual

Thermocouples

Use shielded thermocouples whenever possible to minimize the presence of noise
on the thermocouple wire. Ground the shield wire at one end only. For grounded
thermocouples, connect the shield at the sensor end. For ungrounded
thermocouples, connect the shield to the 0V (common) terminal.
Grounded Thermocouple Assembly
A grounded thermocouple provides better response time than an ungrounded
thermocouple because the tip of the thermocouple junction is in direct contact with
the protective case.
Ungrounded Thermocouple Assembly
An ungrounded thermocouple is electrically isolated from the protective case. If the
case is electrically grounded it provides a low-impedance path for electrical noise to
travel. The ungrounded thermocouple provides a more stable and accurate
measurement in a noisy environment.
Exposed Grounded Thermocouple
The thermocouple does not have a protective case and is directly connected to a
device with a higher potential. Grounding the thermocouple assures that the
thermocouple remains within the common mode specifications. Because a
thermocouple is essentially a wire, it provides a low-impedance path for electrical
noise. The noise filter has a response of >100dB @ 50/60 Hz.
WARNING: A thermocouple can become shorted to a high voltage potential.
Because common terminals are internally connected together, whatever voltage
potential exists on one thermocouple will exist on the other channels.
Ambient The F4-08THM
Variations in temperature range of 0_C to 60_C.
Temperature
The cold junction compensation is calibrated to operate in a still-air environment. If
the module is used in an application that has forced convection cooling, an error of
2–3_C may be introduced. To compensate for this you can use ladder logic to
correct the values.
When configuring the system design it is best to locate any heat-producing devices
above and away from the PLC chassis because the heat will affect the temperature
readings. For example, heat introduced at one end of the terminal block can cause
a channel-to-channel variation.
When exposing the F4-08THM module to abrupt ambient temperature changes it
will take several minutes for the cold junction compensation and terminal block to
stabilize. Errors introduced by abrupt ambient temperature changes will be less
than 4_C.
F4-08THM 8-Channel Thermocouple Input
module has been designed to operate within the ambient
10–9