Rockwell Automation 1756-XXXX ControlLogix Analog I/O Modules - Manual (Page 132)

132 Publication 1756-UM009C-EN-P - December 2010
Chapter 6 Temperature-measuring Analog Modules (1756-IR6I, 1756-IT6I, and 1756-IT6I2)
The table lists the cold junction error from actual temperature, depending on
the type of cold junction compensation that is used.
Cold Junction Compensation
When using the thermocouple (1756-IT6I and 1756-IT6I2) modules, you must
account for additional voltage that may alter the input signal. A small voltage
generates at the junction of the thermocouple field wires and the screw
terminations of an RTB or IFM. This thermoelectric effect alters the
input signal.
To accurately compensate the input signal from your module, you must use a
cold junction sensor (CJS) to account for the increased voltage. Because there
are differences if you choose to connect sensors via an RTB or IFM, you must
configure the module (via RSLogix 5000 software) to work with the type of
CJS used in your application.
Connecting a Cold Junction Sensor Via a Removable Terminal Block
When you connect a CJS to your thermocouple module via an RTB, the
following occurs, depending on module type:
• The 1756-IT6I module uses one CJS in the middle of the module and
estimates temperature deviation elsewhere on the connector.
• The 1756-IT6I2 module uses two CJSs at the top and bottom of the
module and calculates temperature at each channel’s input terminals;
this usage of multiple sensors results in increased accuracy.
Cold-junction Compensation Types
If you use this
module With this type of cold junction
compensation The cold junction error from
actual temperature is
1756-IT6I2 Two cold-junction sensors on
an RTB +/-0.3 °C (32.54 °F)
1756-IT6I2 IFM +/-0.3 °C (32.54 °F)
1756-IT6I One cold-junction sensor on
an RTB
+/-3.2 °C ( 37.76 °F), max
(1)
(1) The cold junction error varies for each channel but 3.2 °C (37.76 °F) is the maximum error any channel
will show.
1756-IT6I IFM +/-0.3 °C (32.54 °F)