National Instruments CFP-CB-4 用户手册

Compact FieldPoint thermocouple input modules can measure the 
temperature of the terminals on the cFP-CB-x connector block. 
The modules use this measurement, called the cold junction 
temperature, to compensate for the thermoelectric voltages 
generated at these junctions. For the most accurate cold-junction 
compensation, use the cFP-CB-3 for connecting thermocouple 
signals.

Heat dissipated by adjacent modules (or other nearby heat sources) 
can cause errors in thermocouple measurements by heating the 
terminals to a different temperature than the sensor used to 
measure the cold junction. The thermal gradient generated across 
the terminals can cause the terminals of different channels to differ 
in temperatures. The resulting measurement creates errors in 
absolute and relative accuracy between channels. The actual 
thermal gradient depends on the connector block you use and 
the details of your installation. The following section provides 
guidelines for minimizing thermal gradients.

To minimize the thermal gradient across the terminals, use the 
cFP-CB-3 for connecting thermocouple signals. The cFP-CB-3 is 
designed with isothermal construction to keep the terminals at the 
same temperature. In addition to using the cFP-CB-3, you can 
minimize thermal gradients by following these guidelines:

•

Do not place power supplies or other heat sources directly 
above or below the Compact FieldPoint system.

•

Do not install high-power modules, such as the cFP-DO-4xx 
and cFP-RLY-4xx, adjacent to the cFP-CB-3.

•

Use small-gauge thermocouple wire to transfer less heat.

•

Run thermocouple wires together near the connector block to 
keep them at the same temperature.

•

Avoid running thermocouple wires near hot or cold objects.

•

Use the foam cushion strain relief to restrict air flow.