FP-TC-120 and cFP-TC-120 10 ni.com
Figure 8. Type B Errors
Cold-Junction Temperature
Measurement Accuracy
Heat dissipated by adjacent modules (or other nearby heat sources)
can cause errors in thermocouple measurements by heating up the
terminals so that they are at 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 be at different temperatures, so the resulting measurement
creates errors not only in absolute accuracy but also in the relative
accuracy between channels. The accuracy specifications for the
FP-TC-120 include the errors caused by a 0.2 °C (0.36 °F)
gradient. The actual gradient you can expect to encounter depends
on the terminal base or connector block you use and the details of
your installation. The following sections provide guidelines for
estimating and minimizing thermal gradients when using the
FP-TB-x terminal bases. If you are using the cFP-CB-x connector
block, refer to the cFP-CB-x Compact FieldPoint Connector
Blocks Operating Instructions.
Estimating Thermal Gradients across the FP-TB-3
Isothermal Terminal Base
The FP-TB-3 is designed with isothermal construction to keep the
terminals at the same temperature. NI recommends that you use the
FP-TB-3 to ensure the best accuracy of thermocouple
measurements. Adjacent FieldPoint modules (either network
0
0.5
1
1.5
2
2.5
3
3.5
4.5
5
0 300 600 900
Measured Temperature (°C)
Error (°C)
1200 1500 1800
4
Max Over Temp
Max at Room Temp
Typ Over Temp
Typ at Room Temp
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