NI USB-9213 User Guide and Specifications
14
ni.com
Temperature Measurement Accuracy Considerations
Temperature measurement errors depend partly on the thermocouple type,
the accuracy of the thermocouple, the temperature being measured, and the
cold-junction temperature. Refer to the
section of the
for the errors for
each thermocouple type when connected to the NI USB-9213. The errors
do not account for the accuracy of the thermocouple itself.
For the best accuracy results, keep temperature gradients across
NI USB-9213 terminals to a minimum and enable the autozero channel.
Refer to the
sections for more information.
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 NI USB-9213
terminals to a different temperature than the cold-junction compensation
sensor. The thermal gradient across the terminals can cause the terminals
of different channels to be at different temperatures, in which case the
resulting measurement creates errors not only in absolute accuracy but also
in the relative accuracy between channels. Refer to the
section for the cold-junction compensation accuracy specifications.
If the
NI USB-9213 module is lying flat or facing upward, the thermocouple
accuracy specifications include the errors caused by the thermal gradient
across the module terminals. Refer to the
section in the
specifications.
Minimizing Thermal Gradients
Thermal gradients can be caused by changes in the ambient air temperature
near the front connector or by the thermocouple wire if it conducts heat or
cold directly to the terminal junctions. For the best accuracy results, follow
these guidelines for minimizing thermal gradients:
•
Use the NI 9940 connector backshell shown in Figure 3.
•
Use small-gauge thermocouple wire. Smaller wire transfers less heat
to or from the terminal junction.
•
Run thermocouple wiring together near the spring-terminal connector
to keep the wires at the same temperature.
•
Avoid running thermocouple wires near hot or cold objects.
•
If you connect any extension wires to thermocouple wires, use wires
made of the same conductive material as the thermocouple wires.
•
Minimize adjacent heat sources and air flow across the terminals.
