©
National Instruments Corporation
21
NI WSN-3212 User Guide and Specifications
Figure 21.
Type T and E Errors
Cold-Junction Temperature Measurement Accuracy
Heat dissipated by adjacent heat sources can cause errors in thermocouple measurements by heating up
the terminals so that they are at a different temperature than the cold-junction compensation 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. If the system is
mounted as recommended in the WSN device user guides, the NI WSN-3212 accuracy specifications
include the errors caused by the gradient across the device terminals.
Minimizing Thermal Gradients
To minimize these errors, follow these guidelines:
•
Use small-gauge thermocouple wire. Smaller wire transfers less heat to or from the measuring
junction.
•
Run thermocouple wiring together near the screw-terminal connector to keep the wires at the same
temperature.
•
Avoid running thermocouple wires near hot or cold objects.
•
Keep the ambient temperature of the device as stable as possible.
•
Minimize adjacent heat sources and airflow across the device.
M
a
x, –40 to 70 ˚C
Typ, –40 to 70 ˚C
Me
asu
red Temper
a
t
u
re (˚C)
Me
asu
rement Error
(˚
C
)
0
1
3
4
5
2
–200
0
200
400
600
800
1000
