Chapter 2
Operating the NI 435x Device
©
National Instruments Corporation
2-7
Figure 2-2.
Effect of the Cold-Junction
With the NI 435
x
, you can perform cold-junction compensation in
software. To do this, you can use the thermistor temperature sensor on the
NI 435
x
accessory to measure the ambient temperature at the cold-junction
and compute the appropriate compensation for the unwanted
thermoelectric voltages using software. The cold-junction sensor is on
analog channel 0 on the PSH32-TC6, TC-2190, CB-27T, TBX-68T,
and CB-68T.
You have several options for performing cold-junction compensation:
•
If you are using VirtualBench-Logger or the NI 435
x
instrument
driver, the software automatically performs cold-junction
compensation on all channels configured as thermocouple channels.
•
If you are using LabVIEW and MAX virtual channels, the software
includes examples that perform these temperature-to-voltage and
voltage-to-temperature conversions for the cold-junction thermistor
and various types of thermocouples based on the National Institute of
Standards and Technology (NIST) standard reference tables. These
examples are located in the DAQ analog input example library and
have NI 435
x
in their titles.
•
If you are not using either of the previous software options, complete
the following steps to perform cold-junction compensation.
1.
Measure the resistance of the thermistor cold-junction sensor,
R
thermistor cold-junction
, and compute the cold-junction temperature,
T
cold-junction
, using the thermistor resistance-temperature
conversion formula.
2.
From this temperature of the cold-junction, T
cold-junction
, compute
the equivalent thermocouple voltage, V
cold-junction
, for this junction
using a standard thermocouple conversion formula.
+
+
+
+
–
–
–
–
V
2
V
thermocouple
V
measured
V
1
V
measured
=
V
thermocouple
+ V
1
– V
2
where V
1
– V
2
= V
cold-junction