Chapter 4
Using Thermocouples
SCB-68 Shielded Connector Block User Manual
4-2
ni.com
The maximum voltage level thermocouples generate is typically only a few
millivolts. Therefore, you should use a DAQ device with high gain for best
resolution. You can measure thermocouples in either differential or
single-ended configuration. The differential configuration has better noise
immunity, but the single-ended configurations have twice as many inputs.
The DAQ device must have a ground reference, because thermocouples are
floating signal sources. Therefore, use bias resistors if the DAQ device is in
DIFF input mode. For a single-ended configuration, use RSE input mode.
For more information on field wiring considerations, refer to the
NI Developer Zone tutorial,
Field Wiring and Noise Considerations for
Analog Signals
, located at
ni.com/zone
.
Cold-junction compensation (CJC) with the SCB-68 is accurate only if the
temperature sensor reading is close to the actual temperature of the screw
terminals. When you read thermocouple measurements, keep the SCB-68
away from drafts or other temperature gradients, such as those caused by
heaters, radiators, fans, and very warm equipment. To minimize
temperature gradients, keep the cover of the SCB-68 closed and add custom
insulation, such as foam tape, to the SCB-68.
Switch Settings and Temperature Sensor Configuration
To accommodate thermocouples with DAQ devices, the SCB-68 has a
temperature sensor for CJC. To power the temperature sensor, set switches
S1, S2, and S3 as shown in Figures 4-1 and 4-2. Notice that this
configuration also powers on the signal conditioning accessory power.
Signal conditioning accessories include temperature sensors and signal
conditioning circuitry.
For single-ended operation, connect referenced single-ended analog
channel 0 to the temperature sensor by switching S5 to the up position.
The signal is referenced to AIGND. Set the switches as shown in
Figure 4-1.
