Measuring Temperature With Thermocouples - National Instruments NI 4350 User Manual

Chapter 3 NI 4350 Operation
Note:

Measuring Temperature with Thermocouples

NI 4350 User Manual
Table 3-3. Using Programmable, Open-Thermocouple Detection
Thermocouples
Voltage signal sources other than
thermocouples
RTDs, thermistors, and resistors connected
to the built-in current source
The default setting for programmable open-thermocouple detection in volts
and 4-wire ohms measurement modes is off.
The thermocouple is the most popular transducer for measuring
temperature. Because the thermocouple is inexpensive, rugged, and can
operate over a very wide range of temperatures, it is a versatile and
useful sensor.
A thermocouple operates on the principle that the junction of two
dissimilar metals generates a voltage that varies with temperature, or
thermal EMF. However, just measuring this voltage is not sufficient
because connecting the thermocouple to the NI 4350 instrument
accessory creates the reference junction or cold-junction, shown in
Figure 3-2. These additional junctions act as thermocouples,
themselves, and produce their own voltages. Thus, the final measured
voltage, V
measured
and the cold-junction voltage, V
compensating for these unwanted cold-junction voltages is called
cold-junction compensation.
Signal
Source
, includes both the thermocouple voltage, V
cold-junction
3-6
Programmable
Open-Thermocouple
Detection
On or Off
Off
Off
thermocouple
. The method of
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