Measuring Temperature With Thermocouples - National Instruments NI 435 Series User Manual

Chapter 2 Operating the NI 435x Device
Note The default setting for programmable open-thermocouple detection in volts and
4-wire ohms measurement modes is Off.

Measuring Temperature with Thermocouples

NI 435x User Manual
Table 2-3. Using Programmable, Open-Thermocouple Detection
Signal Source
Thermocouples
Voltage signal sources other than
thermocouples
RTDs, thermistors, and resistors connected
to the built-in current source
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
electromotive force (EMF). However, just measuring this voltage is not
sufficient because connecting the thermocouple to the NI 435x accessory
creates the reference junction
additional junctions act as thermocouples and produce their own voltages.
Thus, the final measured voltage, V
voltage, V , and the cold-junction voltage, V
thermocouple
method of compensating for these unwanted cold-junction voltages is
called cold-junction compensation.
or cold-junction, shown in Figure 2-2. These
, includes both the thermocouple
measured
2-6
Programmable
Open-Thermocouple
Detection
On or Off
Off
Off
. The
cold-junction
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