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Chapter 2
Operating the NI 435x Device
2-6
ni.com
Note
The default setting for programmable open-thermocouple detection in volts and
4-wire ohms measurement modes is Off.
Measuring Temperature with Thermocouples
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 435
x
accessory
creates the
reference junction
or
cold-junction
, shown in Figure 2-2. These
additional junctions act as thermocouples and produce their own voltages.
Thus, the final measured voltage, V
measured
, includes both the thermocouple
voltage, V
thermocouple
, and the cold-junction voltage, V
cold-junction
. The
method of compensating for these unwanted cold-junction voltages is
called
cold-junction compensation
.
Table 2-3.
Using
Programmable, Open-Thermocouple Detection
Signal Source
Programmable
Open-Thermocouple
Detection
Thermocouples
On or Off
Voltage signal sources other than
thermocouples
Off
RTDs, thermistors, and resistors connected
to the built-in current source
Off