Chapter 3
NI 4350 Operation
3-6
©
National Instruments Corporation
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 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
, 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 3-3.
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
