Programming instructions

Chapter 21 Common SCXI Applications
LabVIEW Data Acquisition Basics Manual 21-2
© National Instruments Corporation
Analog Input Applications for
Measuring Temperature and Pressure
Two common transducers for measuring temperature are thermocouples
and RTDs. A common transducer for measuring pressure is strain gauges.
Read the following sections on special measuring considerations needed
for each transducer.
If you use the DAQ Channel Wizard to configure your analog input
channels, you can simplify the programming needed to measure your
channels. This section describes methods of measuring data using named
channels configured in the DAQ Channel Wizard and using the
conventional method.
Note
For more information about using the DAQ Channel Wizard to configure your
channels, refer to the Configuring Your Channels in NI-DAQ 5.x, 6.0 section of
Chapter 2, Installing and Configuring Your Data Acquisition Hardware. For more
information about using channel names, refer to the Channel Name Addressing
section of Chapter 3, Basic LabVIEW Data Acquisition Concepts.
Measuring Temperature with Thermocouples
If you want to measure the temperature of the environment, you can use the
temperature sensors in the terminal blocks. But if you want to measure the
temperature of an object away from the SCXI chassis, you must use a
transducer, like a thermocouple. A
thermocouple
is a junction of two
dissimilar metals that gives varying voltages based on the temperature.
However, when using thermocouples, you need to compensate for the
thermocouple voltages produced at the screw terminal because the junction
with the screw terminals itself forms another thermocouple. You can use
the resulting voltage from the temperature sensor on the terminal block for
cold-junction compensation. The cold-junction compensation voltage is
used when linearizing voltage readings from thermocouples into
temperature values.
The SCXI modules used to measure temperature in this section are
SCXI-1100, SCXI-1102, SCXI-1120, SCXI-1120D, SCXI-1121,
SCXI-1122, and SCXI-1141. All of the terminal blocks used with these
modules have temperature sensors which can be used as cold-junction
compensation.