National Instruments Module SCXI-1125 - Manual (Page 38)

© National Instruments Corporation 5-1 SCXI-1125 User Manual
5
Using the SCXI-1125
This chapter discusses typical applications for the SCXI-1125. While this
list is not comprehensive, it provides some guidance on how to improve
measurement accuracy for some of the most popular applications of the
SCXI-1125. Advanced operations such as calibration and using the CJC
channel are discussed as well.
Temperature Measurements Using Thermocouples
Making isolated temperature measurements from thermocouples is a
common use of the SCXI-1125. This section discusses how to use
thermocouples, CJC, and how to calculate the temperature accuracy of the
SCXI-1125.
NI recommends using the SCXI-1328 terminal block to make
thermocouple measurements with the SCXI-1125. Although you can use
many of the SCXI terminal blocks for thermocouple measurements, the
SCXI-1328 has an isothermal design that reduces temperature gradients
within the terminal block housing. This design reduces the CJC errors
which might reduce the accuracy of your temperature measurement. Most
SCXI terminal blocks available for the SCXI-1125 contain a cold-junction
temperature sensor, which is used for measuring ambient temperature. This
sensor connects to a special channel on the SCXI-1125 inside the terminal
block close to where the thermocouple connects to the screw terminals.
Note Place the SCXI chassis away from extreme temperature gradients to minimize the
temperature gradient inside the terminal block and maintain its isothermal nature for
accurate CJC.
A thermocouple relies on the principle that a small voltage that varies with
temperature is produced at the junction of two dissimilar metals. CJC is
necessary because the junction between the end of the thermocouple lead
wires and the screw terminals produces a small potential difference, adding
error to the thermocouple voltage. Knowing the temperature at the point
where the thermocouple is connected to the measurement instrument
allows you to determine the correct temperature reading at the
thermocouple junction. Due to the nonlinear relationship between