Cold-junction compensation – National Instruments SCXI-1321 User Manual
Page 14
SCXI-1321 Terminal Block Installation Guide
14
ni.com
gauge with a gauge factor of GF = 2, V
exc
= 3.3 V, and R = 120
Ω produces
the following:
V
change
= 0.3321 mV
Replacing the strained voltage by V
change
in the quarter-bridge strain
equation produces an equivalent 199
µε of change. Refer to the NI-DAQ
User Manual for more information on voltage to strain conversion
equations.
Cold-Junction Compensation
CJC is used only with thermocouples and provides improved accuracy of
temperature measurements. The CJC temperature sensor, mounted in the
SCXI-1321, outputs 10 mV/°C and has an accuracy of ±0.9 °C over the
0 to 55 °C temperature range. To determine the temperature, use the
following formulas:
where V
TEMPOUT
is the temperature sensor output voltage, and T (°F) and
T (°C) are the temperature readings in degrees Fahrenheit and degrees
Celsius, respectively.
Note
Use the average of a large number of samples to obtain the most accurate reading.
Noisy environments require averaging for greater accuracy. You cannot use virtual
channels to take the readings.
You can enable the CJC sensor in one of two ways depending on the input
mode configuration of the SCXI-1121. Jumper W5 switches the
temperature sensor output between MTEMP (multiplexed mode) and
DTEMP (parallel mode) modes. In MTEMP mode, you must scan the
cold-junction temperature independently of the other AI channels on the
SCXI-1121 using the LabVIEW Getting Started Analog Input VI, available
in
examples\daq\run_me.llb
, with the channel string
ob0 ! sc1 ! md1 ! mtemp
. This reads the temperature sensor on the
terminal block connected to the module in slot 1 of SCXI chassis 1.
You then can average several measurements of the cold-junction
temperature and use this average to compensate for the cold junction of the
thermocouple. Using this averaging method compensates for temperature
variations during the measurement period and makes the CJC temperature
more accurate.
T (
°C)
100 V
TEMPOUT
×
=
T (
°F)
T (
°C) 9
Ч
5
-------------------------
32
+
=