Chapter 3
NI 4350 Operation
3-8
©
National Instruments Corporation
3.
Measure the voltage, V
measured
, and add the cold-junction
voltage, V
cold-junction
, computed in step 2.
4.
Convert the resulting voltage to temperature using a standard
thermocouple conversion formula.
Connecting Your Thermocouple
The NI 4350 accessories—the PSH32-TC6 and the CB-27T for the
NI 4350 (PCMCIA), and the TC-2190 and the TBX-68T for the
NI 4350 (ISA) and the NI 4350 (USB)—are designed to be used with
thermocouples. Consult your accessory installation guide for
instructions on how to connect your thermocouples. To make accurate
measurements, make sure that the common-mode voltage of the
thermocouple is within the input common mode limits of the selected
input range.
The NI 4350 instrument analog inputs are protected against damage
from voltages within ±42 VDC in all ranges when powered up and
±17 VDC when the NI 4350 instrument is powered down. You should
never apply voltages above these levels to the inputs.
Caution:
To prevent possible safety hazards, the maximum voltage between any of
the analog inputs and the computer ground should never exceed ±42 VDC
when the NI 4350 instrument is powered up and ±17 VDC when the
NI 4350 instrument is powered down.
Input Ranges
Choose the
±
625 mV range in volts mode when you are measuring
thermocouples. You can measure both the thermocouples and the
thermistor cold-junction sensor on the NI 4350 accessory in the same
scan by choosing the 25 k
Ω
range for measuring the thermistor. This
range offers the best resolution, noise rejection, and accuracy.
Optimizing Measurements
To make accurate thermocouple measurements, set the onboard
programmable ground-referencing and open-thermocouple detection
appropriately. Also consider problems associated with AC noise
effects, thermal EMF, and other errors as discussed in the following
sections.
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