Chapter 2
Operating the NI 435x Device
2-8
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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 the Thermocouple
The NI 435
x
accessories—the TC-2190, TBX-68T, and CB-68T for the
NI 435
x
for PXI, PCI, and USB—are designed to be used with
thermocouples. Consult the accessory installation guide for instructions on
how to connect the 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.
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 435
x
is powered on and
±
17 VDC when the NI 435
x
is powered off.
The NI 435
x
analog inputs are protected against damage from voltages
within
±
42 VDC in all ranges when powered on and
±
17 VDC when the
NI 435
x
device is powered off. Never apply voltages above these levels to
the inputs.
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 435
x
accessory in the same scan
by choosing the 25 k
Ω
range for measuring the thermistor. These ranges
offer the best resolution, noise rejection, and accuracy.
Note
If scanning thermocouples and other transducers, the NI 453
x
device uses the widest
range for all channels.
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.