Connecting The Thermocouple; Input Ranges; Optimizing Measurements - National Instruments NI 435 Series User Manual

Chapter 2 Operating the NI 435x Device

Connecting the Thermocouple

To prevent possible safety hazards, the maximum voltage between any of the
Caution
analog inputs and the computer ground should never exceed ±42 VDC when the NI 435x
is powered on and ±17 VDC when the NI 435x is powered off.

Input Ranges

Note If scanning thermocouples and other transducers, the NI 453x device uses the widest
range for all channels.

Optimizing Measurements

NI 435x User Manual
3. Measure the voltage, V
V , computed in step 2.
cold-junction
4. Convert the resulting voltage to temperature using a standard
thermocouple conversion formula.
The NI 435x accessories—the TC-2190, TBX-68T, and CB-68T for the
NI 435x 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.
The NI 435x analog inputs are protected against damage from voltages
within ±42 VDC in all ranges when powered on and ±17 VDC when the
NI 435x device is powered off. Never apply voltages above these levels to
the inputs.
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 435x accessory in the same scan
by choosing the 25 kΩ range for measuring the thermistor. These ranges
offer the best resolution, noise rejection, and accuracy.
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.
, and add the cold-junction voltage,
measured
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