Converting NI 9213 Data (FPGA Interface)

CompactRIO Reference and Procedures (FPGA Interface)

Edition Date: June 2010

Part Number: 370984T-01

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Set the Calibration Mode to Calibrated in the C Series Module Properties dialog box for the NI 9213 if you want the FPGA I/O Node to return calibrated, fixed-point data from the module in units of volts. If you set the Calibration Mode to Calibrated, you must convert the CJC data from voltage to temperature. Set the Calibration Mode to Raw if you want the FPGA I/O Node to return calibrated, binary data from the module. If you set the Calibration Mode to Raw, you must convert the binary thermocouple and CJC values to voltage and then convert the CJC data from voltage to temperature. You must convert these values in the host VI.

Note Note  The equations below apply only to the NI 9213. If you are using the NI 9213E board-only module, refer to the KnowledgeBase for the correct equations.

Using a VI to Convert Data to Temperature

Refer to the NI 9213 Convert to Temperature polymorphic VI in the labview\examples\CompactRIO\Module Specific\NI 9213\NI 9213 Getting Started\NI 9213 Getting Started.lvproj for an example of converting calibrated or raw data to temperature. You can use the NI 9213 Convert to Temperature VI as a subVI in the host VI.

Using an Equation to Convert Binary Values to Voltage

You can use the following equation in the host VI to convert the binary thermocouple and CJC values to voltage:

Voltage = Binary Value × 78.125 mV ÷ 8,388,607

where Binary Value is the value returned by the FPGA I/O Node.

Using Equations to Convert CJC Data from Voltage to Temperature

You can use the following equations in the host VI to convert CJC data from volts to temperature:

Calculate the resistance of the thermistor:

RT = (10000 × CJC Data × 32) ÷ (2.5 – CJC Data × 32)

Calculate the CJC temperature:

T = [ 1 ÷ [A + B(ln(RT)) + C(ln(RT))3]] – (273.15 + OffsetConstant)

where T = temperature in °C
A = 1.2873851 × 10-3
B = 2.3575235 × 10-4
C = 9.4978060 × 10-8
RT = thermistor resistance reading
OffsetConstant* = 1

*The OffsetConstant is the typical temperature gradient between the CJC sensor and the thermocouple cold junction.

Refer to the National Institute of Standards and Technology (NIST) Monograph 175 thermocouple reference tables for more information about converting and adjusting thermocouple values.

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