Converting NI 9213 Data (FPGA Interface)

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.

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.

 Open example

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:

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

Calculate the CJC temperature:

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

where	T = temperature in °C
	A = 1.2873851 × 10-3
	B = 2.3575235 × 10-4
	C = 9.4978060 × 10-8

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