Convert Thermocouple Measurements Acquired With NI-DAQmx Into Voltage

Updated May 24, 2022

Environment

Hardware

  • NI-9213

Software

  • LabVIEW

There are some cases where we acquire thermocouple measurements using an NI module with the NI-DAQmx driver, but then we need to convert those values back to voltage after the acquisition. This is common when the task is programmed to measure a type of thermocouple that is not the same as the one connected.

For example, if a J-type thermocouple is configured, but a K-type thermocouple is connected, the values acquired in temperature units will not be correct, so going back to the voltage values may help to recover the data and convert it properly to temperature according to a K-type thermocouple. 

To calculate this manually, it is possible to use the Tables from the NIST ITS-90:Thermoelectric Voltages ​​​​​​to get the voltage values from the temperature readings. The goal is to pass from temperature measurements to millivolts (mV).
1. Find the temperature value in the table- For example, for 24°C you go to the respective range, in this case, 0° in the first column.
2. Then find in this row, which is the nearest value to your measurement, in this case, 25°C.


Therefore, for a temperature value of 24°C, the equivalent value is 1.277 mV according to this table and a determined thermocouple type.

Note: You can also use the Convert Thermocouple Reading VI in LabVIEW to achieve this conversion.