Thermocouples with the T8 (App Note)

This application note describes the steps necessary to take thermocouple measurements using a LabJack T8.

Getting Started, T8 & Type J Thermocouples

The following instructions will help you connect a Type J thermocouple to a LabJack T8 using Kipling. After that has been done, there are instructions for how to read the value of a Type J thermocouple and save the data to a file using LJLogM.

1. Go through the T8 Quickstart Guide.

2. Run our Kipling software and Open the T8.

3. Thermocouple Wiring.

Connect the positive wire of the thermocouple to the AIN0+ terminal on the T8 and the thermocouple negative wire to the AIN0- terminal.

4. Configure the Analog Input Channels in Kipling.

For thermocouple measurements on the T8, the only device configuration that you should need to set is the AIN range. The ±0.075V range setting is suitable for most thermocouple types as a catch-all configuration. We recommend using the lowest range setting that will work for your measurement. See our analog input documentation for related information.

To configure the AIN0 range in Kipling, do the following:

Navigate to the register matrix tab in Kipling.
Search for the register AIN0_RANGE and add it to the active registers by clicking on it.
Click on the "write mode" button for AIN0_RANGE in the active registers section. It looks like a small pencil icon under the VALUE column. Set the value to reflect the range setting you want to use. For example, write 0.075 for the ±0.075V range setting. Click the "write" button to apply your new range setting.
To save these settings so that when the device is powered-on, go to the Power-Up Defaults tab and follow the steps to save "Current Device Settings" as it's power-up defaults.

5. Open LJLogM.

Close Kipling and then open the device in LJLogM.

6. Configure a Thermocouple Measurement.

Ensure that AIN0 is in the Names column for row0. This will have LJLogM measure AIN0 in row 0.

7. Configure a CJC Measurement.

The temperature at the AIN0 terminal should be the thermocouple cold junction temperature, and should be found to do cold junction compensation. Replace the register name in the Names column of row1 with TEMPERATURE0_CAPTURE. This will set LJLogM to measure the temperature of the AIN0 terminal. See the Internal Temp Sensor section of the T-series datasheet for more information about the device temperature registers.

8. Apply Scaling.

The scaling equation column of LJLogM can be used to convert a thermocouple voltage to temperature. See the Scaling Equations description in the LJLogUD/LJLogM documentation. The equation you want for the AIN0 measurement in row0 is:
y=TCVoltsToTemp[J:a:b]                             // degrees K
y=TCVoltsToTemp[J:a:b]-273.15                 // degrees C
y=1.8*(TCVoltsToTemp[J:a:b])-459.67       // degrees F

... where J means Type J thermocouple, a is the raw value measured in row0 (the raw thermocouple voltage), and b is the the raw value measured in row1 (cold junction temperature in Kelvin). The TCVoltsToTemp function supports B, E, J, K, N, R, S and T type thermocouples.

Note that the scaling equation variables are a-p corresponding to row0-row15. For example, row4 corresponds to scaling variable e. The scaling equation described above should be adjusted accordingly if you are applying it to channels beyond row0 and row1.

9. Check the Thermocouple Temperature and Troubleshoot if necessary.

After finishing the steps above, you should see a good thermocouple temperature reported in the Scaled column of row0. If you do not see good measurements, see our thermocouple application note Troubleshooting Tips section.

10. Optional: Configure Additional Thermocouples.

Repeat steps 2-8 for any additional thermocouples you want to configure on other AIN channels, except we suggest replacing the regular AIN# register in LJLogM with AIN#_CAPTURE in order to take the new AIN measurement simultaneously with the AIN0 measurement. For example, set the LJLogM row2 Names column to AIN1_CAPTURE to get the raw thermocouple voltage of a thermocouple on AIN1. Also be sure to change # Channels in LJLogM to reflect the number of rows you want to take measurements from.

11. Log Data to File.

Once you are getting all thermocouple readings that you want in LJLogM, you can start logging data to file by clicking the small Write to File radio button near the Exit button in LJLogM. This will save data as a tab delimited ASCII file with a timestamp in the first column. See the LJLogUD & LJLogM Timestamps page for additional information.