Using a Thermocouple with the T8

Overview

The T8 excels at thermocouple measurements with its 24-bit analog to digital converters (ADCs), built-in amplification, and channel-to-channel isolation. The 24-bit sigma-delta ADCs also filter 50/60 Hz noise, which is a common problem in thermocouple applications. A single T8 can measure up to 8 thermocouples.

Resolution

The voltage range for a type K thermocouple is -10.8 mV at -270 °C to +54.89 mV at 1372 °C. This equates to an output of roughly 40 µV/°C. The corresponding temperature resolution when using a T8 is described in Table 1.

Table 1: Approximate Type K Thermocouple Measurement Resolution Using the T8.

In-depth Breakdown: From Appendix A-3-3-2 of the T8 User's Guide, the typical device resolution at resolution index 8 and ±0.15V range is ≈12 μV noise-free and 2.0 μV effective.

12 µV / 40 (µV/°C) = 0.3 °C noise-free from a type K thermocouple.

At resolution index 16 the device resolution is ≈3.0 μV noise-free and 0.5 μV effective. 

Most temperature samples (1 standard deviation) are expected to fall within the effective resolution value.

Accuracy

Table 2: Approximate Type K Thermocouple Measurement Accuracy Using the T8.

In-depth Breakdown: From Appendix A-3-3-1 of the T8 User's Guide, the T8 is calibrated to an absolute accuracy of ±19 µV at the ±0.075 V range. This value corresponds to around ±0.475 °C for a type K thermocouple (19 µV / 40 µV/°C).

Higher accuracy can be achieved by using a higher T8 range setting. Note that this could limit the measurable temperature range. The ±0.036 V range setting has an accuracy of ±13.3 µV. This value corresponds to around ±0.333 °C for a type K thermocouple (13.3 µV / 40 µV/°C).

Tutorial: T8 and Type K Thermocouples in Kipling

Follow these instructions to connect a Type K thermocouple to a LabJack T8 and acquire readings using Kipling. The “Going Further” section provides guidance for other software options.

1. Go Through the T8 Quickstart Tutorial.

2. Connect the Thermocouple to an Analog Input.

This tutorial will use AIN0 on the T8. Connect the thermocouple as shown below:

• Thermocouple+ to AIN0+

• Thermocouple- to AIN0-

3. Run Kiplingand Connect to the T8.

Connect the T8 to your computer via USB, then click Refresh Devices in Kipling. Click the Green USB button to open the T8.

4. Configure AIN0 in Kipling.

The AIN can be configured in Kipling by using the Analog Inputs tab wizard, or by manually writing device registers using the Register Matrix tab.

Wizard Configuration (Easiest Method)

The simplest way to configure AIN is using the wizard in Kipling. Go to the Analog Inputs tab, press the + button for AIN0, and set Extended Feature (EF) to TypeK Thermocouple. Set Range to -0.075 to 0.075 V.

Register Configuration (Alternate Method)

This method may be useful if you eventually intend to do the AIN configuration programmatically.

1. Navigate to the Register Matrix tab in Kipling.

2. Add AIN0_RANGE and AIN0_EF_INDEX to the active registers list. To do so, search for each register using the search bar. When the register appears in the search results, click on the check mark to the left of the register name to add it to the active register list.

3. Set AIN0_RANGE to 0.075 to use the ±0.075 V range setting and set AIN0_EF_INDEX to 22 to configure the type K thermocouple measurement feature (AIN_EF). To do so, click on the "write mode" button for both AIN0_RANGE and AIN0_EF_INDEX in the active registers section, update each register value, then click the “write” button for each register. The “write mode” button looks like a small pencil icon under the VALUE column.

5. Read the Thermocouple Measurement.

Thermocouple measurements can be read in Kipling by using the Analog Inputs tab wizard, or by reading registers in the Register Matrix tab.

Wizard Method

You should see the thermocouple temperature reading just below the AIN0 voltage in the Kipling Analog Inputs tab. As an example, Figure 3 above shows an AIN0 voltage of 0.001185 V and thermocouple temperature of 300.26 K. This measurement was taken at room temperature on a hot summer day, and the value was as expected.

Register Matrix Method

Add AIN0, AIN0_EF_READ_A, and AIN0_EF_READ_C to the active registers list in the Register Matrix tab of Kipling. These return the raw thermocouple voltage, the thermocouple temperature, and the CJC (cold junction compensation) temperature, respectively.

Figure 5 below shows example measurements that were taken at air temperature on a hot (high of 312 K) summer day. The temperature units are Kelvin (AIN0_EF_CONFIG_A set to the default value of 0). All readings match the expected results.

6. Troubleshooting Readings.

If you are not sure what temperature to expect or your reading are not consistent with what you expect, see the tests and other guidance provided in the “Wrong Temperature” section of the Thermocouple Troubleshooting page.

7. [Optional] Configure Additional Thermocouples in Kipling.

Repeat steps 2-6 for any additional thermocouples you want to configure on other AIN channels.

8. [Optional] Configure Settings as Power-Up Defaults.

Most AIN settings such as the input ranges and thermocouple measurement features can be saved so that they are automatically applied when the T8 is powered on. To configure power-up defaults using Kipling, go to the Power-Up Defaults tab and follow the instructions on the page to save "Current Device Settings" as the power-up defaults. See Device Configurations in the T-Series Datasheet for more information.

Going Further

The T8 is compatible with multiple thermocouple types including B, C, E, J, K, N, R, T, and S. For more information, see the Thermocouple AIN_EF section of the T-Series Datasheet. If another thermocouple type is required for your application let us know.

The thermocouple AIN feature (AIN_EF) can be configured programmatically by reading and writing AIN_EF registers, similar to what is described in the “Register Matrix Configuration” sections of the tutorial above.

Software Options

There are four main software options to acquire thermocouple measurements:

1. LJLogM: LJLogM is the simplest logging solution. See the “Log Data with LJLogM” section below.

2. Custom LJM Software: LJM is recommended if you need to create custom software. Programming examples are available on the LJM Example Code page. Registers such as AIN0_EF_INDEX and AIN0_EF_READ_A can be written and read using the LJM Single Value Functions or Multiple Value Functions.

3. DAQFactory: DAQFactory is more customizable than LJLogM. It may require a small amount of programming, but much less than LJM or Modbus TCP programs. See "Device Configuration" and "Linking LabJack Inputs/Outputs to DAQFactory Channels" sections on the DAQFactory for LJM page.

4. Direct Modbus TCP Communications: LabJack recommends using LJM over direct Modbus TCP communications whenever possible. See the Direct Modbus TCP Getting Started guide.

See the T-Series Software Options page for a full list of software options.

Thermocouples in Stream Mode

It is possible to read thermocouples while using stream mode acquisition, but the configuration is more advanced and it should be avoided whenever possible. The AIN extended features (AIN_EF) are not supported while streaming.

• Typically the AIN# and TEMPERATURE# registers are streamed to capture the thermocouple voltage and CJC temperature, respectively.

• The measured thermocouple voltage and CJC temperature can be converted to thermocouple (remote end) temperature using the LJM TCVoltsToTemp function.

• The T8 sub-stream functionality may be required to sample the TEMPERATURE# registers at the requisite <250 Hz sample rate.

Log Data with LJLogM

LJLogM is a simple solution to log thermocouple measurements. Close Kipling and then open the T8 in LJLogM.

Thermocouple Readings in LJLogM

Set the value of the row0 Names column to AIN0_EF_READ_A. This will configure LJLogM to measure the AIN0 thermocouple temperature in row0, reported in the Value column. Repeat this step for however many thermocouples you configured in Kipling. Also be sure to change the # Channels control to reflect the number of channels you want to read in LJLogM. For example, if you set up a second thermocouple on AIN1 you could set the value of row1 Names column to AIN1_EF_READ_A to read the thermocouple temperature in row1. Set # Channels to a value >=2 to read at least the first two channels/rows.

Save LJLogM Data to File

Once you see good thermocouple readings in LJLogM, you can start logging data to file by clicking the small Write to File radio button. 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.