Connect Thermocouples to a Data Acquisition (DAQ) Device
Included in the Section
This document provides step-by-step instructions for wiring and configuring your NI data acquisition device for use with a thermocouple. Before you begin using your NI data acquisition hardware, you must install your application development environment (ADE) and NI-DAQmx driver software. Refer to the Installing NI LabVIEW and NI-DAQmx document for more information.
Thermocouples are the most commonly used temperature sensors. A thermocouple is created when two dissimilar metals touch, producing a small open-circuit voltage that corresponds to temperature. This thermoelectric voltage is known as Seebeck voltage and is nonlinear with respect to temperature.
Thermocouples differ in composition and accurate range:
Table 1. Thermocouple Types
Thermocouples require some form of temperature reference to compensate for unwanted parasitic thermocouples. A parasitic thermocouple is created when you connect a thermocouple to your measurement hardware. Because the terminals on the hardware are made of a different material than the thermocouple wire, voltage is created at the junctions, called cold junctions, which changes the voltage output by the actual thermocouple. You can measure the temperature at this reference junction with a direct-reading temperature sensor, such as a thermistor or an IC sensor, and then subtract the parasitic thermocouple thermoelectric contributions. This process is called cold-junction compensation. You must specify your CJC source or a constant value (typically 25°C) when you configure your thermocouple measurement in software.
Before connecting any signals, locate your device pinout.
Figure 1. Device Terminals Help
You can use NI Measurement & Automation Explorer (MAX) to quickly verify the accuracy of your measurement system setup. Using an NI-DAQmx Global Virtual Channel you can configure a thermocouple measurement without any programming. A virtual channel is a concept of the NI-DAQmx driver architecture used to represent a collection of device property settings that can include a name, a physical channel, input terminal connections, the type of measurement or generation, and scaling information.Follow these steps to begin:
Figure 2. Creating an NI-DAQmx Virtual Channel
Figure 3. Device Physical Channels
Figure 4. Setting up a Thermocouple Channel in MAX
The next step is to physically connect the thermocouple to your DAQ device.
Figure 5. Thermocouple Connection Diagram
Each thermocouple wire has a positive lead and a negative lead. The connection diagram indicates which pins on your DAQ device should be wired according to the physical channel you selected. Connect the positive lead of the thermocouple to the TC+ terminal and the negative lead of the thermocouple to the TC– terminal. If you are unsure which of the thermocouple leads is positive and which is negative, check the thermocouple documentation or the thermocouple wire spool.
If you are using a shielded thermocouple, connect the COM terminal of your device to the shield and the shield to a common-mode voltage reference of the thermocouple. A common-mode voltage reference is a voltage that is within ±1.2 V of the common-mode voltage of the thermocouple. If you are using a floating thermocouple or a thermocouple within ±1.2 V of earth ground, connect COM and the shield to earth ground. The shield grounding methodology can vary depending on the application. Refer to Figure 6 for an illustration of a typical shielding configuration.
Figure 6. Connecting a Shielded Thermocouple
NI-DAQmx global virtual channels allow you to preview your measurements.
Figure 7. Previewing a Thermocouple Measurement in MAX
You can choose to view the signal in tabular form or as a graph by selecting Graph from the Display Type dropdown. You also have the option of saving your NI-DAQmx Global Virtual Channel should you wish to refer to this configuration screen again in the future.