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How Do I Use the Cold Junction Compensation on the SCB-68?

Primary Software: Utility Software>>Measurement & Automation Explorer
Primary Software Version: 3.1
Primary Software Fixed Version: N/A
Secondary Software: N/A
Hardware: Counter/Timers (TIO)>>Connectivity>>SCB-68

Problem:
How do I use the cold junction compensation (CJC) on the SCB-68 to perform a differential measurement?

Solution:

In Hardware:
  1. Configure the switches on the SCB-68 as shown below. This enables the temperature sensor on differential channel 0, making both AI 0 and AI 8 unavailable for other connections. To make this information easily accessible, tape the correct quick reference label for your device (Related Links) to the SCB-68. This label also shows how to configure the switches to either enable the temperature sensor on single-ended channel 0 or disable the temperature sensor.
  2. Input your temperature sensor into ai1 and ai9.


In Software:
  1. Open Measurement and Automation Explorer.
  2. Expand Devices and Interfaces NI-DAQmx Devices.
  3. Right click on your device and select Properties.
  4. Click on the Accessory tab.
  5. Select the SCB-68 as your Accessory and select Configure.
  6. Check the box for Temperature Reference Enabled.
  7. Click the OK buttons to save the changes.

  8. Expand Data Neighborhood.
  9. Right click on NI-DAQmx Tasks and select Create New NI-DAQmx Task.
  10. Create a task for the correct temperature sensor.
  11. Select Built In as the CJC Source.
  12. Save the task and test it.



Notes:
  • These steps will also work with M Series boards with two connectors. Just add two accessories instead of one and input the signal into ai17 and ai25 instead of ai1 and ai9.
  • If you configure the switches on the SCB-68 to enable the temperature sensor on single-ended channel 0 (AI 0) rather than differential channel 0 (AI 0 and AI 8), you cannot use a DAQmx temperature task. Instead, you will have to use an analog input task to read both AI 0 (the CJC) and your temperature channel, and then scale the raw voltage in software. The example program “Manually Scaling Thermocouple Measurements in DAQmx” linked below provides a framework for how to do this in LabVIEW. This procedure is only necessary if you need to use every analog input channel on your DAQ board and cannot afford to use two channels to read from the CJC in differential mode.
  • This steps will not work for the mass termination USB-6251, this is a know issue already in their readme.html files for daqmx versions.




Related Links:
KnowledgeBase 3O0DOQRM: Where can I Find SCB-68/SCB-100 Quick Reference Labels for Various Cards?
KnowledgeBase 11SCAVHL: Thermocouples, the SCB-68, and NI-DAQ Virtual Channels/NI-DAQmx Tasks
KnowledgeBase 2MO86AMQ: How Can I Read the Value of the Cold Junction Compensation of the SCB-68 (SCB-100)?
Developer Zone Example: Manually Scaling Thermocouple Measurements in DAQmx
Product Manuals: SCB-68 68-Pin Shielded Connector Block User Manual

Attachments:


switches.gif - switches.gif
accessory.gif - accessory.giftask.gif - task.gif

Report Date: 08/26/2005
Last Updated: 10/26/2007
Document ID: 3OPCLPBP

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