Difficulty with CJC using on board temp sensor - SCB-68

[TinkerBell]

I am trying to do Cold Junction Compensation using the built in temperature sensor on my SCB-68 connector block.  I am using the 6224 M Series PCI DAQ card.  I am having two problems:

1.  I found a procedure on this forum for reading the temperature sensor which, in my case, is at channel 16.  So, I have my connector block switches set so the Temp sensor is enabled, and I am in differential mode.  I created a global channel called "TempSensor" and set it to channel 16.  I applied a scaling of m = 180 and b = 32 to get farenheit.  When I do this, and I test the channel, it reads a temperature of 75 degrees.  This seems a little hot for my office, which is probably more like 68 degrees.  Any idea what might be wrong?  I don't have to connect anything to the board to read the temp sensor, do I?

2.  I've connected a type T thermocouple to channels 20 and 28 (differential).  For CJC, I can set it manually by estimating the temperature in my office as 68 degrees and puting that value in the CJC Source Constant field.  When I do this, the TC seems to work great.  It reads around 68 degrees in my office, and more like 85 if I squeeze it between my hands.  So, now I want to use the "TempSensor" global channel to do my cold junction compensation automatically.  When I select my CJC Source as "Channel" and enter "TempSensor," I run the test and get temperatures of 176 degees F.  What am I doing wrong?

Thanks for any help!

Kevin

[Marni]

Dear Kevin,

There is a really easy way to test your thermocouple using the Built-In CJC of the SCB-68. The only time I set up a global channel for the CJC is to make sure that it is working correctly. The CJC temp does not indicate the temperature of your office, it indicates the temperature of the SCB-68 board. My CJC value is 2 degrees warmer than the room temperature. 75 degrees is a reasonable temperature to get.

After you check the CJC to make sure that it is working, you can set up the task in the Measurement and Automation Explorer (MAX). Expand Devices and Interfaces>>NI-DAQmx Devices. Right click on your device and select Properties. Click on the Accessory Tab and add the SCB-68 to the second connector and click Configure. Check the box for Reference Temperature Enabled. Click the two OKs to save your changes. Then create your thermocouple task normally and select your CJC source as Built-In.

Now, it will detect temperature changes that you add. Please let me know if you have any questions. Have a great day!

Sincerely,

Marni S.

[TinkerBell]

That works great and is much easier!  Thank you.