BNC-2120 with T-type thermocouple can't measure negative temperatures

[sbuxton]

I have a BNC-2120 attached to a PCI-6221.  I am trying to measure temperatures from appox. +35 C to -40 C using a T-type thermocouple.  When i set up the channel and try to set the minimum value to anything less than zero I get an error that tells me the absolute minimum is zero.  Is there something I need to set up differently or is this a hardware/software issue?

I have the built-in CJC compensation activated and everything seems to work as long as the temperature is above zero.  If the temperature goes negative, the output pegs at zero.

Any ideas?

Sean

[sbuxton]

Never mind, I solved the problem.  Turns out you can't change max/min values once you've enabled a channel.  Deleting and reenabling the channel with different values solved the problem

[Sagarmatha]

can you give me your example vi? I am using bnc-2120 and 6221 board, and can not seem to read the temperature. I am using J type TC. Thanks.

[samantham]

Hello,

 I don’t have this code, but I can direct you to the LabView Example Finder under the Help menu.  In the Browse tab of the Example Finder open the folders <Hardware Input and Output> <DAQmx> <Analog Measurements> <Temperature>.  These are great example that are meant to be modified to your specific application.

[sbuxton]

I'm assuming that you are new to LabView so this might seem a little basic.

Check the switches on the BNC-2120.  There are two that you need to use for a thermocouple.  One selects the thermocouple connector or the BNC connector for AI1.  The other selects the Temperature Reference or BNC connector for AI0.  You will also have to set up the BNC-2120 in Measurement and Automation Explorer.

1. Open Measurement and Automation Explorer.
2. On the left hand panel select Devices and Interfaces/NI-DAQmx Devices/NI PCI-6221. 
3. Then, on the right hand panel select Properties at the top of the screen.  In the window that pops up select the Accessories tab. 
4. Select BNC-2120 from the drop-down menu and then click the Configure button. 
5. Check the Temperature Reference Enabled box and click OK to close the window.
6. Click OK to close the Device Properties window.
7. Close Measurement and Automation Explorer.

Now in a blank VI on the block diagram, drop a DAQ Assistant Input VI.
1. In the window that opens, select Analog Input/Temperature/Thermocouple.
2. Select ai1 and click Finish.
3. Enter your Maximum and Minimum Teperature Ranges and select degrees C or F.
4. In the Thermocouple Type drop-down menu select  J.
5. In the CJC Source drop-down menu select Built In.
6 .Enter your timing and Acquisition parameters and click OK

Add a display and you are ready to go.

Hope this helps

[Sagarmatha]

Thanks! It is working.  -

[AnalogKid2DigitalMan]

You are now 'Proven'