Thermocouple scaling incorrect in Veristand?

Question

Hello, I am using the PXI6289 to control an SCXI1102 AI card with the SCXI1303 terminal block.
VeriStand's channel scaling appears to be incorrect for at least K type thermocouples. When I provide a controlled 0.1 mV voltage, I expect to get about 25C reading. However, the scaled value ends up being 480-520C instead (and oscillates wildly as if there is a large gain on some small voltages).
I have checked the hardware by using MAX to test the devices and read the temperatures, and the results through this way is fine. I also get close to the right temperature by entering the K type polynomial's first few coefficients.
Also, on a related note, I am not sure how to point to the cold junction compensation temperature signal. When I choose "IC" which I assume means "internally compensated," it asks me for a channel, but the CJC temp channel does not show up in the channel list. The CJC temp channel does show up in MAX.
Has anyone verified the thermocouple scaling in Veristand? How do I access the CJC temp channel through VeriStand. Thank you.

Answer 1

Hi Harris,
I assume that you are using NI VeriStand 2009, right?
There is no way in System Explorer (SE) to
configure other types than voltage or current. In your case, there are
basically two workarounds to go with.
Option 1 is the more
accurate one, whereas the effort to get to it is higher than option 2,
which is just an approximation.
Therefore, you might want to not bother
reading option 1 but go directly with option 2.
If you still want
to get a fairly accurate value for option 2, you could only read the
internal channel, calculate the temperature and set up a user channel
like I describe it under option 2.
Option 1:
In
the SE add an 'Internal Channels' to your cDAQ. In case of the 9211 the
internal channel that we need is called '_cjtemp'. Once you have
created the internal channel for your cDAQ1Mod1 device, check out in the
pull-down list if there is a channel that is called _cjtemp, if not,
just start typing a new value in the pull-down list. The returned CJC
value is in volt, which means that you have to convert it to a
temperature. In order to get the convertion use a calculated channel.
The following VI will help you out to get the formular right:
<LabVIEW>\examples\CompactRIO\Module Specific\NI 9211\NI 9211
Support Files.llb\NI 9211 Convert to Temperature (Calibrated).vi. This
VI is used by FPGA though, which means you gotta strip out the binary
conversion. At the end the result should be the same: Temperature in
Celsius. The last step of this calculation is to take this result and
divide it
by 100
Option 2:
Create a user channel and set its
value to a general ambient temperature divided by 100.
In
both options, the final value of C needs to be divided by 100 which is a
specific step that is required in order to use the 'Channel Scaling and
Calibration' tool (this tool is acessible through the Workspace). In
the Channel Scaling and Calibration tool, pick the AI channels of the
9211 module (right now, it is only possible to pick one channel at a
time though). On the 2nd page you may fill out the cal information if
you feel like, if not just proceed on to the next page, change the scale
type to 'Thermocouple' and click next. Specify your thermocouple type
and the units. Set the CJC type to IC sensor and pick as CJC sensor
channel the SE channel that contains the final result (value in celsius
divided by 100).
I know, all these steps sound
ridiculous for just measuring temperature. We are looking into it to
improve the workflow asap though.
Thanks,
Tom
http://www.newgistics.com