NI9213 with cRIO gives inaccurate values

[Lubos]

Hello,

I am developing an application to measure and log thermocouple data with NI 9213 in cRIO 9074 (with FPGA mode), LabVIEW 2011 and NI RIO 4.0 installed and here is a problem I ran into:

I put the end of thermocouple on a place with constant room temperature (around 21°C).

When I insert NI 9213 to the NI USB 9162 and run the attached .VI I get correct data from which I can compute correct temperature (works also for other temperatures). But when I use NI 9213 with cRIO the data are not the same and are wrong (the resulting error is on 21°C -0,5°C). 

For the cRIO I use example program "labview\examples\CompactRIO\Module Specific\NI 9213\NI 9213 Getting Started\NI 9213 Getting Started.lvproj".

With all given card settings.

Obtained data are:

with USB 9162: CJC temperature [°C] = 25,47 Thermocouple voltage [mV] =  -0,197

with cRIO: CJC data = 0,0250423 CJC computed temp [°C] =  25,33Thermocouple voltage[mV] = -0,215Autozero Channel [mV] = 0

And I also discovered, that with bigger absolute value of thermocouple voltage this error grows.

Does someone please have suspicion what can cause these error?

Thanks for any advice or opinion.

Best regards

Lubos

[okubik]

Hello Lubos,

based on my previous experiences it is better to use identical TC type to measure CJ reference value (for example on the other AI channel of your C module). You will approach better CJC at all. I also recommend you to set up polymorphic VI "Create DAQmx channel" to "AI / Voltage / Thermocouple". Because after that it is possible to set up parametres like thermocouple type (ie J, K, N, ...) and CJC value.

I also recommend you to go through thermocouple examples on NI developers zone, such as https://decibel.ni.com/content/docs/DOC-11724 or https://decibel.ni.com/content/docs/DOC-2016. There are also many usefull built-in examples in LabVIEW.

Ondrej