thermocouple measurements using USB-6259

[RafalM]

Hi everyone, 

I need to take thermocouple temperature measurements using USB - 6259. 

LabView version I use is 8.5.1 but 8.6 is available to install if really needed. 

My system:
NI USB-6259 High Speed Multifunction DAQ
LabView 8.5.1 (but 8.6 is available if needed)
Thermocouple Type K: I have two wires that I make the thermocouples of. (Chromel and Alumel).

I tried soldering and spark welding them - no difference.

What I want to do:
I want to take temperature measurements with as high resolution as possible. (ranging from room temperature to, say, 100*C) 

I need to see the temperature measurement results on a graph. I am particularly interested in the slope of rising of the temperature.

What I do:
I make the thermocouple type K
I prepare channels ai0 and ai1 by inserting 33k ohm resistors between the ai0 low and ai0 ground and between ai0 high and ai0 ground; same for ai1. (2 resistors for each channel)
I attach the thermocouple to high and low of the channel.
I attached one thermocouple to channel ai0 and one to channel ai1. One in channel ai0 I want to use as Cold Junction Compensation (CJC)  The other as the temperature sensor.
Now in Measurement and Automation software I create virtual channels for CJC and for temperature measurement.

For the temperature measurement channel I choose the CJC channel in the option for cold junction compensation. 

What I've tried:

I have tested thermocouples using a nano volt meter and an ice bath. They seem to be showing the right temperatures.

I have  tried DAQ assistant to build a VI for this measurement. It does not seem to be working right. The results are jumping around as seen in the figure attached.

I tried attaching a commercially made K type thermocouple to the same set up and the results were the same as with my thermocouples which indicates something is wrong with the setup. Or maybe this is the right result and I am just not handling it right. 

I do not know what to do next. 

I have been asking and searching for a while now. My first post concerning this you can see here.

Thanks for help

[mickeyw]

Hi,

If there is fluctuation with the measurement it is generally caused by a Cold Junction Compensation issue. If you look at the thermocouple examples in LabVIEW (Help>>Find Examples) you should be able to use  If it is not built in then an approximate figure of room temperature as the CJC Constant should give enough accuracy.

Thanks,