Problem with Signal Stability on certain computer

[Chris Rakus]

I'm using LabVIEW 8.5 along with an SC-2345 and a DAQCard-6036E to connect it to my computer.  The program simply reads the temperature from 4 type T thermocouple probes that are hooked up with a set of SCC-TC01 modules, displays the readings on a chart, and saves the points to file once every second. The method for this is a DAQ assistant containing all four Thermocouple channels.

The problem I'm encountering is that the Temperature readings seem to fluctuate over a range of 1-2°C when left to sit over a half hour or so.  It is a slow, but constant slope before reaching some minimum value and then starting back upward., This is making it impossible to calibrate the sensors to the sort of high accuracy I need for their application.  Ordinarily I would assume the proes are bad, but when I repeated the test on an older laptop with LabVIEW 7.1 using the same DAQ Assistant arrangement, the temperatures came out perfectly constant.  They fluctuated slightly, but within a much smaller and acceptable range of error.

The chiller keeping the water bath in which the probes are submerged is very accurte and only fluctuates about +/- 0.01°C.  The problem seems to lie somewhere between the components of the module and the newer laptop I am using.

The laptops both use Windows XP and were hooked up with the same DAQCard. The staility tests were performed on the same day with the only difference being which laptop was connected to the SC-2345.

Has anyone ever encountered this problem before?  And can anyone suggest what might be the actual heart of this issue?

Thanks for your time,

-Chris Rakus

Rowan University Student

[Corby_B]

Hello Chris,

Thanks for your post! I see you are using the thermocouples to a SCC-TC01 then a SC-2345 and finally to a DAQCard-6036E. It seems that other people have had similar issues with grounding on laptops. My initial thought is that there might be a grounding issue with the two different laptops. Check out this post below:

Daqcard 6036e affected by laptop power supply?

Does the same result happen no matter how the laptop is powered? Have you tried running the laptop from just the battery and not from a standard wall outlet? Take a look at this knowledge base that talks about measuring a differential voltage with a laptop. Some additional biasing resistors might be needed to help keep a steady input.

My Laptop is Reading a Floating Voltage from a Grounded Source in Differential Mode

Let me know if any of this helps you. Have a great day!

Corby_B
http://www.ni.com/support