05-12-2011 12:17 PM
Hi Everyone,
Once again I need a little guidance from the community. I am working on a new apparatus and am using a cRIO-9074 chassis with a 9211 TC module. I am attempting to connect a mini IR thermocouple (link below, part number OS36SM-K-140F) for one of my measurements. The part of my apparatus I am trying to measure I cannot make physical contact with. Supposedly this IR TC reads out as a K type thermocouple, it works great on my Fluke 52II TC reader. When I attach it to the 9211 I get a much higher reading that is accurate, something around 60C when the regular TC I am comparing it to reads 24-25C. I have tried playing with the on board scale factors but can't get an accurate reading. The IR TC should be able to read between 20-90C accurately. I noticed on the data sheet that it has a high impedance that may interfere with some readers. Would this affect the 9211? Any help or guidance here would be very much appreciated.
http://www.omega.com/ppt/pptsc.asp?ref=OS36SM&ttID=OS36SM&Nav=
Regards,
PChemist
Solved! Go to Solution.
05-12-2011 12:28 PM
The impedance is probably the issue. Look at pages 11 and 20 of the NI9211 Operating Instructions and Specifications.
From the specs on page 20:
"Gain error from
source impedance..............................Add 0.05 ppm per ohm when
source impedance >50 ohms"
Lynn
05-12-2011 01:16 PM
I thought that was the issue, just wanted to confirm it. Thank you for the response.
Regards,
PChemist
05-12-2011 06:09 PM
I have measured the impedance of my IR TC, it is 20.4kOhm. According to the 9211 manual this will result in an error of 1.02mV in the measured signal. Unfortunately I can't figure out how to input this into the channel scaling offered by the 9211. I have searched the help files and can't find and hints. Does anyone have any?
Regards,
PChemist
05-13-2011 09:40 PM
Hi Everyone,
I have manage to work out a solution for the problem I described above. It seems that a C-Module on a cRIO has a problem adding a scale since the DAQ Assistant doesn't work. My solution is in the picture below. I compared it to a calibrated IR thermometer and it is better than the std dev of a typical K type thermocouple.
Regards,
PChemist