SCXI-1121 - RTD 3 deg F diff between cards

[wantacobra]

Background:

I have installed 2 new SCXI-1121 cards in a SCXI-1001 chassis.  All channels are configured to measure temp using RTDs.  The cards are configured identically (Gain 10x/10x, filter 4 Hz/4Hz, Excitation current mode = 0.15mA).  Using MAX or LabView, the temp readings differ by 3 deg F at ambient (70 deg F) between the two cards.  I have placed a precision 100 ohm resistor on the TBX-1328 terminal block and connected it to the 1121 and confirmed that the temp difference between cards is 2+ deg F.  One card reads 31.7 deg F, while the other reads 29.2 deg F.  All 4 channels have about the same offset.  I have swapped the cards in the chassis and the temp diff moves with the card. 

Questions: 

Do I have a calibration issue?  Could I have a conflict between the physical jumpers on the card and setup in MAX?  What is the recommended gain/filter/current for this application? The calibration procedure addresses each channel separately.  I have checked the excitation current and it appears to be correct at 0.15mA using a handheld meter.  It reads between 0.14 and 0.15 mA.  In order to perform a calibration, it appears that the 1121 card needs to be extended in the chassis.  I don't find a SCXI card extender on the web.  Do you have a part number for this item?

[David L.]

Hi Wantacobra,

Great job with the debugging! Now that we know the issue follows one 1121 module, does this difference happen on all channels of the 1121? If not, than I would suggest switching around RTD’s as you may have a bad one. If it does happen on all channels, calibration is a good idea. Please see this knowledgebase: Compensating for the Offset on an SCXI-1121. It has links for self calibration and Calibration Solutions from National Instruments.

Message Edited by David L. on 08-24-2007 12:12 PM

[wantacobra]

Yes all four channels have about the same offset.  I just found the article on compensating for the offset on an SCXI-1121 and will be trying the second method, using the EEPROM on the 1121.  Thanks for the prompt response and help David.