Multiple TC-2095s, but only 1 Reference junction being used

Question

In my LabVIEW program, I create the DAQmx task programaticly just before starting data acquisition. I use SCXI 1102s with TC-2095s, with multiple sets in a given test stand. I add each used channel to the DAQmx task individually using the DAQmx Create Virtual Channel vi. I added a new 1102 to one of my systems, but a TC-2095 was not handy. I added the new channels to the scan list. When I ran the system, all channels, those in the old module/interface set as well as the new  incomplete set, showed  temperatures too high by an amount that would be expected by an open reference junction channel. When a TC-2095 was connected, the values changed to normal.
This seems to indicate that the system is using only the last junction reference for sensing the junction temperature. This is not optimal. While the TC-2095s are stacked in the rack, and should be close in temperature, there could be several degrees error due to temperature differentials between the boxes.
Is this due to the way I create the task? Is there a way around it.

Answer 1

Hi David,
There a couple of things that are happening here that could cause the unexpected result. If you're new channels in the 1102 are added to the scan list, but are not actually connected (they are floating), you can expect to see incorrect voltages. Also, since you noticed a change back to an expected value when you added the 2095, there could have been an impedence matching problem.
Try removing the new channels from your scan list and adding the 1102 without the 2095 again to see if the voltages are correct. Please post back if you have any questions. Have a great day!
Ryan D.
District Sales Manager for Boston & Northern New England
National Instruments