Thermocouple measurements that after a period of time suddenly go bad

Question

Good Afternoon,
I originally posted this on the multifunction DAQ thread but haven't gotten any response. I don't think the problem is software but...
I'm making long term temperature measurements using a PCI6040E, SCXI1001, SCXI1102 32-channel thermocouple amp and a SCXI1303 isothermal block. These are making readings using J type t'couples. My customer's problem is that the measurements will be fine for a period of time (minutes, several hours) and then suddenly some channels will start giving really bogus temperatures, i.e.: -270 DegC vs the more realistic 35 DegC. I came in the last time it was doing this and when I went down into the code to my DAQ portion the DAQmx Read vi, set as analog, temperature, J type measurement was returning those numbers for the offending channels. Some were still giving valid numbers while others were giving the weird numbers and when a Fluke meter was used to measure the output of the "bad T'couple" it too was giving valid numbers. When I stop the LabVIEW program and go into DAQmx to try and troubleshoot it, there usually seems to be no problem. If I restart the LabVIEW program there also seems to be no problem, but when there is those numbers are coming out of the lowest level of code that I have programmed (the standard DAQmx read). I'm suspecting something "electromechanical" like connectors in/on the SCXI chassis, but the fact that it goes away when software is restarted is confusing me, as well as making troubleshooting difficult.
Thanks,
Putnam Monroe
Putnam
Certified LabVIEW Developer
Senior Test Engineer
Currently using LV 6.1-LabVIEW 2012, RT8.5
LabVIEW Champion

Answer 1

Like I said, you have a bear of a problem.
bad values at the lowest level I can access (in the LabVIEW program the actual DAQmx read, or running test panels in MAX).
That pretty much eliminates your LabVIEW code.
seem to remain bad until the SCXI-1001 chassis is reset
What exactly do you mean by "chassis is reset"? (I'm not a SCXI expert, but I do have a SCXI-1000 chassis here). There is no reset button. It's not hooked up at the moment, so I can't tell if there's a software "reset" function.
Do you mean power Off & On ?
Do you mean a reset button (maybe the 1001 has one, I don't know).
Do you mean a software reset from LabVIEW / Max ?
I don't see how any channel could hang up at 25°C and stay, that's not likely the top or bottom end of your TC range.
Possibly some sample/hold or track/hold amplifier quits sampling, and stays holding.
It IS about room temperature, I wonder if your CJC channel is somehow being read instead?
Ground loops would not likely be the problem, unless there's something else happening when the channels fail (like a welder firing). Even then, it's not likely that a rest would fix that.
My first thought would be to disassemble the SCXI stuff, and look for the usual suspects: dirty connectors, chips not in sockets, loose screw terminals, broken wires. Sounds like hardware to me.
Steve Bird
Culverson Software - Elegant software that is a pleasure to use.
Culverson.com
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