NI-9214 Fails at above 700C. Has any one measured above this?

[samtbacon]

I hoping some one has some expertise in this.

I have a NI-9214 module mounted in a cRIO-9063 chassis and I am having an issue on being able to read temperature at elevated temperatures.  My issue is that the temperatures drop out to an over voltage reading at around 700-800 degrees C.  I am using a k type thermocouple on a ceramic sample on top of a ceramic heater.

  I have taken a look at this problem with an oscilloscope, and two non-national instrument k-type readers.  Both read the correct expected values based on the NIST chart for k-type thermocouples.  I then checked every connection on my system for a potential voltage leak.  I then tried a different cRIO-9063 and different NI-9214 module in case it was a faulty unit.  Same issue.

  I suspect similar to the other post I have read on people having a similar problem the module is having a grounding problem, but I have not seen any one who has solved this problem.  The fact that I can read the right voltages with other hardware has made me suspicious this problem is unique to the NI-9214 circuit design, but I have no idea how to fix it.

  Has anyone had success getting the NI-9214 to measure above ~700/~800 degree C, or is this the real upper limit of the hardware?  Any advise is appreciated!

Thanks! 

[Henrik_Volkers]

Some tests  using a scope with a  10:1 (10M) ti. Your scope migth tie it's Ch-/shield to earth ground (PE)

while @ hi temp:

measure earth ground to COM voltage

measure Channel to COM max voltage(1.2V!): measure voltage COM to Ch+ and Ch- . this might tie COM to earth GND (scope), so while doing it monitor the 9214 reading.

[samtbacon · Accepted solution]

Hey Henrik,

  I apologize for not responding for months.  But in the event some one else is looking for solutions to the same problem here is what I ended up doing:

  Checking the grounds, while an excellent suggestion, did not illuminate a potential solution.  What I figured out though was the issue was most likely noise.  It would be helpful is the 9214 made it easier to distinguish the noise rejection case.  It appears as though the meter I was using had such a slow time constant it was averaging out the noise.

  This lead me to the idea of "what if I did the same thing to my system?".  To do so, I mounted a capacitors in the terminals for the thermocouple inputs.  The electrical "smoothing" effect of the capacitor let me to take measurements at higher temperatures.