open thermocouple detection on the 1120

[GlennXS]

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@Neal M wrote:
Hello GlennXS,

All of the open thermocouple features I know of in NI hardware is taken care of in the driver.  In addition to this, the hardware needs to support this functionality as well.  The previous discussion should work well, but I do not know of a library that does this for you.  All it really is is just 2 separate tasks (a resistance measurement and then a thermocouple measurement).  You should be able to program this yourself using the DAQmx calls relatively easily.  I would suggest looking at the example programs (C:\Documents and Settings\All Users\Documents\National Instruments\NI-DAQ\Examples).

If you give me more details about the hardware you have and what you have tried, then I can try and help you out more.

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Thanks for the advice Neal!

I'm using a USB-9211. I didn't dig too deep into the task abilities (as of yet), however, i do know that my app' reads TC data from those respective channels and it knows no difference as to whether or not the TC is plugged in; i.e. it posts meaningful data to the User Interface or else ~66,656 values (and it just keeps on posting). I searched the niMS Books Online (and the forum) for Open TC/channel detection (and similar phrases) and only founds hits on the forum. The forum hits didn't mention anything about any C#/CLR method/property that serve that purpose. So what i did was code a condition trap that tests for values that exceed a corresponding threshold (e.g. x < -66,000). So are u saying that there is such a C#/CLR method/property that will serve this purpose?

Thanks,
/Glenn

[Neal M]

Hello again Glen,

The 9211 has some open thermocouple circuitry in the form of a current source between inputs.  What this does is force the max voltage across the terminals if there is an open thermocouple.  A diagram of this can be found on page 12 of the 9211 Operating Instructions:



The rest is up to you in software.  You seem to have found this out empirically, and the way you are compensating should work very well for your application.  That method is the way that I would suggest you do it.


Message Edited by Neal M on 06-12-2008 02:04 PM