How do I get MAX to use table vs. polynomial conversion for thermocouples?

[Unplugnow]

My most recent project involves updating some undocumented code that I have inherited.  I have recently updated some drivers for a PXI system that is (now) using 7.1.1 and MAX 4.3.  Several B-type, R-type, and C-type thermocouple channels are defined.  After updating the various software , though, the bottom of the range of the B-type thermocouples is no longer OK at 32F.  I had to change it to 482F (250C) for the software to stop throwing Error -201171.  The suggestion that comes with this error is to change the Thermocouple Scale Type property to table rather than polynomial.  The input channels are defined as analog input -> temperature -> thermocouple -> B-type (with built in CJC).

1) Why is the bottom of the range no longer OK at 32F?  What changed?  (I sem to remember that B-type thermocouples are not reliable there, but that is not the point in this case.)

2) How do I change the property in MAX to Table vs. Polynomial?  I don't see anywhere I can make this change...  If there is no answer to this, how do I recover the indicating range I had previously?

Thanks

[Neal M]

Hello Unplugnow,

I looked up the error you referenced (-201171) and it said:

Measurements: Minimum temperature specified for the thermocouple measurement falls outside of the accuracy limit when using polynomial scaling. Specify a value greater than the minumum temperature for  polynomial scaling with this thermocouple type, or set the Thermocouple Scale Type property to Table.

The behaivior you are describing implies that this is the error you are talking about.  This KnowledgeBase appears to explain one possible reason why you are seeing this after your upgrade.  It basically says that version 8.2 of DAQmx changed the way that it handles the thermocouple scaling.  As you say, B-type thermocouples are inaccurate in the range you are talking about, and the newer versions of DAQmx recognize this and prevent you from trying to read there.

There are a couple of ways to work around this.  The method mentioned in the error you cannot do in MAX, you must use a DAQmx channel property node in LabVIEW to make this change:



The "correct" way would be to use different types of channels in your task and use the correct thermocouple type.  This would scale the voltage to the correct temperature.  A third way to revert back to the old way is to use a voltage task and then apply a custom scale to do the conversion to temperature. These last two could be done in MAX or LabVIEW.

It is important to note that the reason that this was put in was due to the fact the scaling may be incorrect in these ranges.  This could mean that trying to work around this issue may make your readings slightly off if you are in that range.  I hope this helps you out, if I misunderstood the question or you have any others, feel free to post back.


Message Edited by Neal M on 11-16-2007 03:50 PM

[Unplugnow]

Neal M,

Thanks for the info!  I may have to try re-defining the channels as voltage instead and using the table like you suggest if I can't get the temperatures to respond as before.

Unplugnow