How do you make thermocouples read below -200C? LabVIEW won't let me.

[astro]

I'm using SCXI1102 thermocouple module with K type thermocouples.  This goes into an M series DAQ card in a PXI chassis.  My problem is that I want to measure temperatures down to 35K and LabVIEW won't let me.  When I try to set the min value less than -200 C then I get the following error when I try to run my program:

... min temp falls outside accuracy limit for polynomial scaling ...

... set the Thermocouple Scale Type property to Table ...

How do I do this?  I am more than willing to enter the lookup table of values from NIST, but I don't know how.

Also, it doesn't work when I try to use Kelvin units.  But I don't mind just adding 273 to work around that problem.

[Ian_Ren]

It appears the range of the K type thermocouple is from -200C to 1250 C.

(http://www.omega.com/prodinfo/thermocouples.html)

[astro]

That doesn't help me.  I have K type thermocouples embedded in a superconducting magnet coil.  I didn't put them there, if it had been my decision I would have incorporated temperature sensors better suited to cryogenic temperatures.  But they're there now and I have to work with them.  I know for a fact that they can work, I have previous test data which shows temperature measurements down to 35 Kelvin.  I even have a lookup table directly from NIST that will convert sensor millivolts to temperature down to -270 C. 

http://srdata.nist.gov/its90/download/type_k.tab

I know from the error message that LabVIEW is trying to use polynomial scaling and it's not good below -200C.  So my question is, how do I put this lookup table data into the LabVIEW DAQmx Create Channel VI.  This is a "polymorphic" VI that you can use for configuring analog inputs.  I select AnalogInput>Temperature>Thermocouple.  How do I change the Thermocouple Scale Type to Table and how do I put the data in?

[Henrik_Volkers]

Basically you can measure the EMF (voltage) of the TC and the temperature of your CJ and do the math by your own.

If you have the lookup table or a polynominal solution valid for your range, all you have to do is drawing the VI 🙂

(I remember doing so for the "new" ITS90 in LV3.1.....  This was 1993??!)

A good source for TC knowledge:
Manual on the use of thermocouples in temperature measurement,
ASTM PCN: 28-012093-40,
ISBN 0-8031-1466-4

(Page1): 'Regardless of how many facts are presented herein and regardless of the percentage retained,

                all will be for naught unless one simple important fact is kept firmly in mind.

                The thermocouple reports only what it "feels." This may or may not the temperature of interest'

[astro]

I found the channel property for scale type.  It is not under AnalogInput>Temperature>Thermocouple like the article at http://digital.ni.com/public.nsf/allkb/98EE9F9C730FDF09862572F800603D6B says it should be.  It is under AnalogInput>GeneralProperties>ChannelCalibration>ScalingParameter>ScaleType.  But even when I change this property from Polynomial (the default) to Table I still get the same error, so LabVIEW is not accepting my change.  Do I have to input the Pre-ScaledValue and ScaledValue arrays that I want to use?  Where do I do this?

[EEsRULEtheWORLD · Accepted solution]

Hi, Astro.

The AI.ChanCal.ScaleType (Analog Input»General Properties»Channel Calibration»Scaling Parameters»Scale Type) property is different from the AI.Thrmcpl.ScaleType (Analog Input»Temperature»Thermocouple»Scale Type) property.  The AI.ChanCal.ScaleType property will allow you to change the method that the calibration scale uses, while AI.Thrmcpl.ScaleType property allows you to change the method that the thermocouple scale uses.

If the AI.Thrmcpl.ScaleType is missing from your list of properties, check out this Knowledgebase article for more information regarding this missing DAQmx Channel property.  After you are able to locate this DAQmx Channel property, incorporate it into your code, as shown in the following screenshot:

Let me know if this works.  I hope you're having a great day!

[astro]

Thank you very much Sara.  By right clicking on the property node and selecting the Filter option I was able to show properties for all devices and find the right property.  Now my thermocouples work down to -270 C and I can continue my superconducting magnet test procedure.

Thanks again!

Astro