Thermocouple Readings

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I am using a LabJack U3-LV with an LJTickInAmp to read temperatures from a Type T thermocouple.  The value in the LJ Control Panel shows a voltage of 2.5V when no thermocouple is connected and 0.4V when a thermocouple at room temperature is connected.  In DaqFactory, however, the readings are 0.02 with no thermocouple and -2.1 with a room temperature thermocouple (this is without any conversions on the channels).  It is my understanding that the Control Panel and DaqFactory values should be the same, so what am i doing wrong?  Also, shouldn't the voltage read close to 5V without a thermocouple attached?


Thanks in advance

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I'm going to let LabJack reply to most of this.  My guess is that Control panel is doing a differential reading or is running at different resolution.


As to the last question, inputs that have nothing attached will float to just about any voltage, though usually they will read near a neighboring none-floating signal because of the multiplexer used internal.  You would need a pull up or pull down resistor on the input to have it go to a specific value when disconnected.  Also, the U3-LV only goes from 0 - 3.6.  If you need a larger range including 5V, you want the U3-HV.  LV = low voltage range, HV = high voltage range.

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When the LJTIA input is not driven, it likely floats up to the positive rail.  The max output of the LJTIA is about 4.5 volts, but the max of a normal low voltage input on the U3 is about 2.4 volts, so you would see something around 2.4 volts.



1.  Do your initial testing without the LJTIA.  Rather, jumper DAC0 to the FIOx in question.  In the test panel in LJCP set DAC0 to 1.0 volts, and confirm that you read similar from FIOx.  Then close LJCP and see if you read 1.0 volts from the same FIOx in DAQFactory.  In addition you can test by jumpering FIOx to GND (should read near 0) and jumpering FIOx to VS (should read about 2.4 on a low voltage analog input using the normal range).



2.  To test the basic operation of the LJTIA, see the following:



3.  To test thermocouple operation with the LJTIA, see the following:

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So I've jumpered the FIO to DAC0 and in the Control Panel things work fine.  Whatever the DAC0 is set to the FIO reads the same thing.  I then jumpered the FIO to VS and in the Control Panel it reads 2.5V, however, in DAQFactory it reads 0.22V.  I'm assuming this means something is wrong in DAQFactory and I have a hunch it may have to do with our startup sequence (though I could be wrong on that too).  Upon starting DAQFactory the following sequence runs:


include("c:\program files (x86)\labjack\drivers\labjackud.h")


global ID1=1


AddRequest(1, LJ_ioPUT_CONFIG, LJ_chAIN_RESOLUTION, 12, 0, 0)

AddRequest(1, LJ_ioPUT_AIN_RANGE, 0, LJ_rgUNI5V, 0, 0)



This was copied from another application using a U9 and was written by an engineer that is no longer here, which is unfortunate since she was the competent one when it came to programming.  Any other suggestions?  


Again, thanks for all your help.

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So I took your advice and started a new CTL with no scripting of any kind, added an A to D channel and a corresponding variable value, and now everything appears to be working fine.  


The startup sequence I gave above is the only scripting in the old CTL and even with it disabled the problem persists, so if the problem isn't there where else could it be?  Also, if we can't figure it out and I need to make a new CTL from scratch what lines from the above script should I include or exclude from the startup sequence? I have an idea of what each line does I'm just not sure why I need them, especially when new CTL works without any of it.   


Thanks and sorry for my lack of programming skills

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If you want us to see why your CTL is not reading correct, I suggest you post it so we can try it.


As for script, for basic operations you don't need any script, so if you don't have a reason to add script then just proceed without it.


To use a thermocouple with a U3+LJTIA, see the example "Thermocouple LJTIA U3.ctl":

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