How to get LabView to recognize where the amplification

[sman]

Hello there.  Right now  I am writing a program to read the thermocouple temperature (Type K) and I also have the polynomial coefficients but some how DAQ Ass. is not recognizing the amplify signal and it's outputting a very low temperature in Kelvins.  What do I do to let LabView recognize the amplified signal?  Thank You.

[Tom_W_[DE]]

Hi-

Rather than using the polynomial coefficients to scale your data I would suggest using a thermocouple input channel.  K Type thermocouple (and most other common types) scaling is built in with the NI-DAQmx driver and the DAQ Assistant.  When creating your measurement type just choose "Analog Input>>Temperature>>Thermocouple" and then configure the thermocouple type when the task window pops up.  You should be able to see better results using this method than entering the constants yourself.

I hope this helps!

Regards-

[sman]

Thank you very much Tom, it helps a little, but I still can't incorperate the amplification and so my temperature is not what it's supposed to be.  Here is my set up:

I have my thermocouple wires connected to Lab Works II Interface to implify the signal (voltage).  Supposely, I should receive different voltage readings as the temperature changes.  The signal from the Lab Works II  then goes to the Data Acq. Card.  How do I tell the DAQ Assis. to convert that implified signal back into temperature (Kelvin). 

Please do ask me if you need more details on my program. Thank you.

[Tom_W_[DE]]

Hi-

I haven't used a LabWorks II board before, but it sounds like you're applying some external amplification before interfacing with the DAQ board.  This setup is not recommended because the DAQ board itself applies amplification to the signal to utilize the full resolution of your DAQ board's input circuitry.  If you are applying external gain then you will not be able to use a thermocouple input channel.

If you must use the LabWorks II system to apply some gain then you will need to read the input as a voltage and apply a scale to the reading to counteract the gain applied externally by the LabWorks II card.  This means that if the LabWorks card applies a gain of 100 you will need to scale your data with a custom scale in MAX by 1/100 or 0.01.  After you have the voltage you can use the Convert Thermocouple Reading VI that can be found on the All Functions>>NI Measurements>>Data Acquisition>>Signal Conditioning palette to scale the voltage reading to a temperature based on thermocouple type.

Of course the recommended method is to connect directly to the DAQ card without external gain and then to just use a thermocouple input channel.

I hope this helps!

Thanks-

[LV_Pro]

I will add that when he says "connect directly to the DAQ" he probably means with the appropriate signal conditioning interface, otherwise you will end up with inaccurate temperature measurements caused by the parasitic thermocouple junctions. This is caused by the connection of the thermocouples "signal" wires to terminals of dissimilar metal, forming a thermocouple type junction. In most systems this is compensated for using "cold junction compensation", either by a calculation that uses the expected temperature of the screw terminal connection and subtracts the expected thermocouple voltage it would generate from that returned by the actual TC or having a seperate temperature sensor mounted on the same terminal block and using the measured temperature of the terminal block in the calculations. As he says, if you amplify the TC's signal you can't specify a TC junction to MAX as it is expecting the low level signal, not an amplified one.

P.M.

[sman]

Thank you Tom, that helps a lot, i appreciated that.  Do I divided by what I gain in a0, a1, a2, etc...Thanx again.

[sman]

Thank you for your help.