NI USB6008 DAQ module, AD595 and Polynomials

[Georduc]

I am trying to linearise an output from a thermocouple, using the configuration on a DAQ input. The equipment involved is a NI USB6008 DAQ module, a type K thermocouple, and an AD595 amplifier. The data I’ve used is NIST polynomials, which I have typed into the input configuration. Before typing in the data I had a value for temperature which was in the right area, 18C. After entering the polynomial data I have a value of -10C.

Now where I may be going wrong is that the output from the AD595 is 10mV/C where the Thermocouple sensitivity is 41µV/C a gain of 245. I’ve tried scaling the polynomials, but same incorrect output. Any Idea where I’m going wrong?

[LV_Pro]

Can you post what you're doing? Show us the code? Is what you have done so far in LabVIEW or in MAX (Measurement and Automation Explorer)?

[hecmar.arreola]

Without looking at your code it is hard to say.  I do have some experience with the AD595, and I found that if you crank up the gain too high it becomes more and more non-linear...  I also remember the AD595 not working well with grounded thermocouples... but it may have just been my interface circuit.

[Georduc]

Hi

The code is a simple test code to evaluate the effect of linearising with polynomials. Find attached screenshots.

Many thanks for your reply

George

[johnsold]

Did you read the section in the AD595 datasheet on interpreting output voltages?  It specifically mentions an offset voltage of -11 uV.

Lynn

[Georduc]

Hi

I have attached the simple test code I used to evaluate the effect of applying the polynomials. The value without the polynomials in the scaling is where you would expect around 20 C, with the polynomials -447C.

Thanks

George

[johnsold]

I cannot help with that as I do not have the DAQ stuff.

Lynn