incorrect thermocouple data from 6034e cb-68lp labview 5.1

[RT4CY]

You seem to have everything wired properly, unfortunately breadboards can introduce their own problems.  Let's eliminate the breadboard, and simply put the resistors across the terminals on the CB-68.  Simply insert the ends of Resistor 1 across 68 & 67, Resistor 2 across 34 & 67.  Then connect the leads from your thermocouple directly to pins 68 and 34.  Let me know if this shows an improvement in your data

[MarvinTsoi]

Following your suggestion resulted in the following graph in MAX at 1000Hz with 1000 samples.

[RT4CY]

Let's take a step back and perform some troubleshooting steps that I mentioned before.

1.  Put a known voltage across the AI channels.  You can bring in a known voltage from any battery, and wire a differential input.  If you have a function generator, that would be even better (since we can also check an AC signal). 
2.  Try different channels on the DAQ device, or different hardware (DAQ device, terminal block) altogether (if that is an option).
3.  Try backing off the sampling rate.  We may be acquiring frequency components in our signal that are not related to the temperature.  Even the best thermocouples will not respond at 1000 Hz (we usually acquire at 10 Hz from thermocouples).

You might also try performing a Fast Fourier Transform (FFT), to see if we are acquiring any high frequency noise.  Use the Spectral Measurements VI (Express palette»Signal Analysis) which will give you the frequency of the noise induced as a peak in the FFT waveform.  If we see noise at a high frequency, we can add a software filter to remove this noise. 

[MarvinTsoi]

I appreciate your help.  I have done all you have mentioned on channels ai0, ai1, ai2, ai3.  Thermocouple readings are the same, known voltage of a 9 volt battery reading is correct (8.85 volts).  I'm am working on the spectral measuements, i'm new to labview so i need some time to figure this one out. 

[AnalogKid2DigitalMan]

What is the expected voltage range of your signal? Have you set the gain on the card that best matches that range? It may improve the resolution.

-AK2DM

[MarvinTsoi]

I am not familiar with how to set the gain and expected voltage range.  i assumed that would be in the MAX but i can't seem to find it.

[RT4CY]

Hi MarvinTsoi,

The gain that AnalogKid is referring to is programatically set for you by NI-DAQmx when you configure your input limits.

The above screenshot shows where you set these limits when using test panels.  The screenshot below shows where you set the Signal Input Range, which tells DAQmx what gain setting to apply to the DAQ device.  Make sure that these are configured correctly.



We could also try acquiring the voltage output of the thermocouple.  You can read this in with either a Voltage task or the Test Panels.  It would be interesting to take a look at a graph with the actual voltage fluctuation.  The FFT will also be useful.  In LabVIEW, all you would need is a DAQ Assistant, the Spectral Measurements VI, and a graph indicator.  Let me know if you have issues with this. 

Message Edited by RT4CY on 12-29-2008 06:01 PM

[MarvinTsoi]

I have been setting the range the whole time.  I have a screen shot of a voltage reading of a type T thermocouple cause i thought something was wrong with my K type thermocouple.

After running the temperature task in MAX in continuous, i received the following graphs, sometimes it stays that way until i apply a temperature change, like changing from air temp to water, or just touching the thermocouple.

[MarvinTsoi]

The previous post of the strange measurement graphs, those are without the resistors in differential.  With the resistors it seems fine.  I have also attempted on getting the spectral measurement.  It does not seem right to me, but then again i've never done this before.  The following was taken with 20Hz sampling rate for 200 samples.

[MarvinTsoi]

I am out of ideas, does anyone know if this problem is normal or something that is completely not right?  Can it be fixed by myself or do i need to contact NI support?