Sampled Data not correct

[Fantester]

I developed a VI to sample three different chanels from my DAQ board.  I set up the channels in MAX and they read correctly there, but when I am aquireing data in Labview I get a small voltage value from one of my transducers when it should be almost zero.  The othe transducer which is about the same reads correctly.  Atached is the basic acquisition set up in Labview 6.1.

Do I need to change my settings in Max?

The transducers are basic pressure sensors with a digital display.  The output 0 to 5 volts DC. 

[Andy F.]

Fantester,

MAX and LabVIEW should be reading the same information, so it is odd that you are not getting the correct readings.  How small of an offset are you seeing?  I would recommend testing the channel with a power source (battery or something) and see if you still see the offset.  I have looked at your code and it doesn't seem to be a programming issue, and settings in MAX should not cause an offset, unless you set up a scale. 

Andy F.
Applications Engineer
National Instruments

[Fantester]

I modified the vi a little and it seems to work correctly.  I added input config & coupling and input limits to the AI 1-scan vi.  In the atached vi the element3 was the output that was not correct.  I also added a value on the index array function for  element3.

Not sure what helped.

In Max the sensor setting is "Map ranges" with 0 to 5 volts input range.

[Fantester]

My last DAQ channel is still incorrect.  I changed my order of daq channels going into the array and the last channel is incorrect.  Why does the last channel start out reading the correct value and then become incorrect when the voltage is decreased to a low value?  I am wondering if it is a timing issue?  I created a simple vi (picture attached) to verify that the last daq channel becomes incorrect when going from higher voltage to very low voltage.   

The quick and dirty fix would be to sample an additional channel that is not needed and make it the last input since the last input is the only one that reads incorrect. 

I tried increasing the acquisition rate and this did not help either. 

I have a DAQCard-6024E with a SCB terminal block running Labview 6.1.  Any help would be appreciated.  The first DAQ traditional MAX channel is a thermocouple the second two are 0 - 5 volt DC Referenced single ended inputs.

[Otis]

Hi Fantester,

I see what it is you are trying to do, but it still does not make any sense why the one channel would be faulty, but everything else is fine.  I really don't think that it has anything to do with the timing or anything along those lines.  Instead of setting up Virtual Channels you might want to try and take a look at the raw voltage values by simply looking at channel 0, 1, or X.  Check to make sure that your raw voltage is correct, because if it is out of range then your scaled measurements will be off too.

If you are still having some problems could you tell us what type of Virtual Channels you are using?  How are you configuring those channels?  If you keep monitoring those channels in MAX do you see the same behavior that you are noticing in LabVIEW?

Essentially, LabVIEW should not be reading anything differently than you do in MAX.  The VI that you are using doesn't really use any timing; it just looks for a single sample when you demand it.  Another option might be to use an AI Mult Scan.vi to sample at a rate of 3 Hz instead, or whatever rate you want.

Basically try to figure out what the differences are, and then focus on those differences to find out where the error occurs.

Regards,