NI 9205 voltage dropping to 0 during measurement

[ProteinSimple]

Hello everyone,

I'm a self taught LabView user so I'm not too sure that I'm doing this correctly.  But I am using a NI 9205 with my cDAQ system to measure a voltage coming from a mass flow controller.  I have the V+ wired into ai5 and the ground into ai13.  During my measurements, the voltage will constantly change between the expected V_out and 0 V.  I connected the mass flow controller to a DMM and see a constant voltage so I don't believe that it is due to the mass flow controller.  

I have attached my code that I'm using to measure and a picture of what I see. Anyone see this issue before?

[GerdW]

Hi Breathometer,

what about setting a sample rate for your DAQmx task?

What about reading blocks of samples using a "n samples" mode of DAQmxRead?

What about using a "differential" channel mode when wiring ai5 and ai13?

Why do you use a "n channels" mode, when your task is initialized to use only ai5?

Do you know this one?

[ProteinSimple]

Hi GerdW,

Thanks for the reply.  I'll give your suggestions a try! As for the N samples, I saved it on the wrong setting.  I first tried 1 channel 1 sample but I saw the same thing so I tried switching it and forgot to change it back.  I have also tried using a differential sampling mode but that didn't seem to fix my issue either.

Best,

Ellery

[ProteinSimple]

Hi GerdW,

I added a DAQtiming VI before my DAQread and set it to 10Hz.  Now it just has ~10sec of expected voltage readings and then it drops back down to 0V for another ~10 sec and so on and so on.  If I change it to 5Hz, then the cycle is about 20 sec expected, 20 sec @ 0V

Best,

Ellery

[GerdW]

Hi Ellery,

when changing your VI you should attach it again…

[ProteinSimple]

Oh sorry about that will attach.

[StevenD]

A few thoughts:

• If you are wired for differential, the input terminal configuration needs to be set to differential
• The minimum and maximum should be set as well
• The DAQmx timing setup should be outside the while loop, there is no reason to set it every iteration of the loop
• Your chart has the x axis labeled as time, but it's really # of data points collected (which if you know your rate can be easily scaled to time, but I've seen it throw a few people off)

[ProteinSimple · Accepted solution]

Hi all,

I appreciate all your input in trying to help me figure out the issue.  Looking online more, I found out that it was a wiring issue.  Because the module is isolated, the AI channels are floating when compared to chassis ground.  So besides having the AI+ and AI- wired, I needed to wire the COM channel to chassis ground as well.

Hope this helps others with similar issues.

Best,

Ellery