PCI 6251 Analog input not measuring voltage of a mass flow controller

[alitarraf]

Hey,

I have a PCI 6251 M DAQ with a break out board SCXI 1302.

I am trying to measure a 0-5v analog output voltage from a mass flow controller (Check picture of pin out)

When I measure with a Digital Voltage Multimeter the flow signal (PIN2) and signal common (PIN12) I get a right stable voltage between 0 and 5v depending on the flow set up. I can control the amount of flow by supplying a 0-5V Analog output from the DAQ to set point PIN8 and signal common PIN12.

Now when I hook up flow signal PIN2 and signal common PIN12 to AI and AIGND on my DAQ, I do not get any readings on my labview VI for AI.Moreover the flow does not respond to the voltage set point anymore, it get stuck into a range of values no matter how I vary  the AO 0-5V from the DAQ into setpoint PIN8, and signal common PIN12.

I should add that I tried different AI ports with sam results, and I also tried measuring my power supply voltage with my AI and all work well.

It seems like the AI input is affecting the AO output voltages in my load. What would cause this? Would this be an impedance matching issue?

Any thoughts or direction are appreciated.

Thanks

Ali T.

[JÞB]

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@alitarraf wrote:

Hey,

I have a PCI 6251 M DAQ with a break out board SCXI 1302......

Thanks

Ali T.

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That would be the problem right there!

Just how are you connecting a 50pin SCXI 50 pin breakout board to a 68 pin PCI DAQ device?

Go get yourself a SCB-68A (Or any compatable breakout device listed above) and you will have much better results:

[alitarraf]

Hey Jeff,

Thank you for your reply. I do have a SCB68 breaout board. I use the DAQ with dynamic pressure sensors and for triggering cameras and other solenoid valves, so everything is working from that stand point.

I am just surprised that reading a 0-5v through AI is not working, and is actually affecting my load electronics. I am defitively missing something here. I also asked for help from the mass flow controller manufacturer, but the problem is from reading out data with the daq AI input, as I can read voltages with a multimeter.

Thanks for your help,

Ali T.

[JÞB]

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@alitarraf wrote:

Hey Jeff,

Thank you for your reply. I do have a SCB68 breaout board. I use the DAQ with dynamic pressure sensors and for triggering cameras and other solenoid valves, so everything is working from that stand point.

I am just surprised that reading a 0-5v through AI is not working, and is actually affecting my load electronics. I am defitively missing something here. I also asked for help from the mass flow controller manufacturer, but the problem is from reading out data with the daq AI input, as I can read voltages with a multimeter.

Thanks for your help,

Ali T.

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Thanks for clarifying the hardware setup.

From your latest screenshot I see a few things that trouble me about that 6521;

1. The device has not been to an external calibration facility for 8 years and is 6 years overdue for cal.
2. the last external cal was accomplished in a 130 deg F environment This is right on the upper rail of the devices environmetal operating range!  (To risk the obvious joke- that is un-cool dude)

It is way past time to let a technical metrologist take a peek at that thing and verify it functions properly.  In fact, I would hesitate to connect it to a sensitve instrumentation system at all until traceability is re-established.  I suspect the AI front end is damaged.

[alitarraf]

Hey Jeff,

Thanks for your input. You are right regarding the last calibration date June 2006. But I am using the MFC in a transient set up not for continuous flow, and I will be calibrating it with a water displacement method.

I dont think the AI front end is damaged, because I am getting measurements  with a digital voltmeter. Check pictures below for 300 sccm and 0 sccm. I compared voltmeter with oscilloscope measurements, the mean value are pretty close, although I have some AC noise issue, I am wondering if that would affect the readings in the DAQ AI.

[alitarraf]

I just realised that you were talking about the DAQ device calibration. Yes it hasnt been calibrated for some time 🙂

Values might not be super accurate but the AI input are working. I checked them with a power supply voltage and they all match the voltage.

Thanks

Ali T.

[alitarraf · Accepted solution]

Update for everyone who might be interested:

I did not connect the power ground of the MFC to the signal ground of the MFC.

Once I did that the DAQ is reading all the data as expected.

So it turned out not to be a DAQ NI issue at all, but a set up issue from my part, as I suspect is the case with most problems.

Thanks Jeff for your input.

Ali T.