Problem in getting accurate measurement using NI 9227

[Deepa_Krishnamurthy]

Hi everyone,

I am measuring 4-20mA current using NI 9227.

The current output is 4.02mA when measured with multimeter (which is exact output given by the sensor), but when connected to NI9927 and programmed, the acquired current reduces by 1mA, sometimes its more by 0.2mA. Could you please tell me why does this happen.?? For my project even 0.1mA is very important.

Thanks in advance

Best Regards

Deepa

[CFER_STS]

Questions:

1) Is the NI 9227 within calibration?

2) Are you measuring a floating or ground-referenced signal? If floating, what is the common mode voltage?

3) If you place the multimeter in-line with the NI 9227, do they indicate differently?

4) What acquisition rate are you using in your program?

5) cRIO or cDAQ?  Scan Engine or FPGA mode if the former?

[Deepa_Krishnamurthy]

Thanks a lot for your reply

1. Yes it is within calibration

2. Its ground referenced signal, I have grounded the shield of the Lapp cables and connected A+ and A- respectively.

3. Yes, they are indicating differently when multimeter is placed inline with NI9227

4. Acquisition rate 1ks and no of samples per channel is 100

5. I am using cDAQ 9185

[CFER_STS]

Can you post your code?

[Deepa_Krishnamurthy]

Sorry for the delay

Please find the attached file below

[CFER_STS]

I would begin by making sure that you're comparing apples to apples.  A Fluke digital multi-meter indicates measured current as an RMS value, but in your code, you're taking a straight arithmetic mean.  You should replace the mean VI's in your code with code which produces the RMS mean, in order for the values to be comparable.

See:

Root Mean Square - LabVIEW 2017 Help

In the Mathematics / Probability and Statistics palette, where you find the Mean.VI, simply replace those with the RMS.VI from the same palette.

If they still don't match, continue troubleshooting from there.

[Deepa_Krishnamurthy]

I have compared the values even before averaging them. But then its different.

But, till now I was using only three ports(AO-A2)of the NI 9227. So I connected all the signals one by one to A3+ and A3- in oder to test the values , I am getting the exact values.

Is this the problem of calibration.? May I calibrate the device.?

But, the next suggested date for calibration is Nov 2018.

[CFER_STS]

I have a NI 9265 current output module which I use in FPGA mode with a CompactRIO, and I noticed that commanding a 0 mA output to an unused set of output terminals would result in an error if the circuit was open.  I actually had to jump the unused terminals together (I used a 100 ohm resistor, but a dead short might also have worked) in order for the unit to behave as intended.  Just a shot in the dark, but I wonder if shorting your unused input terminals together would result in a stable zero current measurement on the unused channel, and potentially improved behavior of the other channels?

[Deepa_Krishnamurthy]

But, now I am using all 4ports for the data acquisition from 4 different sensor, but then why am I getting the correct output only at A3.?

[CFER_STS]

You mentioned that the current input signals were ground referenced.  Where is the ground sink?  And are all four identical in that regard?