Measuring 4-20mA Pressure Transducer with NI 9227

[MClayp]

Hello, we are trying to measure our GP50 model 370 pressure transducer at a high sampling rate. Can the NI 9227 measure a 4-20mA sensor? I connected a 24Vdc power supply in series with the NI module and transducer as shown below:

I opened NI Max and clicked on the Test Panel option. With the default options, it displays just noise even when I push on the sensor:

I changed it to a lower sampling rate and still saw just noise:

Does the NI 9227 support this type of sensor? I tried the same type of configuration with the NI 9219, and it showed good graphical data in NI Max. We just need a higher sampling rate than the 100Hz from the NI 9219. Also, I tried using DAQExpress with the NI 9227, but it did not show the device. Is the NI 9227 module compatible with DAQExpress? One more thing, the NI 9227 module was purchased back in 2012. Do you think it could be an issue with drivers or the module's firmware? The NI 9227 module still shows up though in the NI Max device list. 

Thank you so much for your help. I look forward to hearing from you.

Michael

[santo_13]

1. Can 9227 measure 4-20mA current loop? - Yes, of course, it is "current"
2. Can your GP50 model 370 work properly with 9227? - Probably, yes
3. Did you connect sensor correct? - going by block diagram, looks good
4. What is different about 9227? - has a very low value sense resistor 12mOhm, compared to typical 249 ohm for 4-20mA current loop

Looks like the sensor is powered up evident from the 2.59mA measured by 9227

What next? hook up a multimeter in series and ensure you see the same current value.

[MClayp]

Hello Santhosh, 

Thank you for your reply. I connected a Multimeter in series this morning, and it read 3.98 mA with no load applied to the sensor. Here's the output from NIMAX:

When I pushed on the pressure transducer, the multimeter read 4mA. Here's the output from NIMAX:

Here's the output when I pushed harder and the multimeter read 4.02 mA:

You can see a bit of a slight upward trend in the NI Module values from these pics but the noise makes it hard to detect. 

Also, tried sampling at a lower sampling rate of 2500 Hz and adjusting the shunt resistor to 12mOhms. Here is the output with no load applied to the pressure transducer (multimeter reading 3.98 mA):

With load applied (multimeter reading 4 mA):

Do you have any recommendations for producing a clearer signal (further adjusting the shunt resistor, sampling rate,...) or do you think this will be as good as it can get in NIMAX?

Thank you so much for your help,

Michael

[MClayp]

Did some more testing, and I think the NI Module is just picking up noise. With no load applied to the pressure transducer and the Ammeter reading 3.98 mA, the output ranges from (2.44 mA to 3.03 mA):

Also, I tried using our NI 9219 module that samples at 100 Hz to see how it responds to the pressure transducer and observed the following:

No load at 3.98 mA Ammeter reading:

Load applied at 4 mA Ammeter reading:

As you can see, the 9219 readings are very accurate. Are there any settings in NIMAX that I can adjust to yield an accurate output for the NI 9227? 

Thank you,

Michael

[santo_13 · Accepted solution]

Further investigation reveals that the NI 9227 is not best suited for 4-20mA current loop measurements due to the large offset errors.

Calibrated typical offset error of +/-0.05% translates to +/-3.5mA which is substantial for 4-20mA current loop measurements.