Fluctuations/ Noise while DAQ using USB 6211

[Vishal_vk]

I am using NI USB 6211 card for data aquisition on a Boiler. My OS is WIn7 and Labview 2011. The sensors i am using are as follows:-

1. Drum level- 1151 Rosemount DPT

2. Feed flow- Rosemount DPT

3. Steam flow- Rosemount DPT

4. Drum pressure-Siemens Pressure transmitter  All transmitters give analog output of 4-20mA.

I am using DAQ assistant for continuous data rec as foll- DAQ- Differential mode,249 ohms shunt resistor for each channel, 1 sample on demand.  

My VI consists of foll-

1. Flat sequence structure followed by delay after each channel.

2. While loop, TDMS write for data rec.

 I am facing foll problems:-

1. The USB card shows continuous fluctuations around the actual parameter. eg if the current sensor output is 6mA steady( measured by multimeter), the DAQ assist shows fluctions from 4 to 8mA continuosly.

2. The sensor output seen in an oscilloscope also shows similar fluctuations( like a sine wave).

3. Similar problems were also seen when i connect a thermocouple.

 What measures do i take to acquire and record the simultaneous data through multiple channels with time stamp? I have attached the VI.

[Henrik_Volkers]

What you see is most probably power line interference ...

Your multimeter has a build in lowpass filter .... so that is the thing you will need to add to your setup.

The nice thing is that you can do that in software.

capture the channels with a higher scanrate (1kHz ... 20kHz) and decimate (with mean) the samples over one or more periodes (like 300ms or 1s.. will fit for 50Hz and 60Hz)

[dsasorin]

Of course is power line interference.

You can add a grounded Faraday box or shielded cables.

[Vishal_vk]

Henrik,

thanks for ur reply.

1.I could not change the sampling rate in my VI as i am using 1 sample on demand in DAQ asst.

2. I decimated the signal using median and used a shielded cable.

 The problem still persists and i am getting a near sine wave. could u elaborate?

[dsasorin]

try changing sample mode:  Differential, RSE, NRSE, with respect to the Ground.

[Henrik_Volkers]

1. Use one Daq Express vi

2. read all Data channels at once, read 300 values with 1kSPS

3. Apply your median filter (against spikes)

4. calc the mean of the 300 values (eliminating line noise)

5. save one value per channel

This will work if you want to record up to 3 samples per second.

If you drill deeper  you can go up to one powerline cycle (50 or 60 Hz) and I claim that you don't need it faster since your sensors/systems aren't 😉

 Why do you use locals??