Measurement using NI cDAQ

[Irene777]

I am now trying to do measurements using NI cDAQ.  And using input module 9205.  And what I want to measure is temperature, Voltage, Current , Pressure, etc,.

So I used DAQ assistant to take the measurements. But there is only one data out (in the diagram).  I would like to know that do I need one DAQ assistant for one measurement(eg. temperature) or what I have done in my attachment picture is correct?  If I am correct by using split signals, How can I distinguish which channel is for voltage, which is current and so on.  Thanks for your help.

best regards,

Irene

[David L.]

Irene,

The way you are doing acquisition and using the ‘split signals.vi’ is correct. The best way for you to keep track of which channels are being graphed is to rename the channels in the DAQ assistant, these names will then follow on to the graph. I have attached a word document demonstrating this with further explanation.

One note is that the 9205 can only do analog input. ( your document showed analog output as well ). If you are using another module to do output, depending on what your DAQ assistant is set for (single point, continuous, etc), you may need to input arrays to the 'merge signals.vi' and not just single points.

Message Edited by David L. on 04-06-2007 10:47 AM

[byte]

Hi David,

I have a question about 9205 and 9215

I am trying to get reading by that modules from K-type thermocouple and results I am getting look abnormal. there is a jumps in readins +-200 dergees C, if I am trying to measer voltage (6-7V) 3rd digit is not stable (like 6.2 stays same and 3rd digit is changing all the time significly). Could you suggest what's wrong?

best regards,

Anton

[Hani R]

Hi Anton,

What you described about the error when measuring 6-7V seems to be within specification of both devices.  Refer to the manuals for the NI-9215 and NI-9205 for details about the accuracy specifications. 

As for the temperature measurements, these particular devices don't have built-in CJC.  What value are you using for the CJC when configuring your task?  Can you also provide some details about how you are configuring the thermocouple task?

[byte]

Hi, I am trying to get reading by M&A 4.2 by creating task for thermocouple readings. I am using K-type thermocouple so I set that parameter in task. I know that there is no CJC in that module, that's why I put 24C (appr. room tempr.). but I am not expiriencing wrong reading but fluctuations within 300-400 degrees, regular multimiter shows 1030 C from that thermocouple, but my module is jumping within 850-1200 C. We got that module after consultation with NI rep. we dont need 100% reading, +-2-3 degrees acceptable for our application, but here we have something strange.

thank you,

Anton

[Hani R]

Hi Anton,

I have a few more questions regarding the issue.  What happens when you measure lower temperatures (at room temperature for example)?  Do you still see significant fluctuations?  Would you characterize the fluctuations as sinusoidal or as random spikes?  Also, have you tried other K type thermocouples (perhaps with different gauge wires)? 

[byte]

Hi Guys,

We have updated problem 🙂

to avoid problems with reading of thermocouples we got 9211, and ok now.

but with 9205 there is still a problems. It gives just strange readings. I have created 8 channels to do RSE readings.

nothing is connected yet to a terminals but device already shows -3.xxx V on each channel. further more... I got connected 5V signal to 1st channel and its shows me 5V - OK, but rest 7 channels will show something aroud 5V as well, but nothing is connected to them. what is wrong? I tried differential connection mode - same problem. I tried 9215 (4 channel module) - works fine. I decided that my 9205 is not working properly - got exchange , connected new 9205 to my system - again that problem. please advise whats wrong.

thank you

Anton Baysov

Message Edited by byte on 04-18-2008 03:55 PM

[byte]

Take a look at one more picture attached. As you can see there is steady signal on line 0 (channel 0), but rest 7 have kind of sinus signals. still need help.

thank you,

Anton

Message Edited by byte on 04-18-2008 04:00 PM

[Brooks_C]

Hello Anton,

The floating signal behavior you're seeing on non-connected channels is expected behavior.  When you don't have anything connected to these channels the voltages are floating.  Since the inputs are all multiplexed through one analog to digital converter (ADC) when you connect a 5V signal to one input the other unconnected channels will have a similar reading due to residual charge on the ADC.  This charge on the ADC can't be dissipated when there is nothing connected to the line.  If you wanted to read 5V on one channel and 0V on the other channel then you'd need to ground all of the other channels.

Here are a couple KnowlegeBase articles that talk about this behavior:  Troubleshooting Unexpected Voltages, Floating, or Crosstalk on Analog Input Channels and Incorrect Readings when Scanning Unconnected or Open Channels (Option 3 in the second KB is the one I mentioned above).

I hope this helps, and have a great day!