Daqmx signal range exceeded?

[JaRay200]

It might be me not quite understanding on the ADC range works in signal express.  

I am using an SCXI 1001 chassis, SCXI-1600 USB DAQ, and an array of SCXI 1520 and 1314's for strain measurements.  Within daqmx I have my range set for -500 to 3500 uStrain with an excitation of 10V and gage factor of 2.2.  Upon a recent test I measured over 4000 uStrain.  Real or not (we think it could be a bad gage) I feel like the ADC should have railed at 3500.  What am I missing here?  If the range doesn't reflect the actual max and min then what does it mean.  

[TheEngineeringBear · Accepted solution]

The range that you set just tells DAQmx what range of values to cover.  With your setup, you could be able to read much larger reading that 4000 depending on the configuration.  From the info given, I couldn't figure out which range the device is set to currently, but I'm sure you could do a little digging in the specifications with your setup to find out what range the card is using in your computer.

If you do feel though the hardware may be acting odd, there is a great KB to help with trouble shooting strain measurements with your specific hardware.

How Can I Troubleshoot Inaccurate Strain Gage Measurements with the SCXI-1314?

http://digital.ni.com/public.nsf/allkb/625E1FE5C6DD2A0C862579600073009E

-Bear

[JaRay200]

I used that KB when I was setting up this test, and I feel like my setup is correct.

I will use one of my channels as an example to hopefully make a little more sense.

I have a strain gage named "Strain-34N1."  I have it setup as "Quarter Bridge 1", 10V excitation, gage factor of 2.2, initial voltage of 5.2653u, signal input range max 3.5m, and signal input range min -500u.  On this test we should see very little in the negative range, but could see upwards of 2.5 to 3 mstrain.  I might bump that max up a bit to be on the safe side but that's besides the point.  In the initial loading test we did, we noticed that this particular sensor was rising exponentially faster than any other strain gage.  I suspect a bad install.  Anyways, eventually this sensor does reach a max range, but it wasn't the 3.5 mstrain I expected, its was about 4.3 mstrain.  My only guess as to why this is so is that the ADC has preset voltage ranges that it must adhere to, so it rounds up to the nearest one.  

Was that more helpful as to what I am trying to figure out?  I know that in theory I can read much much higher than 3.5 mstrain, but that is not my concern.  

[JaRay200]

I think I answer my own question.

http://digital.ni.com/public.nsf/allkb/E69C47B0BB063B548625794A004B8354

Definitely seems like it will select the next available gain setting. 

[TheEngineeringBear]

I'm not sure how that KB answered your question, but it is along the same lines of what I was trying to explain.  

If you understand that creating a channel that has a range of -500 to 3500 doesn't actually mean that you will only measure between those values, then you understand that daqmx will assign a range that covers the desired range.  Daqmx does not adjust the ADC to scale in that range.  You could very well have a range much higher than 3500 and much lower than -500 depending on what voltages they correlate to.  

You are on the right track of understanding the gain settings!

-Ben

[JaRay200]

Now that I reread your reply, I see what you were saying.  When I first read it I was under the assumption that you just meant the Daqmx is capable of reading strain values much higher than that (since the differential voltage is small).  I am squared away now.  Thank you for the input!  I will accept your first response as the answer.