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NI 9236 strain measurement scaling

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I am setting up several strain gauges in quarter bridge configuration. I am confused about the output range of the measurement. I set the channels up with all the characteristics of the gauge (GF = 2, resist = 350 etc.) The measurement returns values ~ 5 x 10^-3. I don't know the units of this measurement. Is it 10^-3 microstrain or 10^-3 strain. I would not believe 10^-3 strain. 10^-3 microstrain is too low. I am expecting 1-5 microstrain in this measurement. Since I input all the scaling information into the measurement task, I expect a properly scaled output but this is difficult to interpret. Changing the GF will change the output in response, so again this points to the fact that the output is indeed scaled by my input parameters and should represent strain.

 

Any help would be greatly appreciated.

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Further information:

 

I have setup virtual channels in Measurement and Automation. These results are the same as from my code. They are tagged with m for 10^-3, but I am still unsure as to what the units are. When I calibrate the simulated strain is ~2m.

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Solution
Accepted by topic author ludhaber

Hi ludhaber,

 

If you are using a strain task, the values returned should just be in units of strain. Thus, you should be seeing values with either an 'm' (representing milli-) or a 'u' (representing micro-) next to the value such as "2m". One good method of verifying the scaling is to use the "Custom Voltage with Excitation" task type. This type is very similar to a strain task, except that it will return a value in terms of voltage (it is actually volts per voltage of excitation, as changing the excitation value of a gauge will change its output voltage, even though the amount of strain may not be changing). The value being returned here should match up to the equation for a quarter-bridge strain gauge for a given strain value, shown below. For example, with a strain value of 5u, you should see an output of 0.0025mV/V being returned using the custom voltage with excitation reading. If you are reading a strain of 5m within the strain task, you should be reading a value of approximately 2.4876mV/V in the custom voltage with excitation task. Hope this helps,

 

 

Message Edited by dansch on 10-09-2008 10:18 PM
Daniel S.
National Instruments
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