Resistance change of the strain gages (NI 9235)

[lester14]

Hi all,

I am currently using NI 9235 to measure the strain from the strain gages. The strain measurement is straight-forward, but I am wondering how do I know the real resistance change of the strain gage after I got the strain data from NI 9235.

is the general definition the gage factor, and it can be used to calculate the resistance change. How about if I do not know the GF in advance, can I calculate the GF from the measurement data from NI 9235?

Thanks.

Lester

[enderWiggin10]

By definition of Gauge Factor, you would need to measure the physical strain(dL/Lo) and compare to the electrical strain (dR/Ro).  Here is the wikipedia article for gauge factor, which contains an equation that also accoutns for temperature effects.

Best,

[dousley]

If I was just interested in the resistance of the 1/4 bridge element, and not actually interested in the strain implied by the resistance, I would set the measurement up to return V/V. In DAQmx that would be done with a "custom voltage with excitation" tas,or in RIO it's the calibrated data before scaling it to strain units.

Once you've got a result in V/V, and you know the value of your completion resistor (120ohms for the 9235). Then it's simply a voltage divider equation.

"V/V output" = Rcompletion / ( Rcompletion + Relement )

Rearranged, that would give you:

Relement = [ Rcompletion ( 1 - "V/V output") ] / "V/V output"

...where Relement is your sensor, Rcompletion is the 9235's 120ohm completion, and "V/V output" is the modules output in Volts per Volt.

Hope that's helpful.

Daniel