strain gage initial voltage and Vex

[WyoEng]

Currently having issues with strain gage readings being half of what is expected when the specimen they are glued to is loaded compressively.  (Note, we're using the DAQ assistant, and therefore DAQmx functions).  One of the things we 're looking into being a possible issue was that the Vex, or excitation voltage, might be incorrect.  Currently we are using a value of 2.0 V, applied internally through the NI 9235 compact daq we're using to read the gages.  They are single gages, used in quarter bridge configuations.  The gage factor is set according to what the gage manufacturer stated.  Is there a certain value of excitation voltage that has to be applied according to your specific type of gage?  

Another possible issue is the initial voltage.  Is this something we have to input?  Or does the DAQ take care of this the first time it is run?  Also, what is this value used for in getting data from the strain gage?

A third possible issue may be that we're calibrating the strain gage wrong, as there are several different options for calibration.  We've been using the Strain Calibration under the Device tab of the DAQ assistant, and within there only the offset nulling, as we don't have an external shunt resistor (unless there is a shunt resister in the NI 9235).

Lastly, we are using the Quarter Bridge I strain configuration option, which I believe is correct, but I know there is a Quarter Bridge II configuration as well, so now I'm starting to wonder about that...

Thanks for any help you can provide!

[_kb]

Hi WyoEng,

The 9235 is only capable of providing a 2V excitation signal, so the Vex Value cannot actually be affected.  Though you're strain gage might specify an optimal excitation level that is different from 2V, the system will still function well -- just not "optimally".

The initial voltage is a null offset that is used to balance the bridge.  It is the output value in the unloaded condition, so we subtract it from measurements before applying scales to the samples.  If this is a known value (sometimes specified by the manufacturer), you can certainly define it.  It can also be discovered and stored with the calibration info when you run a null offset calibration through the Device tab that you've already seen.

It sounds like you are running the correct calibration.  There actually is a 50kOhm shunt resistor built into the 9235, so you can use both offset-nulling and shunt calibration.

The DAQmx help files provide some great documentation on bridge configurations.  A quick review of the information therein should help clear up any concern about which configuration you are using.