Temperature compensation for strain gages

[WyoEng]

I'm trying to compensate for temperature effects on a strain gage placed on a carbon fiber specimen.  The specimen will be loaded uniaxially only.  Temperature is an important concern here, for an electrical current will also be run through it for measurement purposes, which is expected to also heat up the specimen.  I've read in the document "Measuring Strain with Strain Gages" on the NI website (http://www.ni.com/white-paper/3642/en), but though it talks about using a dummy gage, it doesn't talk about just how that second gage, placed perpendicular to the axis of the applied force, it doesn't talk about how to connect that to the strain gage module, in my case the NI 9235, to use it for temperature compensation.  My first thought was to just connect each gage to the module as their own seperate channels, utilizing the quarter-bridge configuration in the LabVIEW software, calibrating both gages while no external physical load or temperature effects are applied, and then, after testing, subtracting the dummy gage measurement values from the active gage.  But then I started wondering, is there a better way of doing this?  Can I use the half-bridge configuration or some other configuration with the 9235 to automatically merge the two gage readings during the testing? 

I'd really appreciate any advice anyone can give me on this.  I'd rather not have to build my own external wheatstone bridge (other than the one built into the NI 9235), and I'd like to make these measurements as accurate as possible.  I've also looked into the possibility of self-compensating gages, but I don't think that would be applicable in this case, due to the complexity of the material--being a composite.

[Izzzzz · Accepted solution]

Hello WyoEng,

The temperature compensation you wish to utilize is referred to as a Quarter Bridge II configuration (image below). This configuration is very similar to a Half Bridge although the second element (R3) is inactive because it is placed transverse to the load direction, unbonded from receiving strain, and in the same location as the active gage (R4) to account for thermal affect.

The NI 9235 only supports a Quarter Bridge I configuration for Strain measurements.  You will get an error if you attempt to use this module for any other Strain gage configurations that is not Quarter Bridge I. Thus the strategy you posed of utilizing two channels to read the active gage and the dummy gage will be the best option for utilizing the NI 9235 in order to compensate for thermal effects on your strain gage.

Another option would be to look into the NI 9237 which supports a Quarter Bridge II configuration.

Best Regards,

Izzy O.

Applications Engineer

National Instruments

ni.com/support

[WyoEng]

Izzy O.,

Thank you!  This helps a lot.  Just a quick related question--when you say unbonded from receiving strain, you don't mean that the gage isn't actually bonded to the specimen, do you?

[Izzzzz]

Yes that is exactly what I mean. It is attached (tape it down) to the specimen in that it will receive the thermal effects that the specimen undergoes, but will not experience the axial load experienced by the specimen i.e. do not bond it to the specimen. 

Regards,

Izzy O.

Applications Engineer

National Instruments

www.ni.com/support

[OSU_Nuke]

If you're still looking for more info on temperature compensation for straing ages, I suggest you check out some of the resources on Vishay Micro Measurements website. Specifically:

http://www.vishaypg.com/docs/11054/tn504.pdf

[WyoEng]

Izzy O.,

Just one last thing to check--if I use the NI 9235 with the active and the dummy gage connected seperately in Quarter Bridge I configuration, it is as simple as subtracting the two values after making sure they're both calibrated to read zero, prior to any thermal or physical strain, right?

[Izzzzz]

Yes that is correct WyoEng. 

Regards,

Izzy O.

Applications Engineer

National Instruments

www.ni.com/support