Thanks for that.
The strain gauge manufcaturer supplies a gauge factor, which defines the gauge characteristics. Where I am unsure is how the overall system is calibrated, ie gauge plus bridge and DAQ.
For example, if I have a gauge with a known factor and I put it on a wing, how can I be sure that the reading on my data acquisition is accurate? I'm not sure how one would apply a known loads / measured outcome to a structure with multiple load paths.