I need to measure tensile strains between 0 and 0.01%, with an accuracy between 1 to 10% within that range.
It seems like all systems that read strain gauges rely on a Wheatstone bridge to amplify small variations around the balanced point of the bridge using a diff amp (which makes sense) so I've made a simple model in excel which gives the output voltage of the bridge VS strain (equations checked online thrice), then the amplification of the differential voltage with half-range offset for a single-ended ADC.
The model basically says I need to measure 0 to 0.7mV with an accuracy of 7 to 70uV, which screams for a gain of 1000. But this means any matched resistor of the bridge can only vary by about 0.1 Ohm before the amplifier saturates...
I can do that for the two dummy resistors (1k in my case) using a matched pair IC accurate to 0.01%, but the best I can do for the strain gauge "mirror" is to trim for each board for a few fractions of ohm with a pot until the bridge outputs zero - but I am sure this is not reliable since the temperature alone will certainly make the strain gauge vary by one or more 0.1's Ohm... I thought of using the active-dummy method and replace that "mirror" resistor with another identical strain gauge roughly at the same temperature, but I do not know how much I can expect another unit to be different in its unstrained resistance to be able to trim it to balance. I don't even know if that will guarantee the conditioner will not saturate.
Is this feasible? Is there better? How is it usually done? I don't think that so few people need to measure these sorts of strain, given that we have to stay clear of the yield strength.