How do I calculate the minimum deflection I can sense with a strain gauge, like the BF350-3AA?

Question

I have a strain gauge (the BF350-3AA), and I will be connecting it to an HX711-based amplifier sensor.

From its low-quality datasheet, I've pulled out the following figures:

1. Nominal Resistance: 350 ohms
2. Sensitivity Coefficient (which I believe is also called the Gauge Coefficient): 2.1
3. Sensitivity Coefficient Dispersion: <+/- 1%
4. Transverse Effect Coefficient: 0.4%
5. Strain Limit: 2%

Ultimately, the quantity that I want to measure is the number of micrometers of deflection over the length of the gauge. What is the minimum sensitivity (or precision) of deflection I can measure?

The gauge is being deployed inside a long flexure block of concrete, with its normal parallel to the axis of the press.

Bonus question: What formula do I use to calculate the deflection as a function of resistance (or resistance change)?

Answer 1

There are several limits to the smallest useful deflection.

One is given by the noise of the system. That varies with the amplifier/ADC that follows the strain gauge, and the bandwidth you are using. The noise level allows you to get a strain via the listed sensitivity.

Even if you use a very quiet amplifier to get a very small observable strain, the long term drift of the gauge, its mechanical hysteresis, and its resetability, will all limit whether the ability to measure such a small strain is actually useful.

At that low level, the gauge resistance is unlikely to contribute meaningfully to the noise of the system, unless you are using a very low noise op amp indeed. I would expect the mechanical parameters to dominate the limits.