Strain Gauges and Load Cells

A strain gauge's resistance changes with strain $\varepsilon$:

$\frac{\Delta R}{R} = GF \cdot \varepsilon$

with gauge factor $GF$ ≈ 2 for metal foil. The changes are tiny (micro-ohms per ohm), so gauges are arranged in a Wheatstone bridge — a load cell typically contains a full bridge — and the millivolt output is amplified by an instrumentation amplifier or a dedicated 24-bit ADC such as the HX711.