Strain gauge fundamentals — How large is the typical resistive change of a bonded strain gauge under normal strain, and what parameter governs it?

Introduction:
Strain gauges convert mechanical strain into a small change in resistance. In precision measurement, these tiny resistance changes are detected with bridge circuits and amplifiers. Knowing the expected magnitude helps select bridge excitation, amplifier gain, and ADC resolution.

Given Data / Assumptions:

• Metal-foil or thin-film bonded strain gauge on a structure.
• Moderate engineering strain levels (hundreds to a few thousand microstrain).
• Temperature effects minimized or compensated.

Concept / Approach:

The fractional resistance change of a strain gauge is ΔR/R ≈ GF * ε, where GF is the gauge factor (≈2 for metal foil) and ε is strain in strain units (e.g., 1000 microstrain = 0.001). For a 120 Ω gauge at 1000 µε with GF ≈ 2, ΔR ≈ 120 * 2 * 0.001 = 0.24 Ω — clearly less than an ohm. Thus the change is small and governed by the gauge factor and the applied strain, not directly by weight alone.

Step-by-Step Solution:

Verification / Alternative check:

Commercial strain conditioners specify bridge sensitivities in mV/V at given microstrain, confirming the minute resistance changes involved.

Why Other Options Are Wrong:

• many thousands of ohms: Unrealistic for bonded gauges; absolute R is ~120–350 Ω, changes are tiny.
• no more than 100 Ω: Overstates change by orders of magnitude.
• positive temperature coefficient: Temperature coefficient exists but does not answer magnitude or governing parameter.
• magnetoresistance dominance: Not relevant for standard strain gauges.

Common Pitfalls:

• Expressing ΔR in percent without relating to microstrain and GF.
• Ignoring temperature-induced apparent strain; requires compensation in bridges.

Final Answer:

is based on the gauge factor, but is typically less than an ohm