Q. Define Resistance Strain Gauge?

Mechanical and civil engineers routinely employ the dependence of resistance on the physical dimensions of a conductor to measure strain. A strain gauge is a device that is bonded to the surface of an object, and whose resistance varies as a function of the surface strain experienced by the object. Strain gauges can be used to measure strain, stress, force, torque, and pressure.

The resistance of a conductor with a circular cross-sectional area A, length l, and conductivity σ is given by Equation,

R = l/σA

Depending on the compression or elongation as a consequence of an external force, the length changes, and hence the resistance changes. The relationship between those changes is given by the gauge factor G,

G = (ΔR/R) / (Δl/l)

in which the factor l/l, the fractional change in length of an object, is known as the strain. Alternatively, the change in resistance due to an applied strain ε(= l/l) is given by

ΔR = R₀Gε

where R0 is the zero-strain resistance, that is, the resistance of the strain gauge under no strain. A typical gauge has R₀ = 350  and G = 2. Then for a strain of 1%, the change in resistance is ΔR = 7 Ω. A Wheatstone bridge as presented in Section 1.4 is usually employed to measure the small resistance changes associated with precise strain determination.