what is Strain gauge?

What is a strain gauge?

Turning a tiny deformation into an electrical measurement

A strain gauge detects small mechanical strains through a change in electrical resistance. It follows the extension or contraction of the surface to which it is attached. It does not independently measure kilograms, newtons or bar. Converting strain into force, mass, pressure or torque requires a suitable mechanical structure, measurement electronics and calibration.

This principle is used in load cells, some pressure sensors, torque transducers and structural testing. The sensing principle may be shared, while the mechanical design and measurement range differ.

Why does resistance change?

A common metal foil gauge has a thin resistive grid on an insulating carrier. Bonding the carrier to the specimen transfers surface deformation to the grid. Stretching changes the conductor's length and cross-section; changes in the material's resistivity can also contribute. With a positive gauge factor, metallic gauges generally increase resistance in tension and decrease it in compression.

Strain is the change in length divided by the initial length: ε = ΔL / L. It is dimensionless; small values are expressed in microstrain (µε). 1 µε = 10⁻⁶. If a 100 mm length extends by 0.05 mm, the strain is 0.0005, or 500 µε.

Gauge factor and the size of the signal

The gauge factor (GF) is the fractional resistance change divided by strain: GF = (ΔR / R) / ε. A value near 2 is common for metal foil gauges, but calculations should use the manufacturer's actual value. With GF = 2, R = 350 Ω and ε = 500 µε, ΔR = 0.35 Ω: just 0.1% of the nominal resistance.

A Wheatstone bridge helps measure this small change. An imbalance produces a small differential voltage that depends on the excitation voltage. When using bridge formulas, the wiring arrangement and output polarity matter; one conversion factor cannot describe every bridge configuration.

Quarter, half and full bridges

A basic quarter bridge uses one active gauge and completion resistors. An additional mechanically unloaded gauge may provide temperature compensation. A half bridge can have two active gauges, and a full bridge four. Gauge orientation and placement in the bridge determine whether the arrangement measures axial strain, bending or torsion.

A suitably arranged full bridge can increase the signal and reduce common temperature effects. However, connecting four gauges does not automatically ensure correct measurement or complete temperature compensation. Their mechanical positions and operating conditions must also suit the measurement.

Editor's note: Strain and stress are different quantities. Strain describes relative deformation; stress concerns internal force per unit area. Under uniaxial, linear elastic conditions, σ = E × ε can be used. For complex loading, a single directional strain reading may be insufficient to determine stress.

How does a load cell measure weight?

In a strain gauge load cell, gauges are attached to a metal body designed to deform predictably. Loading strains the body, changing the bridge output. Understanding how a load cell works requires considering the body, gauges and electronics together. In weighing applications, the displayed mass is obtained through suitable calibration.

mV/V expresses bridge output relative to excitation voltage. A load cell rated at 2 mV/V, excited at a permitted 10 V, produces approximately 20 mV of signal change from its zero-load output at rated load. This is the raw bridge signal, not a 0–10 V output. Different mechanical applications require suitable load cell types and compatible weighing controllers.

Pressure and torque measurements

Pressure slightly deforms a diaphragm; resistive elements bonded to it or integrated into its structure detect that deformation. A pressure transmitter processes the signal and, depending on the model, produces a standard output. In a differential pressure sensor, the pressure difference between two sides can load the sensing element. Pressure sensors do not all use the same technology; mechanical pressure gauges also operate differently.

Torque measurements use torsional strain in a shaft. Gauges are positioned at suitable angles, and a rotating shaft also requires an appropriate signal transfer arrangement. On presses, frames, tanks and supporting structures, local strain measurements help investigate load distribution and design behaviour.

Gauge types, rosettes and nominal resistance

Metal foil is a common construction; wire gauges serve some specialised applications. Semiconductor strain gauges can exploit a strong piezoresistive effect. Piezoresistive refers to resistance changing with mechanical strain; piezoelectric refers to electrical charge generated by mechanical loading. The terms are not interchangeable.

Strain rosettes contain multiple grids measuring different directions in the same region. They help investigate loading that one directional reading cannot fully describe. 120 Ω, 350 Ω and 1000 Ω are common nominal resistances. Selection also depends on grid length, specimen material, temperature range, adhesive and compatibility with the measurement module.

Why temperature and installation matter

Temperature changes affect both gauge resistance and expansion of the specimen. Compensation matched to the specimen material, suitable bridge wiring and proper lead connections reduce errors. An appropriate three-wire connection can reduce lead-resistance effects in quarter-bridge measurements. Excessive excitation can cause self-heating and zero drift.

Surface preparation, gauge alignment, adhesive curing, lead strain relief and moisture protection are part of the measurement system. A poor bond prevents strain from transferring correctly. Zeroing does not repair a poor installation. Accurate measurement requires suitable calibration using a reference load or strain, as well as zero adjustment.

PLC connection and practical selection

The raw bridge output is unsuitable for most standard PLC analog inputs. It usually requires a bridge amplifier, weighing transmitter or dedicated strain measurement module. Modules specifically designed for bridge sensors can accept a direct connection when the sensor and wiring configuration are supported.

The processed value may be transmitted as a 4–20 mA current signal, a 0–10 V voltage signal or digital data. These differ from the raw millivolt output; the analog signal range must match the input type. Check bridge resistance, excitation limits, mV/V rating, measurement speed and filtering together. Route small-signal wiring separately from power cables and terminate the shield according to the equipment instructions.