Strain Gauges Explained: How They Work and Measure Strain Accurately

• A thin metal pattern is bonded to the surface. 

• When the host material strains, the electrical resistance R of the foil changes slightly. 

• Sensitivity is described by Gauge Factor (GF): 

• Laboratory work, machinery tests, modal/FFT studies. 

• Short to moderate cable runs with good shielding. 

• High sample rates (hundreds to thousands of samples per second). 

• A high-tension wire is anchored between two end blocks inside the sensor. 

• Structural strain moves the blocks, changing the wire tension and therefore its natural frequency. 

• The readout briefly excites the wire and measures frequency; strain is computed internally. In practice, strain is proportional to frequency squared and the instrument shows µε directly using the supplied calibration constant. 

• Civil and geotechnical structures (dams, tunnels, piles, struts, bridge components). 

• Long cable runs and sites with electrical noise. 

• Long-term monitoring (months to years), often with an integrated temperature sensor. 

• Quarter bridge: 1 active gauge, 3 fixed resistors. Simple, but more sensitive to temperature. 

• Half bridge: 2 active gauges. Better temperature behavior; common on bending members (one in tension, one in compression). 

• Full bridge: 4 active gauges. Highest sensitivity and best temperature handling when arranged correctly. 

• The logger excites the wire, measures the resonant frequency f, and converts it to µε using a batch calibration constant supplied with the sensor. 

• Many VW systems express calibration as “µε per digit,” where a digit is proportional to f2. 

Foil (ER): 

• Degrease → abrade → degrease → dry. 

• Align the gauge with the target strain direction. 

• Bond using the specified adhesive and pressure/time. 

• Protect with moisture/impact barriers (coatings, covers). 

VW on steel: 

• Install mounting blocks as per procedure (weld/spot-weld/grout). 

• Use alignment jigs and a dummy gauge where specified. 

• Clamp to the recommended torque; avoid bending the body. 

VW embedded in concrete: 

• Fix flanges firmly. 

• Route and protect cables. 

• During pour, ensure good consolidation around the sensor (no voids). 

• The gauge axis must match the direction you want to measure. 

• If directions are uncertain, use a strain rosette (two or three gauges at known angles) to compute principal strains. 

• Always record temperature with strain. 

• Foil gauges: use half/full bridge or a dummy gauge for compensation. 

• VW: the built-in thermistor supplies temperature; software can store and use it. 

• After installation, wait for readings to stabilize (especially in concrete while it cures). 

• Take an Initial Reading (IR) and then work in Δstrain = current − IR. 

• This cancels fixed offsets and improves comparability over time. 

• Use shielded, twisted pairs. 

• Keep power cables away from sensor cables. 

• Use proper grounding and surge/lightning protection for exposed sites. 

• Check continuity and insulation resistance before covering cables. 

• Equation is Young’s modulus (project-specific). 

• For multi-axial stress states, use rosettes and include Poisson’s ratio ν to compute principal values. 

Read more: Strain Gauge Type Load Cell- Introduction & How it Works?

• Setting IR too early: wait for stabilization (especially concrete). 

• Poor surface prep (ER): leads to drift, noise, or debonding. 

• Misalignment: gives lower or misleading readings; use jigs and marks. 

• Ignoring temperature: always log it; apply compensation if needed. 

• Cable issues: protect against cuts, kinks, water ingress; test before cover. 

• Using generic calibration: use the batch factor from your sensor certificate. 

• Sampling rate: 

• ER dynamics: pick a rate that captures content (e.g., ≥200 Hz for traffic; higher for impacts). 

• VW monitoring: minutes to hours; add faster schedules during pours, jacking, or preloads. 

• Context logging: record load steps, environmental notes, and times. 

• Thresholds: set pre-warning and alarm bands in µε based on design limits. 

• Trending: plot Δµε vs time with temperature overlays to recognize seasonal or diurnal effects. 

FAQs

1. What is strain, and why is it important to measure?

Strain is the amount a material stretches or compresses compared to its original length. Measuring strain helps engineers understand how a structure responds to loads, estimate stress, verify service limits, and monitor changes over time.

2. What are the main types of strain gauges?

The two most common types are:

• Electrical-Resistance (ER or foil) strain gauges: These change electrical resistance when strained.
• Vibrating-Wire (VW) strain gauges: These measure strain via the frequency change in a tensioned wire.

3. How do Electrical-Resistance (foil) strain gauges work?

A thin metal foil is bonded to the surface of the material. As the material strains, the foil’s electrical resistance changes. This change is measured through a Wheatstone bridge circuit and converted to strain.

4. How do Vibrating-Wire (VW) strain gauges work?

VW gauges have a tensioned wire whose frequency changes when strained. The sensor’s readout measures this frequency and converts it into strain using a calibration constant. Strain is proportional to the square of the frequency.

5. Which strain gauge type should I use: ER or VW?

• Use ER (foil) for lab tests, dynamic measurements, and short-term projects.
• Use VW for long-term field monitoring, noisy environments, or when long cable runs are needed.

6. How does temperature affect strain measurements?

Temperature changes can affect strain readings:

• ER gauges require compensation via dummy gauges or full/half bridges.
• VW gauges often include built-in thermistors for automatic temperature compensation.

7. What are common installation mistakes to avoid?

• Taking initial readings too soon (especially on curing concrete)
• Poor surface prep for ER gauges
• Misalignment of gauges
• Ignoring temperature effects
• Improper cable handling or protection
• Using generic calibration instead of sensor-specific values

8. What is a strain rosette, and when is it used?

A strain rosette combines multiple gauges at known angles to measure strain in uncertain directions. It allows you to calculate principal strains when strain direction isn’t known beforehand.

9. How do you convert strain to stress?

When material behavior is elastic and loading is uniaxial:
Stress = E × Strain, where E is Young’s modulus.
For complex stress states, use strain rosettes and include Poisson’s ratio in calculations.

10. What are good data practices for strain measurements?

• Use suitable sampling rates (e.g., ≥200 Hz for ER in dynamic tests)
• Log initial readings after stabilization
• Record context (e.g., load events, temperatures)
• Apply thresholds for warnings/alarms
• Monitor trends over time, especially for VW data in long-term projects