---
title: "Short note: Strain gauge"
subject: "Basic Electronics Engineering"
chapter: 7
key: "semester_2/basic_electronics_engineering:short-note-strain-gauge"
source: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/short-note-strain-gauge
site: BCT Engineering
---

# Short note: Strain gauge

Basic Electronics Engineering · Chapter 7: Application of Electronic System

- Asked in: 2073 Magh, 5 marks; 2071 Bhadra, 4 marks; 2070 Bhadra, 5 marks; 2069 Poush, 4 marks; 2068 Magh, 4 marks; 2067 Mangsir, 6 marks (asked 6 times)
- Video: [Electronic Basics #33: Strain Gauge/Load Cell and how to use them to measure weight](https://www.youtube.com/watch?v=lWFiKMSB_4M) — GreatScott!
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/short-note-strain-gauge
- Whole chapter: https://www.bctengineer.com/md/semester_2/basic_electronics_engineering/chapter-7

## Question

Write a short note on strain gauge (strain gauge transducer, application circuit of strain gauge).

## Short answer

A strain gauge is a resistive transducer whose resistance changes with strain, with gauge factor typically 2 for metal foil. The tiny resistance change is measured using a Wheatstone bridge and instrumentation amplifier, commonly applied in load cells and pressure sensors.

## Answer

A **strain gauge** is a resistive transducer whose electrical resistance changes when it is stretched or compressed. It is used to measure strain, and from that, force, pressure, weight and torque.

#### Principle

When a conductor is stretched, its length increases and its cross-section reduces, so the resistance R = ρL/A increases. The gauge factor is

GF = (ΔR/R)/(ΔL/L) = (ΔR/R)/ε

where ε is the strain. For metal foil gauges, GF is about 2.

#### Construction

```text
 Backing (plastic film)
  +----------------------+
  |  ____    ____    __  |
  | |    |  |    |  |  | |  <- fine metal-foil grid
  | |    |__|    |__|  | |
  +--o------------------o+
    leads (soldered)
```

A thin zig-zag grid of constantan foil is bonded to a flexible backing, which is cemented onto the surface being tested. Typical resistance: 120 Ω or 350 Ω.

#### Application circuit (Wheatstone bridge)

The tiny change in resistance (typically ΔR/R ≈ 0.1 %) is measured using a Wheatstone bridge, followed by an instrumentation amplifier.

```text
           +Vs
        +---+---+
        |       |
        R1      Rg  (active strain gauge)
        |       |
   A ---+       +--- B  -> instrumentation
        |       |          amplifier -> ADC/meter
        R2      R3
        |       |
        +---+---+
           GND
```

Initially the bridge is balanced (output 0). When the gauge is strained, the bridge becomes unbalanced, and the output is Vo ≈ Vs·ΔR/(4R) for one active arm. The amplifier raises it to a measurable level, which is shown on a meter or sent to an ADC. A dummy gauge in the adjacent arm compensates for temperature changes.

Applications: load cells (weighing machines), pressure gauges, torque and vibration measurement, and testing of bridges and structures.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why does the resistance of a strain gauge increase when stretched?
   A. Length increases and cross-section decreases, so R = ρL/A rises
   B. Resistivity becomes zero
   C. Length decreases and area increases
   D. The temperature falls
   Answer: A. Stretching increases L and reduces A, so R = ρL/A increases. (Key point: Principle of strain gauge)

2. The gauge factor of a strain gauge is defined as:
   A. ΔL/L divided by ΔR/R
   B. ΔR divided by R only
   C. (ΔR/R) divided by the strain ε
   D. Strain multiplied by resistance
   Answer: C. GF = (ΔR/R)/(ΔL/L) = (ΔR/R)/ε; for metal foil gauges it is about 2. (Key point: Gauge factor)

3. Why is a Wheatstone bridge with an instrumentation amplifier used with a strain gauge?
   A. The resistance change is tiny (about 0.1 %), so the bridge detects it as an output voltage that is then amplified
   B. The gauge produces its own large voltage
   C. The bridge converts AC to DC
   D. It increases the gauge factor
   Answer: A. ΔR/R is very small, so a bridge converts it to a small voltage which the amplifier raises to a measurable level. (Key point: Why a Wheatstone bridge)

4. How is temperature effect compensated in the bridge?
   A. By using a larger supply voltage
   B. By using a dummy gauge in the adjacent arm
   C. By using a dummy gauge in the opposite arm only
   D. By removing the amplifier
   Answer: B. A dummy (unstrained) gauge in the adjacent arm changes equally with temperature and cancels its effect. (Key point: Temperature compensation)

## Related in other subjects

- [Ballast circuit for a strain gauge](https://www.bctengineer.com/question.md?key=semester_4%2Finstrumentation_and_measurement%3Aballast-circuit-strain-gauge-sensitivity) — Instrumentation and Measurement, chapter 6
- [Gauge factor of resistance strain gauge](https://www.bctengineer.com/question.md?key=semester_4%2Finstrumentation_i%3Astrain-gauge-factor) — Instrumentation I, chapter 3
- [Derivation of strain gauge factor](https://www.bctengineer.com/question.md?key=semester_4%2Finstrumentation_and_measurement%3Astrain-gauge-factor-derivation) — Instrumentation and Measurement, chapter 6
- [Elastic transducers and strain gauge load cells](https://www.bctengineer.com/question.md?key=semester_4%2Finstrumentation_and_measurement%3Aelastic-force-transducers-load-cells) — Instrumentation and Measurement, chapter 7