Load Cells vs Strain Gauges — Which One Do You Need?

Q: Which type of load cell should I select—strain gauge, VW, or hydraulic?

Strain-gauge load cells are good for static to moderate dynamics and easy to read. VW load cells are stable over long cables and years, ideal for long-term anchor/strut monitoring. Hydraulic/pressure cells are rugged, suited for distributed pressure and harsh conditions.

Q: What factors should guide my choice between strain gauges and load cells?

Key factors include measurement target (strain/stress vs force), installation constraints (retrofit vs inserting in load path), time scale (dynamic vs static), signal path (short vs long cables), accuracy, environment, durability, and budget.

Q: What are common mistakes when using strain gauges or load cells?

Common mistakes include using strain gauges when direct force is required for acceptance, installing load cells without proper alignment or seating, ignoring temperature effects, taking initial readings too early in fresh concrete, and running long unshielded resistive-bridge cables without sense lines.

In civil and geotechnical monitoring, strain gauges and load cells solve different problems. Strain gauges measure deformation of a structural element (microstrain). Load cells measure force directly (kN/ton). Choosing correctly depends on the measurement target, installation constraints, signal path, time scale (static vs dynamic), and required accuracy and maintainability. This article gives plain, actionable guidance for instrumentation designers, asset owners, contractors, and consultants.

What a Strain gauge measures (and why)?

A strain gauge reports local deformation of a member. Readings are usually in microstrain (µε). With material properties and section data, strain can be converted to stress and, if needed, to internal force. Strain gauges are the right tool when you need:

Distributed information (strain profile along a beam, pile, tunnel lining).

Local peaks near details (weld toes, re-entrant corners).

Dynamic response (traffic-induced vibrations, modal tests).

Minimal intrusion (retrofit on existing members without breaking the load path).

How strain gauges work (practical view)?

Foil resistance strain gauges use a thin metal grid bonded to the structure. Deformation changes electrical resistance slightly. A Wheatstone bridge converts this tiny change to a differential voltage. With proper excitation, shielding, and amplification, you get stable micro-strain. A half or full bridge arrangement improves temperature behavior and sensitivity.

Vibrating wire (VW) strain gauges use a tensioned wire whose resonant frequency changes with deformation of the housing. The readout “plucks” the wire and reports frequency (or f²) that is internally converted to µε via the supplied calibration. VW is favored for long cables, harsh sites, and multi-year stability. Response is slower than foil, so VW is not for high-speed dynamics.

Key implications

Foil: excellent bandwidth; needs careful surface prep, moisture protection, and low-noise acquisition.

VW: excellent long-term stability and noise immunity; needs VW readout; lower sampling rate.

What a load cell measures (and why)?

A load cell reports force directly. You install it in the load path (in series) so that the load passes through the sensor. Typical uses:

Anchor and tieback force at the head.

Strut and prop load in excavations.

Pile and jack load in proof tests.

Support reactions under bearings or temporary jacks.
When you need a number in kN without assumptions about section properties or modulus, use a load cell.

How load cells work (practical view)?

Strain-gauge load cells: a machined metal element with multiple foil gauges wired in a full bridge. As the element elastically deforms under load, the bridge output (mV/V) scales with force. Most units are factory-calibrated, temperature-compensated, and sealed.

Vibrating wire load cells: force changes pressure or flexure that a VW transducer senses as frequency. Output is stable over long cables and years; well suited to remote monitoring.

Hydraulic/pressure-cell style: force creates a fluid pressure change read by a sensor. Used where load distribution and ruggedness matter more than high-rate dynamics.

Key implications

Strain-gauge load cells: easy to read with standard bridge instruments; good for static and moderate dynamics.

VW load cells: best for long cable runs and low drift; sampling is intermittent; ideal for long-term anchor/strut monitoring.

Selection criteria (use this like a checklist)

1. Measurement target

Need force (kN) in a discrete element (anchor, strut, jack, bearing)? → Load cell.

Need strain or stress distribution in a member (pile along depth, girder top/bottom, lining ring)? → Strain gauges.

2. Installation constraints

Can you insert a device in the load path and provide flat bearing plates with alignment? → Load cell is feasible.

Structure cannot be separated or space is tight; retrofit is needed without interrupting load path → Strain gauges.

3. Information density

Many points along/around a member → Strain gauges scale economically.

Few critical points where absolute force matters → Load cells.

4. Time scale and dynamics

Dynamic events (modal, impact, traffic): Foil strain gauges (and strain-gauge load cells with adequate bandwidth) are suitable; VW is not for high-rate continuous dynamics.

Static/slow trends (seasonal, consolidation): VW strain gauges/load cells excel.

5. Signal path and distance

Long cable runs, high EMI, remote loggers → Prefer VW (strain or load cell). If using resistive bridges over distance, use shielded cable, 6-wire sense lines, and nearby conditioners.

6. Accuracy and traceability

Direct, traceable force with factory calibration → Load cell.

Derived force from strain is fine if section properties are known and verified → Strain gauges.

7. Environment and durability

Wet, buried, shotcrete, or pour → VW embedment strain gauges or rugged load cells/pressure cells.

Exposed steel surfaces → Weldable/spot-weldable strain gauges; protect cabling and coatings.

8. Operations and maintenance

Need swappable sensor later? Load cells can sometimes be replaced when unloaded. Foil strain gauges once bonded are not easily replaceable.

Long-term monitoring with limited site access → VW instruments reduce drift and recalibration frequency.

9. Budget

Many measurement points → Strain gauges (especially foil) are cost-effective per point.

Few high-value points where decisions hinge on a force threshold → Load cells are justified.

Typical civil and geotechnical use cases

Strain gauges

Bridge girders: bending strain top/bottom flanges; fatigue hot spots.

Piles/shafts: embed multiple gauges along depth to derive load transfer.

Tunnel linings and segmental rings: strain at crown/invert; ring joints.

Slabs and walls: crack risk and thermal strain.

Dynamic studies: traffic, wind, machinery; modal/FFT tests.

Load cells

Anchors/tiebacks/soil nails: center-hole units at anchor heads.

Bracing/props in excavations: compression load cells inline with struts.

Pile load testing and jack proofing: compression/tension load cells with reaction frames.

Support reactions at bearings: under-bearing load cells during maintenance or load redistribution.

Contact pressures: earth/rock pressure cells where force per area is of interest.

Comparison table (at a glance)

Aspect	Strain gauges	Load cells
What it measures	Local strain (µε)	Force/load (kN)
Output	Foil: mV bridge; VW: frequency	Strain-gauge: mV/V; VW: frequency
Best for	Distributed behavior, local peaks, dynamics	Direct force at discrete points
Install	Bond/weld/embed on member; minimal intrusion	Insert in load path with bearing/alignment
Cabling	Foil needs good shielding/shorter runs; VW tolerant of long runs	6-wire bridges mitigate cable effects; VW tolerant of long runs
Long-term	Foil needs protection; VW very stable	Good; VW very stable; periodic checks
Dynamics	Foil: excellent; VW: limited	Strain-gauge types: good; VW: limited
Cost per point	Low (foil), moderate (VW)	Moderate to high

Implementation pointers (reduce rework and noise)

Design

Define the quantity first (strain vs force). Don’t infer force from strain if a traceable force is required for acceptance.

For distributed questions (e.g., pile load transfer), plan multiple strain stations instead of one load cell at the head.

Mechanical

Load cells: provide flat, parallel bearing surfaces; add load distribution plates; control eccentricity; align center-hole units carefully; specify capacity with headroom (e.g., 120–150% of expected max).

Strain gauges: enforce surface prep (degrease/abrade/clean), alignment to the principal axis, and environmental protection (coatings, sealants, covers).

Electrical

Use twisted, shielded pairs; separate sensor and power cables; bond and ground per site practice.

For bridges/long runs: 6-wire sense for resistive bridges or prefer VW for distance/noise immunity.

Verify polarity and perform continuity and insulation checks before closing works.

Data

Store Initial Reading (IR) after stabilization (especially in concrete); analyze Δ from IR.

Log temperature with every read; apply compensation or at least trend with temperature.

Set thresholds and hysteresis for alarms (pre-warning, warning) based on design limits.

QA/commissioning

Bench test each channel before installation. For load cells, confirm zero and span under known loads. For strain gauges, use controlled bending/axial checks.

Document calibration factors and wiring maps; label cables and junctions.

Selection scenarios (quick decisions)

1. New retaining wall with pre-stressed anchors; owner wants assurance loads stay within limits.
Install center-hole load cells at anchor heads (VW if long cables/remote). Add a few strain gauges on the wall or waler to contextualize behavior.

2. Bridge girder with suspected fatigue hotspot.
Use foil strain gauges near the detail to capture local cycles and do modal/FFT if needed. A load cell is not useful unless you are measuring jack forces.

3. Deep excavation with cross-lot struts; excavation staging over months.
Install compression load cells on critical struts (VW for long-term stability). Add settlement/tilt and a few lining strain gauges to correlate system behavior.

4. Instrumented pile to study load transfer.
Embed strain gauges at multiple depths in the cage (VW for long-term). A single head load cell gives total head load only; it cannot resolve shaft/base distribution.

5. Dam monolith concrete behavior over years.
Use VW strain gauges embedded at key elevations and anchor load cells where anchors are present. Prioritize long-term stability and long cable runs.

Common mistakes to avoid

Choosing strain gauges when absolute force acceptance is required. Use a load cell.

Installing a load cell without flat, aligned seating and adequate distribution plates; expect drift and scatter if you skip this.

Ignoring temperature; trends can be thermal rather than structural.

Taking IR too early in fresh concrete; wait for stabilization.

Long, unshielded runs of resistive bridges without sense lines or local conditioning.

Under-sizing capacity or operating near limit; plan overload margin.

Use strain gauges when you need deformation information, distributed insight, and dynamic response with minimal intrusion. Use load cells when you need direct, traceable force at discrete points and can place a sensor in the load path. For civil and geotechnical work, combining both—strain for behavior, load cells for force control—often yields the most reliable decisions with the least ambiguity.

FAQs

1. What is the main difference between strain gauges and load cells?
Strain gauges measure deformation (microstrain) of a structural element, while load cells measure force directly in kN or tons. The choice depends on whether you need distributed strain information or absolute force.

2. When should I use a strain gauge instead of a load cell?
Use strain gauges when you need distributed data (strain profile along piles, beams, tunnel linings), local peak measurements, dynamic response (traffic, vibration, modal tests), or retrofit monitoring without interrupting the load path.

3. How do foil and vibrating wire (VW) strain gauges differ?
Foil strain gauges provide excellent bandwidth and dynamic response but need careful installation, shielding, and moisture protection. VW strain gauges offer long-term stability, noise immunity, and tolerance to long cable runs but are slower and unsuitable for high-frequency dynamics.

4. What does a load cell measure, and how is it installed?
A load cell measures force directly by being placed in the load path. It requires proper seating with flat bearing surfaces, alignment, and load distribution plates. It is best for anchors, struts, jack loads, and bearing reactions.

5. Which type of load cell should I select—strain gauge, VW, or hydraulic?

Strain-gauge load cells: good for static to moderate dynamics, easy to read.
VW load cells: stable over long cables and years, ideal for long-term anchor/strut monitoring.
Hydraulic/pressure cells: rugged, suited for distributed pressure and harsh conditions.

6. What factors should guide my choice between strain gauges and load cells?
Key factors include:

Measurement target (strain/stress vs force)
Installation constraints (retrofit vs inserting in load path)
Time scale (dynamic vs static)
Signal path (short vs long cables)
Accuracy, environment, durability, and budget.

7. Can strain readings be converted to force?
Yes, strain can be converted to stress and internal force if material properties and section data are known. However, if you need direct, traceable force values, a load cell is the better choice.

8. What are common mistakes when using strain gauges or load cells?

Using strain gauges when direct force is required for acceptance.
Installing load cells without proper alignment or seating.
Ignoring temperature effects on readings.
Taking initial readings too early in fresh concrete.
Running long unshielded resistive-bridge cables without sense lines.

9. Which technology is best for long-term monitoring in harsh environments?
Vibrating wire strain gauges and VW load cells are preferred. They offer low drift, immunity to noise, and tolerance to long cable runs, making them ideal for buried, shotcrete, or remote installations.

10. Can strain gauges and load cells be combined in one project?
Yes. Using both often yields the best results: strain gauges provide distributed behavioral insights, while load cells deliver direct force measurements at critical points. Together, they reduce ambiguity and improve decision-making.