An electronic scale looks direct: place an object on it and the display shows grams or kilograms.
The sensor does not directly see mass. It senses tiny deformation caused by force, then calibration converts that signal into a weight or mass reading.
The first model is: gravity creates load, load deforms an elastic element, strain gauges turn deformation into resistance change, a Wheatstone bridge converts that into tiny voltage, and calibration maps the voltage to weight.
Object weight
-> Elastic element deforms slightly
-> Strain gauge resistance changes
-> Wheatstone bridge outputs tiny differential voltage
-> Amplifier and ADC
-> Calibration converts to weight
Force First, Mass Later
An object experiences gravitational force:
F = m * g
The load cell senses a signal related to F. In ordinary scales, g is treated as constant, so the calibrated result can be shown in kg or g.
High-precision systems must consider local gravity, leveling, air buoyancy, temperature, and mechanical structure.
The Elastic Element
A load cell contains a carefully designed metal elastic element. It is shaped so force creates predictable strain in specific regions.
Common structures include:
- Single-point
- Cantilever beam
- S-beam
- Column
- Spoke
The deformation is tiny and usually invisible. The design should be linear, repeatable, recover after unloading, tolerate eccentric loading, and avoid permanent deformation.
If the elastic element is overloaded or shock-loaded into plastic deformation, the scale may become permanently wrong.
Why Strain Gauge Resistance Changes
A strain gauge is a sensitive resistor bonded to the elastic element. When stretched or compressed, its resistance changes slightly.
Elastic deformation
-> Strain gauge stretches or compresses
-> Resistance changes
The change is very small, so load cells usually use a Wheatstone bridge.
Why the Wheatstone Bridge Is Common
No load: bridge balanced -> near-zero output
Load: resistance changes -> bridge imbalance -> tiny differential voltage
Load-cell sensitivity is often specified in mV/V. For example, a 2 mV/V load cell with 5 V excitation produces about:
2 mV/V * 5 V = 10 mV
at full scale. That is why a low-noise amplifier and high-resolution ADC are required.
Amplifier and ADC Matter
The bridge output is a tiny differential signal. A practical scale needs:
- Stable excitation voltage
- Low-noise instrumentation amplifier
- High-resolution ADC
- Good routing and shielding
- Digital filtering
- Zero and span calibration
If the load-cell output is specified in mV/V, excitation variation affects output. Ratiometric measurement, where the ADC reference tracks bridge excitation, can cancel part of that variation.
Calibration and Tare
Real systems have dimensional tolerances, gauge bonding variation, adhesive stress, amplifier offset, ADC offset, and mechanical preload. Theory alone is not enough.
Calibration usually includes:
- Zero point
- Span using known weights
- Optional multi-point calibration
- Temperature compensation
Tare simply subtracts the current load as a new zero:
Place container -> set to 0
Add item -> show net weight
It is an algorithmic offset, not a physical change in the sensor.
Why Eccentric Load Makes a Scale Wrong
An ideal scale gives the same reading no matter where the object is placed. Real scales can have eccentric-load error if force paths change with position.
Causes include:
- Poor load transfer design
- Uneven multi-sensor support
- Uneven base
- Mechanical friction or binding
- Enclosure touching the pan
- Screw preload differences
The scale measures the whole force-transfer structure, not one isolated sensor element.
Why Creep Makes Readings Drift
Creep is slow reading change under constant load:
Load applied
-> reading reaches a value
-> reading slowly drifts
Elastic elements, adhesive, gauges, and structures can all contribute.
Unload recovery can also be slow, which appears as hysteresis or zero return delay.
Temperature Drift and Zero Drift
Temperature affects gauge resistance, elastic modulus, bridge zero, amplifier offset, ADC offset, and mechanical stress. Good bridge design and compensation reduce, but do not remove, temperature effects.
Eccentric load, creep, and temperature drift are separate problems even if they all look like “the scale is not stable”. Eccentric load is position-dependent. Creep is time-dependent under constant load. Temperature drift follows thermal conditions.
Installation and Overload
Load cells are sensitive to mechanical installation:
- Uneven mounting surface
- Wrong load direction
- Side forces and torque
- Cable pulling on the cell
- Housing interference
- Overload and impact
The scale measures the whole force path, not just the sensor element.
Overload and shock are especially dangerous. If the elastic element is permanently bent, software calibration may hide the zero error temporarily, but linearity and repeatability are damaged.
Industrial scales often use overload stops, flexible connections, leveling, and side-force protection for this reason.
Engineering Takeaway
An electronic scale is not a direct mass sensor.
mass -> gravity
gravity -> elastic deformation
deformation -> strain-gauge resistance change
resistance change -> bridge voltage
bridge voltage -> ADC reading
ADC reading -> calibrated weight
If a scale is inaccurate, do not only blame the chip. Elastic element, mounting, force path, eccentric load, creep, temperature, amplifier, ADC, and calibration all belong to the measurement chain.