The most common type, the strain gauge load cell, works on the principle of the Wheatstone bridge circuit:

1. Strain Gauges: Thin, metallic foil resistors (strain gauges) are precisely bonded to the load cell’s spring element (or body).
2. Deformation: When a force (weight) is applied, the metal body bends or deforms slightly.
3. Resistance Change: This deformation causes the bonded strain gauges to stretch (increasing resistance) or compress (decreasing resistance).
4. Signal Output: These resistance changes unbalance the Wheatstone bridge circuit, creating a tiny, proportional voltage output mV that is measured by an indicator or amplifier.

Load cells and the weighing systems they are part of require periodic calibration. While the load cell itself is a stable measuring element, calibration is necessary to ensure the entire system (cell + mounting hardware + electronics) is accurate.

• Standard Practice: For commercial/industrial use, calibration is recommended at least once per year.
• High Precision/Legal-for-Trade: Applications requiring high accuracy or regulated measurements (Legal-for-Trade) may require more frequent, certified calibration (e.g., every 6 months).
• After Events: Calibration is always required after any mechanical or electrical modifications, repairs, overloads, or reinstallation.

• Rated Output (Sensitivity): This is the fundamental performance specification, usually expressed in mV/V (millivolts per volt). It is the change in the output signal voltage when the cell is loaded to its full Rated Capacity E-MAX, divided by the Excitation Voltage supplied. A common value is mV/V
• Excitation Voltage: This is the stable DC power supplied by the weighing indicator or amplifier to the load cell’s Wheatstone bridge circuit. Standard values are typically between 10 VDC and 15 VDC. The actual output signal (mV) is the product of the Rated Output and the Excitation Voltage. But nowadays few instrument with high precision IC’s & ADC also work on 3.3 VDC excitation.

• > Material: The material (e.g., Alloy Steel or Stainless Steel) determines the cell’s mechanical strength and resistance to corrosion. Stainless steel is required for hygienic, washdown, or harsh chemical environments, while Alloy Steel is sufficient for dry, controlled indoor use.
• > IP Rating: The Ingress Protection (IP) rating (e.g., IP67, IP68) indicates the sealing effectiveness of the internal strain gauges.
  • > IP67 means dust-tight and protected against temporary water immersion.
  • > IP68 means dust-tight and protected against continuous water immersion under pressure.

Creep is defined as the change in the load cell’s output signal over time (t) while the applied load remains constant and all environmental factors (temperature) are stable.

• > It is caused by the viscoelastic relaxation of the metal body under sustained stress.
• > It is usually specified as the percentage of Rated Output measured over a standard period, such as 30 minutes.
• > Criticality: A low creep specification is vital for applications like tank, silo, or hopper weighing, where a load may sit for hours or days, ensuring the displayed weight does not drift over that period.