Load cells are sensors used to measure forces and, through a properly calibrated electronic system, determine the mass of an object or material. They are essential components of industrial weighing systems and are used in scales, platforms, tanks, silos, hoppers and dosing systems.
But how do load cells work? What types are available, and which factors affect their accuracy? To understand their characteristics, it is useful to start with the physical principles of measurement and the technologies used to convert mechanical stress into an electrical signal.
What are load cells?
A load cell is a force transducer, a device that converts an applied force into a measurable signal. In the most common industrial applications, the sensor consists of a metal element designed to deform elastically when subjected to a load.
This deformation is extremely small and generally invisible to the naked eye. It is detected by sensing elements and converted into an electrical signal.
The signal is then processed by an indicator, transmitter or electronic control system, allowing the measured value to be displayed and used in weighing operations.
Load cells do not directly measure the volume of a material: they detect the force exerted by the load. Through proper calibration, the weighing system can express the result in units of mass, such as kilograms or tonnes.
How do load cells work?
The operating principle of a load cell depends on the technology used. One of the most common solutions in industrial weighing is the strain gauge load cell, which operates by measuring the elastic deformation of a metal body.
When a force is applied, the load cell body undergoes deformation proportional to the load, within its designed operating range. This deformation is detected by strain gauges, sensors whose electrical resistance changes as the material deforms.
The role of strain gauges
Strain gauges are sensing elements attached to the surface of the load cell’s metal body.
When the metal deforms, the strain gauges also undergo a change in geometry, resulting in a variation in electrical resistance. Although this variation is very small, it can be detected and converted into a usable signal.
The Wheatstone bridge
In strain gauge load cells, the gauges are generally connected in a Wheatstone bridge, an electrical circuit used to measure small changes in resistance.
When the load cell is not subjected to a load, the circuit has a particular balance condition. Applying a force changes the resistance values and produces a variation in the electrical output signal.
The signal is amplified, converted and processed by the weighing electronics. After calibration, the system associates the signal with the value of the applied load.
The measurement process
- Load application: a force acts on the load cell’s sensing element.
- Elastic deformation: the metal body deforms within its designed limits.
- Electrical variation: the strain gauges detect the deformation through a change in resistance.
- Signal processing: the electronics convert the signal into a measurable value.
- Display: the system provides a force or mass reading, depending on the application and calibration.
What do load cells measure?
From a physical perspective, load cells measure a force, expressed in newtons (N) in the International System of Units.
In weighing systems, however, the result is normally displayed as mass, expressed in kilograms (kg) or tonnes (t). This conversion is performed by the electronics according to the system’s calibration.
It is therefore important to distinguish three quantities:
- Force: the mechanical action applied to the sensor.
- Mass: the quantity of matter, normally expressed in kilograms.
- Weight: the gravitational force acting on a mass.
When weighing industrial containers and structures, the tare must also be considered. Tare is the mass of the empty container. Subtracting it from the total measured mass gives the net mass of the material.
What are the different types of load cells?
There are several types of load cells, each designed for specific operating conditions. A common classification is based on the direction of the applied force and the way the sensing element deforms.
Compression load cells
Compression load cells are designed to measure forces that compress the sensing element. They are frequently used in weighing systems for tanks, containers, silos and structures supported at multiple points.
The selection of a load cell must take into account the required capacity, load distribution and any forces acting outside the intended measurement direction.
Tension load cells
Tension load cells detect forces that tend to stretch the sensing element. They are used, for example, in suspended-load applications and systems designed to measure pulling forces.
Bending beam load cells
Bending beam load cells use the deformation of a beam subjected to an applied force. They are used in various types of scales, platforms and weighing systems.
The geometry of the load cell and its mounting arrangement are essential to obtaining reliable measurements.
Shear beam load cells
Shear beam load cells measure deformation caused by shear forces. They are used in numerous industrial applications, including weighing systems designed for relatively high capacities.
How to choose a load cell
Choosing a load cell does not depend solely on the maximum weight to be measured. The characteristics of the application, operating conditions and accuracy requirements must also be considered.
The main selection criteria include:
- Rated capacity: the load for which the sensor is designed.
- Type of loading: compression, tension, bending or shear.
- Number of support points: particularly important in systems using multiple load cells.
- Required accuracy: based on the measurement and process requirements.
- Environmental conditions: temperature, humidity, dust and potentially corrosive substances.
- Dynamic loads: vibrations, impacts and rapid changes in applied forces.
- Electronic compatibility: connection to indicators, transmitters and control systems.
The design must also account for the weight of the structure, load distribution and potential overloads. The final selection should be based on the sensor’s technical specifications and the weighing system design.
How accurate are load cells?
Load cell accuracy depends both on the sensor’s characteristics and on the conditions in which it operates.
Important technical parameters include:
- Linearity: indicates how closely the sensor’s response follows the ideal relationship between applied force and output signal.
- Hysteresis: describes the difference in response that may occur when a load is applied and subsequently removed.
- Repeatability: indicates the ability to provide consistent results when the same force is applied repeatedly under identical conditions.
- Temperature effects: temperature variations can influence the signal and the mechanical behaviour of the system.
- Calibration: establishes the relationship between the detected signal and the applied load.
The accuracy specified for an individual load cell does not necessarily correspond to the accuracy of the complete weighing system.
Uneven load distribution, rigid mechanical connections, vibrations and mounting errors can affect the result, even when the sensors themselves have high technical specifications.
How should load cells be installed and positioned?
Load cell positioning must be determined according to the structure being weighed and the forces acting on its support points.
In industrial weighing systems, sensors are generally installed at the support points defined by the design, using mounting assemblies compatible with the load cell type and operating conditions.
The design should consider:
- weight distribution across the different supports;
- the mass of the empty structure and the maximum expected load;
- possible lateral forces and accidental loads;
- pipes and mechanical connections;
- thermal expansion of the structure;
- vibrations and environmental conditions;
- inspection and maintenance requirements.
A rigid connection that has not been properly accounted for may transfer part of the load outside the intended measurement path, reducing weighing reliability.
For this reason, installation should be designed and checked by qualified personnel, following the sensor manufacturer’s instructions.
How can load cells be tested?
Load cells and their associated electronic systems require periodic checks to ensure consistent performance.
These activities may include:
- Visual inspection: checking the condition of the sensor, supports and fastening elements.
- Connection checks: inspecting cables, connectors and protective components.
- Zero verification: checking system behaviour under known conditions.
- Repeatability tests: comparing readings obtained by repeatedly applying the same load.
- Reference load checks: comparing the displayed reading with a known reference value.
It is important to distinguish testing an individual sensor from checking the entire weighing system. Measurement errors may also originate from the electronics, mechanical connections or operating conditions.
Calibration and maintenance must be performed according to the relevant procedures by qualified personnel.
How do load cells for silos work?
In silo weighing systems, load cells detect the forces transmitted through the structure’s support points. The electronic system processes the signals and, after subtracting the tare, determines the mass of the stored material.
Measurement accuracy also depends on load distribution, mechanical connections and installation conditions.
For more information about the use of sensors to monitor stored quantities and support industrial processes, read our article What are load cells used for?.
Load cells for hoppers: weighing and dosing
In industrial hoppers, load cells measure the mass of the contained material and its variations during loading and unloading operations.
These measurements can be used in gravimetric dosing systems, including applications based on measuring the loss of weight during material discharge.
Poggi in-line hoppers can incorporate electronic weighing and dosing systems with load cells and an extraction belt conveyor.
What is the difference between volumetric and gravimetric dosing?
Materials can be dosed using different measurement principles. Two common methods are volumetric dosing and gravimetric dosing.
Volumetric dosing
Volumetric dosing controls the amount of material according to the volume delivered or operating parameters related to the flow rate, such as the operating time of a discharge device.
It is generally used when the material has relatively consistent characteristics and the process does not require direct measurement of the mass being discharged.
Gravimetric dosing
Gravimetric dosing uses measured mass to control the amount of material delivered.
In loss-in-weight systems, the material is dosed by measuring the decrease in the mass of the container during discharge.
This principle makes it possible to monitor the actual quantity discharged, even when the material’s bulk density varies.
The choice between the two methods depends on the characteristics of the material, the required accuracy and the needs of the process.
Load cells and industrial weighing systems
Load cells are an essential technology for force measurement and the development of industrial weighing systems.
Selecting the appropriate sensor type, correct sizing, calibration and installation conditions are all important for obtaining reliable measurements.
Poggi designs and manufactures products for bulk material storage and handling, which can be configured with weighing and dosing systems according to specific operating requirements.
Available solutions include primary hoppers and in-line hoppers, which can incorporate load cells and systems for monitoring material quantities.
For further information about available configurations and integration options, contact Poggi.
Frequently asked questions about load cells
Do load cells measure weight or mass?
Load cells detect force. In weighing systems, the electronics process the signal to determine and display a mass value after proper calibration.
What are the most common types of load cells?
Common industrial weighing solutions include strain gauge load cells designed for compression, tension, bending and shear. The choice depends on the structure’s geometry, the direction of the applied force and the required capacity.
How is load cell capacity determined?
Rated capacity must be selected by considering the maximum expected load, the mass of the structure, force distribution, the number of supports and potential overloads. Proper sizing requires a technical assessment of the complete system.
How many load cells are needed for a silo?
The number depends on the structural configuration and the silo’s support points. There is no universal number: the sensors and their mounting assemblies must be defined during the design phase.
Can load cells be used to dose powders?
Yes. When integrated with discharge and control systems, load cells can support gravimetric dosing of powders and other bulk materials by measuring the quantities discharged.
What factors can affect load cell accuracy?
Major factors include temperature, vibrations, lateral loads, uneven force distribution, unsuitable mechanical connections and calibration problems.
