Types Of Weighing Scales Based On Their Working Principles

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Industries from pharmaceuticals to food rely on weighing scales for accurate and reliable weight measurement.The global market for industrial weighing equipment was worth $2.5 billion in 2022 and is expected to grow to $3.6 billion by 2030.

This growth is due to move towards advancements like Industry 4.0 and the Internet of Things (IoT) and automated weighing solutions.

Whether you work in manufacturing, logistics, retail, or R&D, understanding your weighing scales will help you ensure accuracy, compliance, and efficiency in your operations.

In this article, we will look at the different kinds of weighing scales and how they work.

Spring-Based Weighing Scales

Spring scales (or hanging scales) operate on Hooke’s Law. Hooke’s Law states that the force exerted by a spring is directly proportional to its displacement. But what does this mean for spring scales?

In simple words, the weight of an object causes the spring to stretch, and the amount of stretch is proportional to the object’s weight. When an item is attached to the scale’s hook, its weight causes the spring to stretch.

The amount of stretch corresponds to the weight of the item. The scale features a calibrated dial, scale, or digital display that converts this stretch into a weight reading.

Components of a Spring Scale

Hook or Platform: The part where the item to be weighed is placed or hung.
Spring: The primary component that stretches or compresses in response to the weight.
Scale or Indicator: The part that shows the weight measurement, either through a pointer or a digital readout.
Housing: The casing that holds all the components together and provides structure to the scale.

How These Components Interact

Placing the Item: When you place or hang an item on the hook or platform, its weight applies a force to the spring.
Stretching the Spring: The force of the item’s weight causes the spring to stretch (or compress if it’s a compression spring).
Movement of the Scale or Indicator: As the spring stretches, it moves the scale or indicator. This might be a pointer that moves along a calibrated scale. In digital spring scales, sensors measure the stretch and convert it into an electrical signal.
Measuring the Stretch: The scale or indicator measures how much the spring has stretched. The amount of stretch corresponds to the weight of the item.
Displaying the Weight: Finally, the weight is shown on the scale. In mechanical scales, you read the value where the pointer stops. In digital scales, the weight is displayed on a screen.

Advantages of Spring Scales

Simple Design: Spring scales have a straightforward design that makes them easy to use and maintain.
Cost-Effective: They are generally less expensive than other scales, such as electronic or balance scales.
Portable: Due to their simplicity, they are often lightweight and easy to move, making them great for tasks that require mobility.
No Need for Electricity: Spring scales operate mechanically, so they don’t require electricity or batteries.

Disadvantages of Spring Scales

Limited Capacity: These scales usually have a lower capacity, making them unsuitable for heavy-duty weighing.
Wear and Tear: The mechanical components, especially the spring, are susceptible to wear and tear, which can degrade accuracy over time.
Material Fatigue: Repeated stretching and contracting weaken the spring, a process known as material fatigue. As the spring fatigues, it will not return to its original length, leading to less accurate measurements.
Permanent Deformation: If the spring is overloaded (or if the weight is loaded suddenly), it can stretch beyond its elastic limit, causing permanent deformation. This means the spring won’t fully return to its original shape and won’t measure weight correctly.
Loss of Elasticity: Springs lose elasticity with prolonged use. This means the spring won’t stretch as predictably and give a wrong reading.
Not Ideal for High Precision Tasks: For tasks requiring high precision, such as in a laboratory setting, spring scales may not provide the needed accuracy.

How Do Modern Spring Scales Counter These Challenges?

The springs are made of music wire and 302 stainless steel to withstand repeated use without significant wear or deformation.
Some spring scales have dampers or shock absorbers to minimise the impact of sudden loads and reduce wear on the springs.
Spring scales need regular calibration of the scales to ensure ongoing accuracy.
Some manufacturers apply protective coatings to prevent rust and corrosion.

Accuracy And Precision

Typical spring-based weighing scales have an accuracy of about 1-2% of the measured weight. For instance, a spring scale with a 50 kg capacity would be accurate within ±0.5 to 1 kg. Many spring scales have a precision of around ±0.3% of their maximum capacity.

Load Cell-Based Weighing scales

Load cell-based weighing scales work by converting mechanical force (weight) into an electrical signal. When an item is placed on the scale, the load cell deforms slightly under the weight and outputs an electrical signal.

A simple load cell consists of four strain gauges. Strain gauges are small sensors that alter their resistance when stretched or compressed. The load cell strain gauges are arranged in a balanced Wheatstone Bridge configuration. When the load is applied, the resistance of the strain gauges changes, and the Wheatstone Bridge gets unbalanced.

This produces a small electrical signal, amplified and processed by a microprocessor to calculate the weight. The weight is then displayed on a screen, ensuring accurate and reliable measurements.

Strain Gauge: A strain gauge is a sensor whose resistance varies with applied force. It is a small, thin, and flexible device that can be attached to various materials.

Wheatstone Bridge Circuit: A Wheatstone Bridge Circuit is an electrical circuit used to measure unknown electrical resistances by balancing two legs of a bridge circuit.

Components of a Load Cell-Based Weighing Scale

Platform: The flat surface where the item to be weighed is placed.
Load Cell: The primary sensor that detects the weight of the item.
Strain Gauges: Components within the load cell that deform under weight and change their electrical resistance.
Wheatstone Bridge Circuit: A circuit used to measure the electrical resistance changes in the strain gauges.
Amplifier: Boosts the electrical signal from the load cell.
Microprocessor: Processes the amplified signal and calculates the weight.
Display Screen: Shows the weight measurement to the user.

How These Components Interact

Placing the Item: When you place an item on the platform, its mass creates a force that is transmitted to the load cell.
Deforming the Load Cell: The load cell contains strain gauges bonded to it. As the load cell deforms under the weight of the item, the strain gauges also deform.
Changing Electrical Resistance: The deformation of the strain gauges causes a change in their electrical resistance. This change is very small but crucial for measuring the weight.
Measuring Resistance Change: The Wheatstone Bridge Circuit, integrated within the load cell, measures these small changes in resistance very accurately.
Generating Electrical Signal: The changes in resistance create a small electrical signal proportional to the weight of the item.
Amplifying the Signal: The amplifier boosts this small electrical signal to a level that can be processed by the microprocessor.
Processing the Data: The microprocessor receives the amplified signal and processes it to calculate the exact weight of the item.
Displaying the Weight: Finally, the calculated weight is shown on the display screen, providing a precise and clear measurement.

Advantages And Disadvantages Of Load Cell Based Weighing Scales

Here’s a quick look at the advantages and disadvantages:

Advantages

Versatility: It can be used in a wide range of environments and applications.
Durability: Load cell based weighing scales built to last and can withstand harsh conditions.
Wide Range: Capable of measuring a broad spectrum of weights.
Ease of Integration: Easily integrated with other systems and equipment.

Disadvantages

Cost: High-quality load cells can be expensive.
Complex Calibration: Requires regular calibration to maintain accuracy.
Sensitivity to Environmental Factors: Sensitive to temperature fluctuations.
Maintenance: Requires regular maintenance to ensure continued accurate and effective functioning.
Overload Risks: Essential to use within their designed capacity to avoid overload and potential damage.

Accuracy And Precision

Load cell scales usually have an accuracy of 0.1% to 0.01% of their full capacity. For example, a 100 kg scale can be accurate within ±0.1 to ±10 grams. These scales are highly precise, with resolutions of 1 part in 8,000,000 achievable in 20 ms over the scale range.

Surface Acoustic Wave Based Weighing Scales

Surface Acoustic Wave (SAW) weighing scales operate by generating acoustic waves on a piezoelectric material surface using interdigital transducers.

When an object is placed on the scale, its weight alters the wave’s travel time and amplitude due to the mass loading effect. The added mass increases the inertia of the material, causing the SAWs to travel slower (reducing velocity) and lose energy more rapidly (reducing amplitude).

These changes are detected by receiving transducers, which convert the altered waves back into electrical signals.

The variations in the signal are processed to determine the weight of the object with high precision. SAW based weighing scales are highly accurate and have very small time drift.

Components of a SAW Weighing Scale

Platform: The flat surface where the item to be weighed is placed.
Wave Generator (Transmitter): A transducer that converts electrical signals into surface acoustic waves.
Wave Detector (Receiver): A separate transducer that detects the surface acoustic waves after they have travelled across the material.
Substrate: The material surface on which the acoustic waves travel.
Microprocessor: The computer that processes the data from the sensors and calculates the weight.
Display Screen: Where the weight measurement is shown.

How These Components Interact

Placing the Item: When you place the item on the platform, it exerts a force on the substrate.
Generating Acoustic Waves: The wave generator (transmitter) creates surface acoustic waves. These waves propagate along the substrate.
Wave Propagation: The presence of the item changes the characteristics of the substrate, such as its tension and density, which in turn affects how the waves travel. The heavier the item, the more significant the alteration.
Detecting Changes: The wave detector (receiver) picks up the altered waves after they have travelled across the substrate. The detector measures changes in the wave’s time of travel, frequency, and amplitude.
Converting to Electrical Signals: The detected changes in the acoustic waves are converted into electrical signals by the wave detector.
Processing the Data: The microprocessor receives these electrical signals and interprets the changes. It calculates the weight of the item based on how the acoustic waves were altered.
Displaying the Weight: The calculated weight is displayed on the screen, providing a precise measurement.

Advantages and Disadvantages of Surface Acoustic Wave (SAW) Based Weighing Scales

Advantages

Sensitivity: Highly sensitive to small changes in weight, beneficial for light or delicate items.
Durability: Robust and can withstand harsh environments, including extreme temperatures and pressures.
Non-Contact Measurement: Allows for non-contact measurement, reducing wear and tear on components.
Fast Response Time: Provides quick and real-time weight readings, enhancing efficiency.
Low Power Consumption: Consumes less power compared to other electronic scales, making it energy-efficient.

Disadvantages

Cost: Typically more expensive than other types of scales due to advanced technology and components.
Complexity: More complex, making maintenance and repairs more challenging and potentially costly.
Sensitivity to External Factors: Can be affected by environmental factors such as vibrations and electromagnetic interference.
Limited Availability: May not be as widely available, making them harder to find and purchase.
Calibration Requirements: May require more frequent calibration to maintain high accuracy.
Specialised Use: Often designed for specific applications, limiting their versatility compared to general-purpose scales.

Accuracy And Precision

SAW scales are about 20 times more accurate than traditional strain gauge scales. For instance, a 100 lb SAW scale can have an accuracy of ±0.001 lb. These scales offer exceptional precision, with resolutions as fine as 0.001 lb.

Force Motor Based Weighing Scales

Force motor technology-based weighing scales operate by balancing an item’s weight using electromagnetic force. When an item is placed on the platform, a load cell is activated, which sends an electrical signal to the microprocessor.

The microprocessor, a small computer, activates the electromagnetic force motor to counterbalance the weight. A position sensor detects any platform movement and provides feedback to the microprocessor, which adjusts the force to maintain balance.

The microprocessor calculates the required electromagnetic force to balance the weight and converts this information into the precise weight, displaying it on the screen.

Components of a Force Motor-Based Weighing Scale

Platform: The flat surface where the item to be weighed is placed.
Load Cell: A sensor that detects the weight of the item.
Electromagnetic Force Motor: A motor that generates an electromagnetic force to balance the weight.
Position Sensor: A sensor that detects the position of the load.
Microprocessor: The computer that processes the data from the sensors and calculates the weight.
Display Screen: Where the weight measurement is shown.

How These Components Interact

Placing the Item: When you place an item on the platform, the load cell senses the weight and generates an electrical signal.
Detecting Initial Weight: This electrical signal is sent to the microprocessor, providing the starting point for weight measurement.
Generating Electromagnetic Force: The microprocessor activates the electromagnetic force motor, generating a force to counterbalance the item’s weight.
Balancing the Load: The position sensor monitors the platform’s movement. If the platform shifts due to the weight, the sensor sends feedback to the microprocessor.
Adjusting the Force: The microprocessor dynamically adjusts the electromagnetic force based on the sensor’s feedback until the platform is balanced.
Converting Force to Weight: Once balanced, the microprocessor measures the electromagnetic force required to maintain this balance, which is directly proportional to the item’s weight.
Processing the Data: The microprocessor processes the signals from the load cell, position sensor, and force motor to calculate the precise weight.
Displaying the Weight: Finally, the calculated weight is shown on the display screen, providing a clear and accurate measurement.

Advantages and Disadvantages of Force Motor Based Weighing Scales

Advantages

Fast Response Time: Delivers quick weight readings, which is beneficial for dynamic weighing processes.
Stability: Offers excellent stability and repeatability, ensuring consistent measurements over time.
Robustness: Can handle a wide range of weights, making them suitable for various industrial applications.
Low Maintenance: Typically requires less maintenance compared to other high-precision scales due to fewer moving parts.
Integration Capabilities: Can be easily integrated with other systems and equipment, enhancing automation and data collection.

Disadvantages

Cost: Generally more expensive than other types of weighing scales due to the advanced technology used.
Complexity: More complex, requiring specialised knowledge for installation, calibration, and repairs.
Power Dependency: Requires a stable power supply to maintain accuracy and functionality, which can be a limitation in certain environments.
Environmental Sensitivity: Can be sensitive to environmental factors such as temperature fluctuations and vibrations, which may affect accuracy.
Size and Weight: Typically larger and heavier than other types of scales, which can be a drawback in space-constrained areas.
Initial Calibration: Requires precise initial calibration and may need regular recalibration to maintain accuracy, adding to maintenance efforts.

Accuracy And Precision

Force motor scales can achieve accuracy levels of 0.01% to 0.001% of their full capacity under ideal conditions. These scales offer high precision, with resolutions as fine as 1 part in 200,000 (0.0005%) of the full capacity.

Hydraulic Weighing Scale

Hydraulic weighing scales operate by converting weight into hydraulic pressure. When an item is placed on the platform, it applies force on a piston within a hydraulic cylinder. This force displaces the hydraulic fluid, resulting in increased pressure, which corresponds directly to the item’s weight.

A pressure sensor or gauge then measures the fluid pressure, and the system converts this reading into a weight value displayed on the screen. This mechanism provides accurate measurements in heavy-duty environments.

Components of a Hydraulic-Based Weighing Scale

Platform: The surface where the item to be weighed is placed.
Hydraulic Cylinder: Contains hydraulic fluid and reacts to the weight applied to the platform.
Piston: Moves within the hydraulic cylinder in response to the applied weight, altering the fluid pressure.
Pressure Sensor or Gauge: Measures the fluid pressure within the hydraulic system.
Display: Shows the weight measurement derived from the fluid pressure.

How These Components Interact

Placing the Item: The item’s weight exerts a force on the hydraulic cylinder via the piston when placed on the platform.
Displacement of Hydraulic Fluid: The piston’s downward movement into the hydraulic cylinder displaces the fluid within.
Increase in Fluid Pressure: The displacement of hydraulic fluid increases the pressure inside the cylinder, directly proportional to the item’s weight.
Measuring the Pressure: The attached pressure sensor or gauge detects the precise fluid pressure resulting from the weight.
Converting Pressure to Weight: The pressure measurement is converted into a weight value using the known properties of the hydraulic fluid and system dimensions.
Displaying the Weight: The calculated weight is then shown on a screen or gauge for accurate reading.

Advantages and Disadvantages of Hydraulic-Based Weighing Scales

Advantages

Durability: Built to withstand harsh environments and heavy-duty use, making them reliable in challenging conditions.
Resistant to Overloads: Handles overloads better than other scale types, reducing the risk of damage.
Stable Performance: Maintains consistent and stable measurements even in fluctuating conditions.
Low Maintenance: Requires less maintenance due to fewer electronic components.
Temperature Tolerance: Performs reliably in extreme temperatures where electronic scales might fail.

Disadvantages

Lower Precision: Generally less precise than electronic scales, making them less suitable for applications requiring high accuracy.
Bulkiness: Typically bulkier and heavier than electronic scales, which can be a disadvantage in confined spaces.
Slower Response Time: May respond more slowly compared to electronic scales, which can be a drawback for dynamic weighing processes.
Complex Setup: Installation and setup can be more complicated due to the hydraulic components.
Fluid Leaks: Risk of hydraulic fluid leaks, leading to maintenance issues and potential environmental concerns.

Accuracy And Precision

Hydraulic scales are known for their accuracy, typically achieving levels of ±0.1% of the load capacity. For instance, a hydraulic scale with a 10,000 kg capacity would have an accuracy of ±10 kg. These scales can also achieve high precision, often with resolutions around 5 kg or better, depending on the specific design and application.

Conclusion

Hydraulic scales have an accuracy of ±0.1% of the load capacity. For example, a hydraulic scale with a 10,000 kg capacity would have an accuracy of ±10 kg. These scales can also achieve high precision, often with resolutions around 5 kg or better, depending on the specific design and application.

FAQs

Why are load cell-based weighing scales preferred in industrial applications?

Load cell-based scales are preferred because they offer high accuracy, durability, and versatility across a wide range of weights and environments.

How do I choose the right weighing scale for my application?

Consider factors such as the required precision, environmental conditions, capacity needs, and integration with other systems when choosing the right scale.

How often should load cell-based scales be calibrated?

Load cell-based scales should be calibrated regularly, depending on the frequency of use and the accuracy requirements of the application. Routine checks help maintain precision.