Truck Scale Load Cell Technology

Understanding the most important components in your scale

Side view of a truck scale showing the load cellLoad cells are the heart of any truck scale. They are the sensors that measure the weight of objects on the scale deck. Most truck scales require 6 to 12 load cells. They must work together flawlessly to provide accurate weight readings.

A few popular load cell types are currently being sold for use in truck scales. Understanding the differences in their operation and features can help you choose a system that will be accurate and reliable, providing the most value from your investment.

Evaluating Load Cells

You have several choices when it comes to load cells. Because load cells are the components that most closely affect scale performance, it is worthwhile to understand how they work and the guidelines they must meet.

Do regulations require that they all perform similarly? Most scales are built to comply with the legal-for-trade requirements of Handbook 44, OIML R76, and/or other Weights and Measures regulations. Does that mean that different types of load cells perform the same because they meet the same guidelines? No

Load cell performance guidelines in Handbook 44 and OIML R76 include accuracy tolerances, or error thresholds, used for calibrations. However, the performance standards included in Handbook 44 and OIML R76 still reflect the capabilities of mechanical scales, which are largely antiquated. Mechanical scales have limited capabilities compared to more modern load-cell technologies. In other words, some load cells can perform significantly better than the minimum required. So, what benefits do newer systems offer the scale buyer?

  • Accuracy

A system that is designed to establish and maintain a high level of accuracy means that a business can avoid product or profit loss due to weighing errors.

  • Reliability

A stable and resilient load-cell system means a more reliable scale with less downtime, fewer repair expenses and a lower cost of ownership.

Now, let’s briefly discuss the various scale technologies in terms of their accuracy and reliability.

Types of Load Cells

Analog

Analog load cell for vehicle scaleAnalog load cells have been used in truck scales since the 1960s. Each cell contains a precision-shaped piece of metal, often steel or stainless steel, that changes its shape slightly as a force (weight) is applied. The change is monitored by electrical strain gauges and is sent as an analog voltage signal to one or more junction boxes. The combined signal is then transmitted to the scale house, where it is measured and converted to a digital signal that indicates the weight.

Hydraulic

Image of a hydraulic load cellHydraulic load cells are hydraulic pistons that compress a reservoir of fluid. The compressed fluid flows through individual hydraulic lines to a mechanism, sometimes called a “totalizer,” that is in or near the scale house. This mechanism then exerts the accumulated force of the combined fluid pressures onto an analog load cell. This load cell generates an electrical signal that indicates the total weight on the scale

Analog/digital hybrid

image of a hybrid vehicle load cellHere, analog load cells are connected to a junction box that converts the analog signal to digital. A digital signal is stronger and less susceptible to the weighing errors that can occur due to interference from external influences.

Digital

image of digital vehicle scale load cellThis is a load cell that generates an analog voltage, which is converted into a digital signal within the load cell enclosure. The data from the cells are processed to determine the total weight. Utilizing a digital signal at the load cell and beyond provides advantages because the signal is not susceptible to interference as analog load cell signals are.

 

POWERCELL®

Powercell truck scale load cellThese load cells utilize digital electronics and are equipped with signal-processing capabilities at each load cell. The load cell can eliminate errors by monitoring and adjusting the weight measurement based on several criteria. This process is called digital compensation. POWERCELL load cells have also introduced features such as predictive diagnostics, self-monitoring, breach detection, and remote diagnostic that help ensure accuracy and eliminate downtime.

Special note: Before the introduction of electronic components, all vehicle scales were mechanical, supported by numerous levers and pivot components. Today’s designs require less steel, are more easily installed, and are more accurate and reliable than their mechanical predecessors. Some surviving mechanical scales can be upgraded to full-load cell systems.

Load Cell Technology Comparison

 

 

Accuracy Reliability Price Service Active Alerts

 

Short-term and long-term load cell system performance Load cell designed to protect against environmental factors and failure The initial acquisition cost of the load cell system Ease of maintenance and product replacement Proactive alarm system to alert the user of potential errors

POWERCELL

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Image of PDX truck scale load cell

Strong digital signal combined with in-cell compensation algorithms delivers consistent accuracy Stainless steel, hermetically sealed design protects the sensor circuits from harsh environments Compared to other load cell systems on the market POWERCELL has a high initial cost Onboard processors allow users and technicians to quickly identify service needs Self-monitoring system that alerts the user of any inconsistency in load cell performance

Digital

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Image of digital truck scale load cell

Strong digital signal that is less prone to interference from RFI, temperature, and other hazards Varies by manufacturer but the stronger digital signal leads to less system vulnerability Compared to other systems on the market digital cells have a high initial cost Some systems offer diagnostic features that can assist with both regular and emergency service Some systems have self-monitoring features that alert you of communication interruption

Analog

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Analog cells are accurate enough to meet most metrology standards but lack long-term consistent performance Prone to failure due to low signal strength which is easily influenced by the surrounding environment Analog cells are highly commoditized and have the lowest initial cos No diagnostic features, which leads to time-consuming manual adjustments Cannot check the status of individual load cell signals in the system, no alerts available

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Image of a hydraulic load cell

By using hydraulic fluid these load cells are protected from outside interference leading to an accurate weight signal Being non-electronic, hydraulic systems hold up well in harsh conditions Compared to other load cell systems on the market hydraulic cells have a high initial cost Requires specialized maintenance procedures such as changing or bleeding fluid lines Mechanical with no modern or advanced features such as self-monitoring or diagnostics

*This information is based on METTLER TOLEDO service records from 50,000+ in-field tests covering a wide range of load cell technologies.

Load Cell Geometry

Man installing a Powercell load cell in a truck scaleThere are two predominant geometries for load cell systems: compression (vertical) and shear beam (horizontal).

Compression load cells

Compression load cells (analog and digital) measure loading with strain gauges on vertical columns running through the center of the load cell. Typically, those load cells are used in a “rocker pin” design, allowing the weighbridge a small amount of free movement within the restraints of a checking system. That checking system can include shock-absorbing bumpers, adjustable bolts, and check rods. When adjusted correctly, the system allows just enough movement that the weighbridge and load cells are self-centering. That prevents the scale from binding on the foundation or approaches. It also limits the opportunities for physical wear to the components. The rocker pin design should also include anti-rotation features so that the load cells cannot rotate in their positions.

Shear beam and double-ended shear-beam

Shear beam and double-ended shear-beam (including cantilever) load cells also use strain gauges, except they are mounted to a horizontal beam. Single-ended shear beams are fixed on one end and linked to the weighbridge on the other. Double-ended shear beams are typically fixed or supported in the center and linked to the weighbridge on both ends. Either way, those beams typically are linked to a lower mounting point on the weighbridge, creating a suspension system. There, the weighbridge can swing slightly in two or more directions. That system is designed to be self-centering, thanks to the gravitational pull downward on the beam ends. However, the links between the beam ends and the scale structure should be regularly inspected for signs of wear.

Notes: Article from the METTLER TOLEDO Truck Scale Buying Guide

Contact us today to learn how the METTLER TOLEDO PDX and GDD load cells can improve the accuracy of your truck scale.