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1 - Evaluating Load Cells You have a number of 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 are capable of performing 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 ex- penses and a lower cost of ownership.
2 - Types of Load Cells There are five predominant types of load cell systems used in vehicle weighing applica- tions: analog load cells, hydraulic load cells, hybrid analog/digital systems, digital load cells and POWERCELL load cells.
Analog Analog load cells have been used in truck scales since the 1960s. Each cell con- tains a precision-shaped piece of metal, often steel or stainless steel, that chang- es its shape slightly as a force (weight) is applied. The change is monitored by elec- trical 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 mea- sured and converted to a digital signal that indicates the weight. Hydraulic Hydraulic load cells are hydraulic pistons that compress a reservoir of fluid. The compressed fluid flows through individual hydraulic lines to a mechanism, some- times called a “totalizer,” that is located in or near the scale house. This mecha- nism 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 to- tal weight on the scale. Analog/digital hybrid Here, 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 This is a load cell that generates an analog voltage, which is converted into a dig- ital signal within the load cell enclosure. The data from the cells is processed to determine the total weight. Utilizing a digital signal at the load cell and beyond pro- vides advantages because the signal is not susceptible to interference as analog load cell signals are. POWERCELL ® These 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 a number of criteria. This process is called digital compensation. POWERCELL load cells have also introduced features such as predictive diagnostics, self-monitoring, breach detection and remote diag- nostic that help ensure accuracy and eliminate downtime.
Now, let’s briefly discuss the various scale technologies in terms of their accuracy and reliability.
Special note: Before the introduction of electronic components, all vehicle scales were mechanical, supported by nu- merous 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.
Keep in mind that METTLER TOLEDO has designed, built and/or serviced scales with each of these technologies.
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