Scaling Up the Operation
Through a bit of nifty mathematical magic, tons are converted into cubic yards.
Each load of waste is measured in tons as it comes into the landfill and then gets transformed into cubic yards out on the workface. Though profitability is derived from cubic yardage (which determines air-space utilization efficiency and operational lifetime), a landfill’s cash flow is determined by the ton, since tipping fees are charged on a per-ton basis. In any business, the basic financial input is cash flow. Therefore, much of a landfill’s success depends on the accuracy and efficiency of its truck scales used to measure incoming waste amounts.
Truck scales are typically to be found in the ground, but are often available onboard the truck itself. Almost all truck scales used by landfills are in-ground scales. In-ground scales can be installed either in shallow pits or directly on a flat surface. Each has is advantages and disadvantages in terms of cost, ease of installation, simplicity of operation, and accuracy of measurement.
Measurement in any type of truck scale is performed by load cells. A series of load cells is typically installed in the framework of the scale (or the body of the truck for inboard scales). When subject to applied loads from the dead weight (applied stress) of the truck and its waste, these cells either deform (resultant strain) or increase interior pressures. Both the physical deformation and the pressure increase can be read by sensors (strain gauges) attached to the load cells. Given the known properties of the load cell material, the measured strain can be compared with the material’s stress-strain curve and the applied load for each load call can be calculated. Most cells are very stiff and deform only slightly; therefore, the sensor gauges must be relatively sensitive. The sensors then send an electronic or pneumatic/hydrostatic signal back to a summation station. At the summation station, the loads on the individual cells are summed up and a total weight determined. The total weight is then displayed on a computer screen and/or printed out on a hard-copy scale ticket.
Types of Load Cells
There are several types of load cells used by in-ground scales: single-end beam, S-type, double-end shear, tension linked, and compression cylinders.
Single-end beams are designed for side mounting and single linkage to the scale framework. They utilize the cantilever principle with one end fixed at the support and the other attached to the applied load point. As the applied load increases, the single-end beam bend and the resultant deformation can be measured. These load cells are designed for use in low-profile scale applications such as floor scales, platform scales hopper scales, and tank scales. Typically, they are utilized in medium-range load weighing. Since these cells are made in the form of a cantilever design, they are best used in sets of three or four installed around the perimeter of the truck scale with the fixed end along the edge and the cantilever end supporting the weighing platform in the center.
S-beams are used for weight conversion, tension loadings, and general purpose and get their name from their S-like shape. Unlike most load cells, S-beam load cells can provide a load measurement while under either tension or compression. Applications include tank level, hoppers and truck scales. Their design allows them to minimum error due to side loads. Generally higher than other load cells, S-beams may not be useful in low profile applications.
Double-ended shear beams differ from single-ended beams by being fixed at both ends and having their applied load located in the center of the beam. This configuration (due to the physics of static loadings) tends to deform less under equivalent loads. Therefore, they are suitable for heavy load applications such as motor truck scales, railway scales, crane scales, rigid mount applications, and heavy-duty tank weighing systems.
Tension-linked load cells are used to measure heavy lifting by crane or pulley or to measure horizontal pulling loads. Combination compression/tension load cells can be used for applications where the load may shift from tension to compression and back again. They can be used in horizontally restricted locations.
Compression cylinders are used to weigh loads by measuring changes in air pressure within the canisters. With an integral button design they can be mounted where space is restricted. Since they do not permanently deform over time, they offer better long-term stability compared to other types of load cells.
The body of the load cell, no matter what its configuration, merely deforms under the applied loads. In terms of the mechanics of materials, the applied load is referred to a “stress,” and the resultant deformation is referred as a “strain.” Every material has what is referred to as a “stress-strain curve,” where deformations are graphically matched to applied loadings. It is the measurement of the amount of deflection, combined with the load cell’s material characteristics, that allow a determination of the load weight. The actual measurement of the deformation is performed by a part of the load cell called the “transducer.” The transducer is basically a micro-strain gauge connected to the core or frame of the load cell.
An example of how a strain gauge transducer works can be simply stated: The strain induced in the gauge alters the cross-sectional width of the gauge material. This in turn affects the electromagnetic characteristics (resistivity and frequency) of the gauge. The signal passing through the gauge reflects the new characteristics and alters accordingly.
A somewhat different type of gauge is the vibrating wire gauge. Each vibrating wire gauge consists of a wire that has been tension-mounted between two mounting blocks connected to the ends of the cylinder making up the body of the load cell. The ends of the wire are connected to the load cell by means of mounting blocks. Deformation of the cylinder under the applied load weight produces relative movement between the mounting blocks, which causes a change in the wire’s tension. This in turn results in a change in its frequency of the wire’s vibration. The new resonance frequency is measured by “plucking” the wire with an electromagnetic coil. A signal cable attaches this coil to a readout device. The readout device transmits the new signal to a computer that translates the frequency resonance change into applied load for purposes of measurement. The average of the measurements is used as the loading on the cell, reducing the effects of eccentric loading.
The gauges used in air-pressure load cells are also different. These gauges measure changes in interior air pressure within the cell rather than deformations of the cell itself. Air-pressure load cells experienced increased interior air pressures as a result of the applied loads. Though this may seem low tech compared with strain or vibrating wire gauges, the better systems are highly sensitive to even incremental weight changes and can compensate for environmental factors that would alter the interior air pressure, such as changes in ambient temperature.
Types of In-Ground Scale Systems
The in-ground truck scales typically used at landfills are categorized by their type of construction. There are two broad types of scale construction, concrete deck and steel deck. Concrete-deck truck scales are more expensive and difficult to install, but they have a poured concrete slab of 6- to 8-inch thickness with a roughened surface to minimize the potential for skidding. Given that trucks can leak fuel and oil, creating a slippery contact surface, this can be a serious safety concern.
Less expensive and easier to install are steel-deck flattop truck scales. These utilize a surface consisting 12-gauge (or thicker) steel plating that has been bolted or welded to the scale’s mainframe, instead of a poured concrete surface. In all cases, a properly designed scale frame minimizes the deflection to the frame itself, avoiding other deflections that can interfere with the accuracy of the load-cell measurements. A well designed and built scale will have a deflection strain ratio of no less than 1:850.
In-ground scales can be constructed aboveground (with its foundation on existing ground surface) or belowground (with its foundation set in shallow excavation). The deck of an above grade scale is above surrounding grades, while that of an in ground scale is flush with the adjacent ground surface. Aboveground scales can be constructed with either a relatively high profile that allows for ease of maintenance or a lower profile for ease of approach. The lower the profile, the less need for long, steep approach ramps, which improves truck safety. Temporary portable truck scales can also be installed aboveground on a flat grade or pavement. Lacking the need for any approach ramps, belowground scales are designed for greater operational safety.
Each scale is operated by a compatible software program system. These programs are used to create weight data records, print our written reports, and issue payment tickets and perform other billing activities. More extensive and complicated software exists to regulate automated truck loading systems and allow for automated truck loading. These software packages operate the following components: load management, hardcopy printer, integration to work with the landfill’s management information system, radio frequency identification system for truck identification and load tracking, ready-to-install container identification chips, and performance of the annual calibration of the system itself. Optional software packages are available for support activities such as route optimization program and administrative report writing.
Other types of software allow an operator to do more than just track hauling operations; they can control the actual loading process. These are referred to as unattended systems and do not require an operator. They can be complicated, sophisticated systems performing multiple tasks or relatively simple and inexpensive systems performing basic operations. The more complicated systems are used at load-out facilities where trucks are initially loaded and are really two systems in one, simultaneously loading and weighing each truck. The concurrent operation allows a facility to dispense with a second truck scale. The unattended batching software reduces manpower needs because the driver can interface directly with the loading system by way of a card reader/keypad/display unit located in the cab of each truck. The driver can initiate the load sequence and set the weight parameters for loading. A ticket printer provides hardcopy records of the transaction. The entire system is tied into an overall management information system network and report-writing database software.
A simpler system allows for unattended operations and data collection. This is the system most commonly used at landfill to weigh incoming waste trucks. The unattended system merely allows for automated updates of truck hauling data by using a radio frequency card reader/display/keyboard unit. This is not the same thing as actually operating the scale. Near-instantaneous driver identification is made possible concurrent with the entering of data into the management system. Optional features include traffic control lights, scale-mounted vehicle-position sensors, and remote scale displays—all fully automated. To prevent fraud, security codes allowing access are integrated with the automation software. A radio frequency tag-reader antenna can be mounted on the scale to interact with radio frequency tags attached directly to the truck. With this set up, human error is eliminated during scale operations.
Commercially Available Scales and Software
There is a wide range of companies that manufacture, sell, install, and maintain truck scales and their associated commuter software systems. Some of the major players in this market and their products are described below.
PC Scale Tower supports its own off-the-shelf and custom-designed point-of-sale solutions for generating tickets and reports based on weight, yards, and units, using single weighments of single or commingled materials per vehicle, multiple weighments of multiple materials per vehicle, or no weighments at all, allowing customers to track volume/yards per unit for such materials as tires, appliances, drums, bags, or bales. The company’s software also allows the scale system to create and report on an unlimited number of units of measure, which can be custom-named as necessary.
PC Scale’s matching WR 5.0 software allows a truck scale to run reports using any data range, making historical and management reporting simple and fast due to its robust nature, and archiving is no longer necessary for most facilities. Furthermore, scale data can be uploaded to virtually any accounting program, reducing double-entry errors and saving man-hours. PC Scale Tower’s scale and routing software packages integrate all waste management business functionalities such as routing with electronic mapping, dispatch, billing, and electronic white board with full calendar functions, while providing security, audit trails, and data integrity.
Cardinal’s Guardian Hydraulic Tuck Scales are used for the solid waste, aggregate, chemical, grain, and construction industries because they offer a high level of protection against many of the conditions that can adversely affect truck scale installations. Water, lightning, power surges, explosive areas, corrosion, welding, shock loading, extreme temperature fluctuations, and even rodents can cause scale problems, all at a cost to the owner. Cardinal’s hydraulic load cells are immune to electrical surges and lightning damage. The Cardinal model PTG-3K Pressure Transducers, located near the Cardinal weight indicator, are fluidly coupled to the SST hydraulic load cell to convert the pressure signal into an electrical signal.
Carolina Software’s WasteWORKS-SQL is simple off-the-shelf package that offers users and management with expanded reporting capabilities, Windows level security and fast, efficient processing in a wide range of network environments. Microsoft SQL Server 2008 Express is bundled free with WasteWORKS-SQL and can be installed on a server or even a single workstation. WasteWORKS may also be installed on a server running the full version of SQL Server 2005 or 2008. Microsoft SQL Server Express provides high-level data security, simplified administrative tools, and is fully compatible with other editions of SQL Server.
WasteWORKS Reports menu allows customers to take advantage of the Crystal Reports engine for high-speed reporting, editing, and creation of reports in Crystal format. Customers may also create reports using the Microsoft Reporting Services tool, which is included with MS SQL Server.
Synchronization helps keep site servers “in sync,” even when connections are less than reliable. With regular updates between all sites, users will always be on the same page and downtime situations will not occur. Each site maintains a complete copy of all the data, so if any site goes down, all of the other sites can continue to operate. Data from other sites will automatically synchronize when the connection is restored. The Sync Manager ensures that all sites are updated at regular intervals, and if the connection is disrupted, processing continues without interruption.
B-TEK’s Centurion-AT Truck Scale’s thick, heavy decking acts as a wear surface. The weighbridge comes standard with three-eighths-inch decking with modules built with 12-inch-wide flange I-beams equally distributed across the width of the platform and oriented longitudinally, similar to the design of a highway road bridge. The PVS series of scales are constructed with a sturdy fabricated lever system and precision machined pivots and bearings to provide years of dependable weighing.
CapurIT provides onboard intelligence systems for route optimization, waste collection, and truck scale operations. The company’s truck scale system achieves a high level of automation by use of an RFID system, GPS, and GPRS, and truck engine monitoring. The RFID tags improve revenue collection by identifying every customer, provides statistics on container collection and recycling efforts, real-time accurate feedback to optimize productivity, automated recording of operations, and reduced operating costs overall. The company’s GPS allows real-time data communication between the truck and the main office, allowing for pickup verification and location, and the identification of service problems. As part of the GPS, truck engine monitoring allows for efficient fleet maintenance, idle time, poor driving behavior notification, and visual feedback of status and conditions to the truck driver. The operating system’s touchscreen display removes reliance on operator knowledge and paper lists, replacing them with optimized screen routes and collection points.
Core Computing Solutions provides EnCORE, a powerful enterprise-class financial and operational software system designed specifically for the waste management and recycling industry. EnCORE can service small, medium, and large companies that specialize in residential, commercial, and industrial hauling, as well as landfill, transfer station, material recovery facilities, and recycling centers. Its integrated scale management and accounting functions include accounts receivable billing and accounts payable accounting, recycling and MRF management, recycling inventory management, report writing utilizing Microsoft SQL and Crystal Reports, onboard truck computer and handheld barcode technologies, and integration scale management for transfer stations, landfill, MRF, and recycling facilities.
Creative Information Systems produces SMS Turbo, a software package designed to increase ticketing transaction speed, reduce operating costs, automate billing, and increase revenue. The system helps increase transaction accuracy and reduces the risk of theft or loss. Landfill operations receive multiple benefits from SMS Turbo. SMS Turbo allows residential and commercial ticket transactions including hazardous waste ticketing and tracking. Data recording and record tracking capabilities allow the operator to record specific items received to comply with government regulation and tracks on hand inventories of each material. It can handle ticketing for weighted and nonweighted items (each, yard, bail, gallon, etc.). The system runs daily totals to ensure permitted daily allowances are not exceeded and warns operators when daily limits are approached. The system provides support for electronic signature capture, bar code reading, and touchscreen displays as well as support for unattended and driver-assisted transactions. It accepts payment via cash, checks, credit cards, and debit cards, and generates invoices automatically using SMS Turbo’s own Billing/Accounts Receivable module or with more than a dozen interfaces to standard accounting package
Scales from Emery Winslow Scales employ hydrostatic load cells that are non-electronic, making them immune to failure from lightning, water, rodents, power surges, and other impacts. Emery Winslow manufactures truck scales of every shape and size, for either pit installations or above grade, steel deck, or concrete, with unique cleaning features. The company’s Hytronic Technology is a combination of advanced hydrostatic and electronic weighing technologies. Hydrostatic load cells and totalizer outputs are non-electronic and will not drift or change sectional calibrations, drastically reducing calibration cost over the lifetime of the scale. They are NTEP approved and integrated into scales with a free-floating bridge design, reducing end wall headaches. Non-electronic hydrostatic load cells have no electronic apparatus to fail on or under the weighbridge.
Fairbanks Scales Trident is a new fully electronic, heavy-capacity truck scale featuring a factory poured and steam cured, concrete deck scale for above ground or pit-type use. Trident’s TensileCore-engineered concrete system is built for superior structural integrity. Fairbank’s exclusive Intalogix Technology is a standard feature of the Trident.
In addition to the Trident, Fairbanks produces the Titan steel deck aboveground truck scale an industrial leader in gross weight and load cell capacity, deck plate thickness, I-beam construction, CLC rating and sheer steel weight. The Titan is constructed from multiple 12-inch and 16-inch Structural Steel I-beams. These massive supports produce an extremely rigid weighbridge that handles the heaviest truck scale applications.
Mettler Toledo manufactures both steel deck and concrete deck truck scales. Its Model 7563 is a steel deck scale for weighing over-the-road legal trucks and certain off-road vehicles. The design is flexible and can be tailored to match specific application requirements. The orthotropic design provides reliable weighing over a long service life, while its Powercell load cells transmit a powerful signal for accurate, dependable weighing. In addition, the steel deck design is suitable for operations that need their scale up and running quickly.
Model VTC221 is Mettler Toledo’s strongest concrete deck truck scale, combining a concrete driving surface with an orthotropic understructure similar to those used in the company’s steel-deck truck scales. This composite design draws upon the strengths of both concrete and steel to produce a very durable structure. It also eliminates a common cause of premature deck failure: voids in the deck that occur when pouring concrete around I-beam flanges and sharp corners. As a result, it has a weighbridge capable of handling high volumes of traffic over a long service life. Model VTC221 also uses Powercell PDX load cells. This design combines the strength of an orthotropic weighbridge with the reliability of Powercell load cells.
In addition to various types of truck scales, Rice Lake also produces truck scale management software. Truck scale operations currently using a manual ticketing system can automate their process with Rice Lake’s OnTrak data management software. Print tickets or reports as well as store information to a central database. OnTrak is adaptable to a wide variety of industries and compatible with many popular operating systems. OnTrak supports Windows XP SP3 (32-bit), Vista SP 2 (x86 or x64), and Windows 7 (x86 or x64) Systems with an SQL Server 2005 Express Client/Server Database. Its flexible operating structure provides comprehensive report, user, and administrator filters. It provides exclusive data bases for customer, hauler, truck, product, job, price/discount, tax, and transaction.
Soft-Pak Inc.’s “i-Pak” integrated software package handles inventory tracking and material routing. Within the i-Pak’s software suite is the vehicle management module that tracks the performance of trucks, equipment, and other waste operations vehicles. It also measures the performance of stationary support equipment such as bailers, compactors and bins. I-Pak can chose the most efficient route and track the actual productivity of individual trucks. E-Pak is a secure, Web-based extension of the i-Pak system that forgoes the need for additional hardware.
Unitec manufactures a variety of truck scales, including low-profile, portable, shallow pit, off-road, and axle scales. The low-profile scale’s side rail design has been redesigned and improved over the years into a durable truck scale capable of 1,500 to 1,600 loads per day. The low profile of 12 inches from the deck to the top of the pier minimizes the need for ramps. The pier design of the foundation minimizes concrete work. The integrated side rails keep trucks on the scale, resisting shocks from accidental truck hits. The external load-cell mounting system absorbs all reasonable shock load and is efficient to service. The bumper-bolt system keeps scale movement to a minimum.
Author's Bio: Daniel P. Duffy, PE, writes frequently on the topics of landfills and the environment.