By Lynn Merrill

A ton here. A ton there. When it’s coming into your facility and you’ve got to know exactly how many tons are in there, having a scale system that can accurately measure, record, and file is critical to your daily success.

Selecting the right scale system that can handle the number of trucks pounding over it daily, or that can operate reliably at the bottom of a transfer pit in a heavy rainstorm, can be a challenge. Up-front purchase costs are important, but without an ongoing maintenance program to ensure that the scales remain accurate from Monday morning to Saturday afternoon, the read-outs you get might be short-weighing you.

In essence, your scale system can be the most important component of your landfill, transfer station, MRF, or composting facility, since it’s the one piece of equipment that objectively measures what’s coming in and out of the facility. From this information flows all other performance and cost data. Historically, while reasonable estimates of weight were calculated simply by knowing the cubic yards of the load being delivered and then using standard densities for various expected loads, such estimations are subjective and allow for both parties to cheat by overloading or overbilling. A scale provides an unbiased and objective measure and reduces the opportunity for nose-to-nose disagreements about how much weight’s actually in that 40-yard roll-off sitting at the gate.

Scale technology is well developed over the last 150 years, and recent innovations and advancements focus more on improving reliability and durability of the system and its individual components. While the vast majority of scale systems in the past were mechanical in nature, current technology incorporates load cells in order to ensure greater accuracy and more reliable operations.

A Scale Primer
Scale systems are regulated by various certifications, laws, and regulations at both the federal and state levels. Of primary importance is the National Institute of Standards and Technology (NIST) (Gaithersburg, MD) reference book titled Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices. This handbook provides information for scale manufacturers as well as scale owners and operators. The handbook prescribes all federal specifications for the installation and operation of truck scales used for trade purposes. Scales that are approved for use as “legal for trade” are further approved by the National Type Evaluation Program (NTEP). In addition, many states have either adopted the federal requirements or developed their own programs for scale systems used for trade purposes. Certification programs for these scales may be administered by local agencies that will periodically inspect the scale system and certify its accuracy. Consulting with local regulators is an important first step to determining the requirements for a scale system at your facility.

PHOTO: LOS ANGELES SANITATION BUREAU

Scale equipment is broken down into three primary operating categories for use at solid waste facilities. These operating categories include mechanical, electro-mechanical, and electronic. In addition, scales are further delineated by whether they are pit-type or platform scales, and whether they utilize a steel deck versus a concrete deck. In addition, it is critical to determine the appropriate size of a scale system, based on the maximum vehicle length, number of vehicles projected to use the scale daily, and the expected maximum weights of the vehicles when specifying a scale system. A facility that expects to only handle refuse collection vehicles may be able to specify a smaller scale system than an operation that must handle a variety of vehicles ranging from small private vehicles up to transfer rigs. But in all cases, it is better to specify the largest system practical, given budget and physical limitations.

PHOTO: KERN COUNTY

Mechanical scales use a combination of levers to convert the weight of the vehicle into a reading on a dial or beam indicator. These types of scales are extremely durable, but have limitations relating to accuracy and the ability of the scale to transfer data to computerized record keeping. While the simplicity and low cost of a pure mechanical scale system is appealing to the technophobic, the technology has limited applications in a high-volume facility. All transactions would be manually entered into a data system.

When most people refer to a mechanical scale system in today’s operations, they are actually referring to electro-mechanical scale systems that combine the relative durability of the mechanical lever system with an electronic interface. The typical electro-mechanical scale transmits the mechanical weighing function to a single electronic load cell that converts the input to digital data. This allows the data to be converted into various computerized scale management systems.

The third category is a true-electronic system, which eliminates the complex system of levers and incorporates multiple analog load cells that use strain gauges to detect and measure the weight. The analog signal from the load cell is then converted into a digital signal that is transmitted to the display or other data system. While load cells have become the predominant measurement system for scale technology, concerns regarding the reliability of load cells continue to influence purchasing decisions. The cost to replace a failed load cell can be significant, so some facility operators tend to favor a mechanical system. However, many manufacturers have made significant strides in the past few years in improving the reliability of the load cells used in their products, resulting in the ability to offer longer warranties, provided that the system is properly installed and maintained.

Of significant debate regarding load cell technology is the concept of hermetically sealed features. In essence, hermetically sealed means that the entire unit is sealed against infiltration by outside air. Different load cell manufacturers may use different approaches to seal the load cell at different points of the cell. Hermetic sealing prevents water and other elements from entering the cell body, which could damage the cell’s ability to operate properly. Hermetic sealing is accomplished through welding the components to form a leak-proof unit that is subsequently filled with nitrogen.

While various claims may be made regarding the advantages of one load cell over another, for most solid waste operations, the real issue will be the actual failure rate of the particular load cell and, most importantly, the available warranty. While manufacturing warranties have improved dramatically to the point that some are able to offer warranty periods up to 15 years, these warranties may not include the load cell that may be covered under a separate manufacturer’s warranty of significantly shorter duration.

PHOTO: KERN COUNTY

Once you have determined the type of scale operating system, you must determine whether to use a pit-type scale versus a platform-type scale. Pit-type scales are permanent scale installations in which the operating system is installed belowground in a concrete-lined pit. These systems have the advantage of being level with surrounding roadway surfaces and may meet the requirements for ADA compliance at the facility. Disadvantages include a higher up-front cost, and the fact that under-platform maintenance may require compliance with confined space safety regulations.

Platform scales have the advantage of being easier to install, and are movable if the operational needs of the facility change. Maintenance requirements are also simplified due to the open nature of the system. These systems require the installation of approach ramps on either end of the scale, and therefore require additional space when compared to a pit-type scale. Because of the additional height, access platforms along either side of the scale platform may need to be installed in order to allow safe passage of drivers between the scale and the scale house. In addition, guide rails are recommended in order to reduce the opportunity for a distracted collection truck driver to drive off the side and shut down operations while the accident is resolved.

Weighing Loads in the City Of Angels
The City of Los Angeles recently acquired the 15-year-old Central Los Angeles Recycling and Transfer Station that includes scale systems for both weighing-in the collection vehicles as well as measuring the load-out of the transfer trailers. The system has suffered from a variety of deferred maintenance that has required city staff to consider replacement of the existing scale systems. “The city purchased this facility in April 2004,” states solid waste disposal superintendent James Kurz. “The city actually took over operations of the facility on November 29, 2004. Previous to that, we contracted with the previous owner to run that facility for us for about six months. When the city bought this facility, it was in a rundown state due to a lack of maintenance. Currently we have two inbound 70-foot platform scales and one outbound 70-foot platform scale. Down in the loading tunnels we have four 9 by 10 axle scales where we weigh the rear trailer and drive axles of the transfer truck so we make sure that we don’t overload them. While the inbound and the outbound scales are not in too bad of shape, we do plan on replacing them in the future by updating the electronics and the load cells in them. But the scales that we have down in the tunnel are in horrible shape. Since we’ve taken over operations we’ve probably spent about $40,000 just in maintenance to keep them operational.”

PHOTO: KERN COUNTY

The location of the scales in the tunnel demonstrates the importance of proper installation and maintenance, observes Kurz. “The drains in the bottom are all plugged up and we’re trying to get those unplugged to keep any liquids from backing up into the scales themselves,” he says. “There are four load cells with a junction box and circuit board down in the scale itself which creates a lot of problems for us because of the moisture in there. When we took over operations, the circuit boards were just hanging there loosely wrapped in plastic. They were not in a watertight container.” In addition, the scales lack proper seals to prevent the build-up of debris, requiring additional cleaning in order to ensure proper maintenance. “Monthly we have to go underneath the scales and clean them out. There could be up to a foot of dirt and debris or more in there, which plugs up the drains and packs in around the scales. We decided to go ahead and find the money and start putting a specification together to put new scales in there.”

PHOTO: KERN COUNTY

The city currently uses both platform and axle scales—provided by Avery Weigh-Tronix—at it’s green waste facilities. “Our axle scale is an Avery Weigh-Tronix model BMS multiple-axle steel-deck truck scale,” states Javier Polanco, sanitary engineer with the city. “It is actually made up of three individual and independent scales. They can be used as one whole scale or they can be used as individual scales to provide the weights of the rear axle, the drive axle, and the steering wheels. These individual weights are needed to be sure the truck is at the correct legal weight to drive on public roads. We use the scale to weigh trucks arriving and departing. Arriving trucks are weighed to determine the amount of incoming green waste. The departing trucks are loaded with processed mulch while the truck is on the scale. There are two large weight indicators on posts. These two large weight indicators (also known as scoreboards) constantly show the weights of the rear and drive axles and serve to guide the loader operator. The operator can load the transfer trailers to the correct weight, thereby maximizing the payload carried without overloading the trailer. To help signal our drivers during the weighing process, a stop-and-go light was also installed. The lights are controlled from the scale, these lights let the driver know they are in the correct position on the scale and indicate when weight has been registered electronically by the weighing system.”

PHOTO: LOS ANGELES SANITATION BUREAU

One of the features that Kurz likes about the scale system is a technology that adjusts for load-cell failures. “With these newer scales they have a technology they call a ghost system. If a load cell goes out, the computer reconfigures the system where you can still get an accurate weight until you can get the scale fixed, which is a feature that we like. Another feature that we like is that they now hot-dip-galvanized them. There is a lot of moisture down in the tunnel and that’s part of our problem; there’s a lot of rust down there. They take a lot of abuse.”

Two Transfer Stations In Ohio
In Dayton, OH, the Montgomery County Solid Waste District operates two transfer stations that provide transfer services to approximately 1.5 million people. “We receive all the residential solid waste for the entire county at those two transfer stations,” says Dan Graeter, solid waste district assistant manager. “We also receive some of the commercial waste generated in the county and some outside the county as well. We receive approximately 1,200 tons per day at the south transfer station and around 500 or 600 tons per day at the north transfer station. Both facilities have scales both inbound and outbound. We don’t use tare weights. The north plant has one inbound and one outbound scale, and the public and the commercial trucks come in on those scales. The south facility has two sets of scales for a total of four—two for the public, and they come in and weigh in and weigh out on those two scales; and then we have two for the commercial.”

The district is in the process of redesigning the south transfer station in order to reduce the time traffic spends crossing their scales. “Both facilities are old incinerators that have been modified to be transfer stations,” says Graeter. We’ll get 500 public vehicles a day over a weekend. What happens is that we have this huge line of traffic trying to get into our facility. The primary purpose for the modification at the south plant is to build a larger facility. Currently, the two scale houses for the public and the commercial are separated by 400 yards. We’re going to be building the two separate scales right next to each other and we’re going to have just one scale house instead of two servicing both of them. We’re going to have the commercial scales automated to alleviate some of that work for the scale operator.”

PHOTO: LOS ANGELES SANITATION BUREAU

The system will involve the use of a radio frequency identification (RFID) system attached to the commercial trucks. The RFID system will provide information regarding the truck and company into the scale system. A touch screen will allow the driver to input weight origination data, while a weatherproof printer will provide weight ticket printouts. Unusual transactions will be handled by a pneumatic tube system that will allow information to be put into a plastic tube and sent to the scale house, saving the driver the time to walk to the scale house to complete a transaction. In addition, the scale systems will be tied into an automated gate system with color light indication and the data will flow into the district’s computerized scale system management program provided by PC Scale Co., Oxford, PA).

The district plans to install a hydrostatic scale system from Emery Winslow Scale Co. (Seymour, CT). “We’re replacing all of our scales right now with hydrostatic,” says Graeter. “We wanted to get a scale that would be very durable in a situation with a lot of water. It seems like what we saw was the hydrostatic scales are a very durable scale, very reliable. When you get in a transfer station situation and your scales go down, its a major impact on your transfer station. It is very important that we have a scale system that we can really rely on. Again, we’re in the process right now of building the addition to the building, putting in the new scales and then the new software. The hydrostatic scales are a little more expensive upfront, but we looked at the cost that we’d been paying to keep our current scales up. It was substantial, and we’re hoping that a larger investment up front will pay off in the long run—not just in maintenance costs but also in operations.”

Scaling in Central California
The Kern County Waste Management Department (Bakersfield, CA) operates seven landfills that range in size from seven tons to 1,500 tons per day. Five of the sites have 70-foot mechanical scale systems, and the county recently added a second scale system to one of their busiest sites at the Shafter-Wasco landfill. “The traffic was getting up high enough where it was getting too problematic to have people crossing back and forth, so we put in the outbound scale,” states operations manager Doug Landon.

PHOTO: LOS ANGELES SANITATION BUREAU

The county selected an electro-mechanical scale from Fairbanks Scales (Kansas City, MO) for the new installation. “One of the big reasons is just from the logistics of how we laid out our gatehouse with the road in and out,” stated Landon. “We wanted something flat to the road surface as opposed to having a raised platform. There would have been design issues related to drainage. From what our engineers also were telling us about handicapped access, to be able to have handicapped access with the gatehouse would have been more to work with platform scales. With the research we did, we felt that mechanical scales were more durable and accurate and in the long run would turn out to be lower cost.”

One of the critical issues for installation was the pit itself and the fact that the scale system was being installed at an operating landfill site. “With every scale that we’ve ever done prior to this, we didn’t have to worry about continuing operation while it was going in,” comments Landon. “This one was going into an existing facility with traffic going out on the existing lane. One of the first things the contractor had to do was establish a bypass for the traffic that, space-wise, was tight for us. The pit is of prime importance as far as tolerances because it’s got to house the scale. We required that the contractor that was going to oversee that work had to have done this type of work before. On the pit, where you get into problems is that the pit is built to such tolerances. If whoever is building it doesn’t appreciate that well enough and gets a little bit sloppier, then the scale guys get in there and things aren’t laying out right. They are not having the space they need or too much space and then you’re going to have to redo it.”

Landon has been very satisfied with the performance of the scales. “The scale itself has basically been flawless,” he says. “Any problems that we’ve had have been a direct result that we didn’t get in there and clean out after a big storm or some events that brought a lot of water and sedimentation in. As long as we’ve kept a close eye on getting the pit cleaned out and doing the preventative maintenance they’ve performed very well.”

Writer Lynn Merrill is a consultant based in southern California.

MSW - November/December 2005