paul reagan, sampling associates international, us, and

FFor commercial transactions of bulk com-modities, determination of the quantity ofmaterial is critical. Weight discrepanciesare often the source of vigorous commer-cial disputes. This article surveys theimportant issues in the weighing of bulkmaterials and the different weighing tech-nologies available. While draft surveys area widely used method for weight determi-nation for waterborne shipments of bulkcommodities, this technique is notaddressed here, as it is not strictly a weigh-ing technology.

Important issuesThere are a number of significant issues tounderstand prior to any discussion ofavailable technologies.

Custody transfer vs process controlOne very important issue is that there aretwo different general applications for mostweighing technologies. The first applica-tion is for custody transfer, i.e. the officialweight on which payment is made on thecommercial transaction between a buyerand seller. The second is for process con-trol, which means that it is only used fornon-commercial purposes such as loadingcontrol, batching, blending and generalinventory, but may not have or need theaccuracy necessary for payment purposes.

A frequent problem in the bulk materi-als industry is that equipment intended forprocess control is used for custody transferpurposes.

Maintenance and housekeepingAs with all sophisticated equipment, goodpreventive maintenance is important forperformance. As the ‘output’ of scales isthe weight of material, lack of maintenanceleads to erroneous readings.

Good weighing practice requires specialemphasis on housekeeping. One of themost frequent causes of poor scale perfor-mance is the buildup of loose material onor under the scale platform or weigh-bridge. This can add to the weight sensedby the weighing elements, especially if thescale is not tared or zeroed out prior toweighing. It can also cause errors byinhibiting proper scale movement. If notcleaned regularly, such buildup will furtherdegrade scale performance through exces-sive part wear, corrosion and improperresistance to moving parts.

A certifiablescale still needsto be certified‘Certifiable’ refersto the hardware,i.e. the fact that thescale is designed

Reprinted from World Coal • February 2004

Paul Reagan, Sampling Associates International, US, and Jim Freeman,Kanawha Scales & Systems, US, review the basic principles of weighing technologies available, highlighting the strengths and weaknesses of each method.

Table 1. The impact of different levels of weighing errors

Weighing error percentage (±) Cost of weighing error (US$/year)

0.10 150,000

0.25 375,000

0.50 750,000

0.75 1.25 million

1.00 1.5 million

Figure 1. Digital weight indicator.

Figure 2. Load cell assembly for weigh bin system.(Photo courtesy of Kanawha Scales & Systems.)

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and manufactured to achieve the necessaryaccuracy for commercial transactions.‘Certified’ means that, not only does thehardware have the necessary capability,but it is also installed, maintained, calibrat-ed and tested to meet the required toler-ances.

The use of equipment that is certifi-able, but not certified, commonly resultsin scale systems that are assumed to beweighing accurately, but may in actualityperform quite poorly.

Calibration andtestingThere are a number ofprocesses involved inthe quality assuranceof scale systems.Calibration is theprocess by which ascale is checked andbrought into accept-able weighing accura-cy tolerance limits byusing specially manu-factured weights of aknown mass (calibra-

tion weights). All scales need to becalibrated and all need calibrationweights, or valid weight substitu-tion, to do so.

Some applications have calibra-tion weights included, or built intothe installation, whereas othersneed to have special calibrationweights transported to the site eachtime calibration is performed.

Calibration is not the same ascertification. Certification is a docu-mented process where a scale isproven to perform to certain specifi-cations and tolerances according tothe governing authority for thejurisdiction in which the scale isphysically located. It is also usuallyperformed in the presence of a rep-resentative from the authority, andis required for custody transferscales on which payment is made.

Calibration is an important pre-requisite for the certification test butneeds to be performed much morefrequently, to ensure that scales con-tinue to perform satisfactorilybetween official certifications. Forexample, many governing authori-ties require a certification test on anannual or semi-annual basis, whilecalibration is often performed week-ly. Also, although process controlscales that are not used for payment

do not need to be certified, they still needto be calibrated regularly.

The cost of errorsWeighing errors are common. Simply stat-ed, the higher the weighing error, the high-er the error in payment that one partytransfers to another. It is not uncommon tofind systems, in a custody transfer applica-tion, with errors of ± 0.75% or worse. Table1 shows the impact of different levels ofweighing errors for a 5 million tpa

operation for a bulk material with a valueof US$ 30/t. A good scale maintenance andcalibration programme will pay for itselfmany times over.

Technologies availableCertifiable and process control scales needto be installed properly, maintained well,and calibrated frequently. Often, applica-tions do not perform well because they arenot installed properly, or are poorly main-tained. In many cases, these problems (andthe associated weighing errors) are noteven recognised because of a lack of cali-bration and testing.

Static and dynamic weighing systemsWeighing systems can generally be sepa-rated into two broad categories: static anddynamic. Static systems are weighingdevices that weigh the material while atrest; dynamic weighing systems determinethe weight of the commodity while it is inmotion.

As static weighing systems determinethe weight of material at rest, or in a stateof equilibrium, the achievable weighingaccuracy is generally higher than withdynamic systems. Even so, technology con-tinues to improve, and many modern-daydynamic weighing devices can providevery acceptable weighing accuracies forcustody transfer applications at a lowerinitial cost. However, time and resourcesneeded for the ongoing maintenance, test-ing and certification of each of the types ofweighing devices can differ significantly.

Common denominatorsRegardless of the technology used, allscales have the following basic compo-nents:

� A surface upon which either the mate-rial or the container/vehicle holdingthe material rests (often known as theweighbridge), or a vessel that containsand captures the load (often known asa weigh bin or hopper). This compo-nent transfers the applied load to theweight-sensing elements.

� Weight-sensing elements that convertthe mass sensed into a signal that canbe interpreted.

� A weight indicator that captures theabove signal and then displays thedetermined weight.

The weight-sensing element is veryimportant. Today, there are three generaltypes.


Figure 3. Electronic truck scale. (Photocourtesy of Unibridge Scale Systems.)

Figure 4. Certified calibration weights being loaded into certi-fied weight cart. (Photo courtesy of Kanawha Scales &Systems.)

Figure 5. Continuous length single draft rail scale. (Photocourtesy of Rice Lake Weighing Systems.)

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Mechanical lever systems This is an older technology (but still wide-ly used) based on the principles of a simplelever. A load applied on the scale platformproduces a force at the end point of a seriesof levers proportional to the applied load.

Hydraulic/hydrostatic load cellHydraulic cells operate under the simplehydrostatic principle that load applied to aknown area provides a known resultantoutput pressure. While not as common aslever systems and electronic load cells,these can perform effectively in manyapplications.

Electronic load cellThis most common weight-sensing ele-ment is a device that deforms under load,which then produces a small analogueelectrical signal proportional to the appliedload (Figure 2).

Today, digital load cell systems, whichconvert the traditional analogue signal todigital, are becoming widely used. The dig-ital signal is more stable and less suscepti-ble to outside interference, such as radiofrequency and electromechanical signals. Itcan also travel further than its analoguecounterpart, and the technology enablesadvanced troubleshooting capabilities.Many users are upgrading older scales tothis technology.

Static weighing devicesIn static weighing applications, the materi-al only needs to be at rest long enough forthe commodity being weighed to settle.Once a stable scale reading can be attained,an accurate weight reading is captured,and the process continues. All of the staticweighing devices below can achieveweighing accuracies of ± 0.1%.

Truck scalesA truck scale is a widely used means ofweighing material in vehicles (Figure 3).As the material is in the truck, it is alreadyat rest. The truck itself comes to a full stopon a platform or deck (the weighbridge),typically constructed of some form of rigidstructural steel, which is entirely support-ed by multiple (typically 8 - 10) load cellsor a lever system. The force of the load isdistributed over these weight-sensing ele-ments and the signals are captured andsummed to obtain the total weight.

Scale weighbridges range greatly in sizeaccording to the application. Many stan-dard bulk commodity applications rangefrom 20-m-long and 3-m-wide to

36-m-long and 3-m-wide.While there are a

number of variables thatcan affect prices, a com-pletely installed truckscale costs approximate-ly US$ 40,000. In addi-tion to its low cost,another advantage is thatit can be located (or relo-cated) almost anywherethat a level grade can befound. However, it mustweigh the material vehicle-by-vehicle, which can be time con-suming. For large tonnageoperations, this can cause abottleneck.

Testing and calibration of alltypes of truck scales is com-pleted with calibration testweights. These are cast or fab-ricated blocks that are a knownweight, calibrated in a labora-tory, and traceable to knownstandards. Once these certifiedcalibration weights areapplied, the scale indicationreads the applied weight, andthe scale is adjusted to agreewith the known calibrationweight standard.

Calibration weights are nota part of the delivered truckscale hardware, but are trans-ported to the scale wheneverthey are needed. As it is easilyaccessible by vehicle, the deliv-ery of the calibration weights to a truckscale is not a problem. The use of calibrat-ed, certified weight carts used in conjunc-tion with test weights has become verypopular in the US to facilitate a quick andeconomical method of testing and certify-ing rail and truck scales (Figure 4).

One important consideration when pur-chasing a truck scale is the ability of theweighbridge to withstand highly concen-trated loads, generated by single, tandemor multiple adjacent axles groups, over theweakest area of a scale weighbridge. This isgenerally the mid-span area between loadbearing sections of the scale.

While a truck’s total weight may notexceed the scale’s total gross capacity, it ispossible that, due to the truck configura-tion and axle groupings, the truck maygreatly exceed the concentrated load ratingof a scale’s weighbridge. If this is a fre-quent occurrence, the overall scale life willbe adversely affected.

As there are differences in the way inwhich scale manufacturers represent theconcentrated load ratings on their scales,selecting the correct equipment requires acareful assessment of the stresses generat-ed by the various truck configurations andaxle groupings that need to be weighed.Inadequately sized equipment is a com-mon cause of excessive scale wear and pre-mature weighbridge failure.

Static rail scales A static rail scale also weighs its cargowhile at rest. In this case, it weighs a railcarwhile it is stable and stationary on a railscale platform (Figure 5).

Static rail scales used for custody trans-fer are typically seen in one of three config-urations: continuous length, single draft, ordual draft. Continuous length is very simi-lar to a truck scale; there is a weighing plat-form, on which the railcar rests, which cap-tures the weight with a series of load


Figure 6. Certified calibration weights with hydrauliclift mechanism on weigh bin system. (Photo courtesy of

Kanawha Scales & Systems.)

Figure 7. Components of a batch weighing loadout system.(Diagram courtesy of Kanawha Scales & Systems.)

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sensors. The single and double draft con-figurations utilise a modular weighbridgethat is positioned directly under the axlepairing of the railcar. The single draft con-figuration uses two weighbridges and cap-tures the weight of the car in one reading.The double draft uses only one weigh-bridge, therefore the railcar is weighed intwo separate readings by capturing theweight of the first axle pairing and thenadding the weight of the second.

An installed double draft static rail scalecosts approximately US$ 90,000. It is a veryaccurate way to measure bulk materialstransported in rail wagons. However, therailcars must each stop completely in orderfor the weight to be captured, and it istherefore time-consuming. It is also costlyto relocate, so it is important to get the ini-tial location right.

Calibration and certification of railscales is usually performed with spe-cialised test equipment, which provides aknown weight with which the rail scale isto be calibrated. These may essentially bethe same as the test weights used for truckscales, except that more weight isrequired. Often, it is the railroads thatown, maintain and dispatch this spe-cialised equipment. As access to a railscale is usually via the railway itself, a fre-quent complaint regarding rail scales isthe logistical difficulty in scheduling thecalibration unit to the scale. Specialisedtruck-dispatched rail testing equipment isbecoming a suitable alternative for manyrail scale installations.

Weigh bin scalesWeigh bin (or hopper) scales allow staticweighing of bulk materials by collecting a‘batch’ of material, capturing the weight,then subsequently discharging theweighed material. A weigh bin scale is typ-ically a conical-shaped collection vessel,supported by or resting on three or fourload cells. They achieve high accuracy inweight determination. These scales aremuch more costly than other static weigh-ing devices, but they all combine impor-tant process control elements with theirweighing function.

They also incorporate internal calibra-tion weights so that scale accuracy can beverified within a few minutes. As they arestatic scales, they can be certified usingthese calibration weights (Figure 6).

Batch weighingThis system, sometimes referred to as adosing scale, is commonly used in rail and

truck loading applications (Figure 7). Inaddition to accurately capturing theweight of a consignment, batch weighingalso serves an important process controlfunction by allowing the railcars andtrucks to be loaded with a precise amountof material. By preparing a set amount ofmaterial for each rail wagon or truck, theloading function can be highly automatedand efficient by filling the vehicle to themaximum allowable weight, without over-loading or underloading.

A batch loadout system consists of thefollowing common components:

� A feed conveyor to the top of thebuilding provides continuous supplyof bulk material to the loadout.

� A surge bin creates a buffer betweenthe feed conveyor system and thebatching process.

� High-speed weigh bin feed gatesenable accurate batch preparation to adesired target weight.

The weigh bin scale captures the binweight after it has been filled, and thenagain after being discharged, and records anactual loaded material weight. A weigh bindischarge gate releases material from thebin, through a chute, into the railcar or truckbelow. A typical cost of an installed batchweighing system is US$ 1 - 1.5 million, butcan vary significantly depending on theconfiguration.

Pulse weighing system This is a dual weigh bin system used to pro-vide static weighing while allowing thebulk material to continue to flow (Figure 8).The flow of material is cycled back andforth between two adjacent weigh bins andthe consignment weight is the sum of theaccumulated batch weights.

The cost of an installed pulse weighingsystem is typically US$ 550,000 - 750,000.

Garner systemThis is similar to the other two systems, inthat that total weight loaded is captured indiscrete batches. The difference here is thatthere is an extra bin (called the garner bin)that provides greater control of the materi-al’s discharge once it is weighed, enablingefficient throughput with a single bin sys-tem. The garner system is used in direct


Figure 9. Model 10-14-4 precision conveyor scale.(Photo courtesy of Thermo Ramsey.)

Figure 10. Installation of a certified conveyor scale.(Photo courtesy of Thermo Ramsey and KanawhaScales & Systems.)

Figure 8. Diagram of pulse weighing system.

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vehicle loading applications and continu-ous material flow processes such as stock-piling.

Dynamic weighing systemsDynamic scales for the bulk materialsindustries largely consist of two maintypes: in-motion railroad track scales andscales on conveyor belts. Achievable accu-racies are generally less than static weigh-ing systems due to the many differentexternal forces that can affect the weighingsystem performance in dynamic applica-tions.

In-motion rail scalesAn in-motion rail scale is usually a singleweigh module under a section of track(approximately 3.8-m-long). It captures theaxle pairing weights in a dual draft modeof operation. Vehicle position sensors workwith sophisticated electronics instrumenta-tion to determine the weight of each indi-vidual car.

In-motion scales are configured for cou-pled or uncoupled in-motion operation.Uncoupled operations usually gravity-feeda car across a scale module in a controlledmanner. Coupled in-motion (CIM) systemsare more challenging, as the effects of thecouplers tend to induce extraneous forcesand stresses into the vehicle beingweighed. Proper installation, location andfoundation support are key to successfulinstallations, as grade and track curvaturecan have significant impacts. An improperinstallation can be very expensive to fix orrelocate.

An installed CIM rail scale costs approx-imately US$ 150,000. Its advantage is theefficiency gained due to the ability toweigh the cars in motion. However, manydifferent forces can disrupt its ability toperform, and locations available for a suc-cessful installation are limited.

The testing of CIM rail scales is a verycomplicated process that can be expensive.Unlike the individual test car used for thestatic rail scale, the CIM rail scale usuallyrequires a test train consisting of a mini-mum of 10 cars, run over the scale at leastfive times in any of the four modes ofweighing that it uses: pushing in eitherdirection and pulling in either direction.

Conveyor belt scalesOne of the most widely used devices forweighing bulk materials is the belt convey-or scale. As most bulk materials are movedover a conveyor belt at some point, this is avery convenient way to capture the weight

for process control and custody transfer,without the need to interrupt the flow ofmaterial.

In its simplest form, a belt scale is adevice that makes a determination of beltloading per length (i.e. kg/m), captures abelt travel rate (i.e. m/min), and then inte-grates this information into a weight pertime. This rate of flow can then betotalised, so that an accumulated amountof material that has passed on a conveyingsystem over the scale can be determined.

While there is a range of designs of beltscale systems, the typical belt scale systemincludes the same components as a staticsystem, e.g. weigh frame, load-sensing ele-ments and a data integrator, but has anessential additional sensor to determinethe speed of the belt.

The weigh frame supports a set of idlersmounted on a frame (Figure 9). This struc-tural frame is mounted to the conveyorstructure so that the load imparted throughthe idlers is applied to the load-sensing ele-ments (either a lever system with a loadcell, or a series of load cells). The result isthat as the conveyor belt travels overidlers, the load from the conveyed materialon the belt exerts forces on these idlers andthe weight-sensing elements. The weightreading on a known length of conveyor ismatched with the conveyor speed to cap-ture the weight of the material.

There are two major advantages of abelt scale. The first is its convenience, as thematerial is weighed while moving on theconveyor belt. The second is its cost; a cer-tifiable belt conveyor scale fully installedcosts approximately US$ 42,000 and canweigh to ± 0.125% accuracy.

However, a belt conveyor scale requiresa lot of attention in order to ensure its per-formance. In addition to the normal scalemaintenance requirements, many parts ofthe conveyor belt can affect weighing accu-racy.

In fact, it is essential to view the entireconveying system as part of the scale. Anyforces that act on the conveyor and impedethe scale’s ability to sense only the forcesfrom the material travelling on the convey-or belt can lead to poor weighing perfor-mance. This means that factors such as beltalignment, skirting, unrestricted rolling ofidlers, training idlers, structural deflectionsand gravity take-up are important aspectsof weighing with belt conveyor scales.Often, scales do not perform due to lack ofattention to the conveyor system.

The location of a belt scale is also a crit-ical factor. A frequent mistake in the

application of conveyor belt scales is locat-ing them in the wrong place (such as tooclose to a transfer or feed point).

Finally, conveyor belt scales are particu-larly susceptible to poor housekeeping.Material spillage is common with conveyorbelts in general; most of the spillage in thearea of the scale ends up on or under theweighbridge.

Many modern scales have the ability toperform an electronic calibration (called anE-cal or R-cal). This is often completeddaily, but is not a check of the actual accu-racy of the scale system (as is oftenassumed). Rather, it is simply a test of thestability of the electronics of the scale.

Another calibration technique is to use aknown static weight, which can periodical-ly be applied to the weighbridge. Thisweight allows a calibration of the weigh-bridge and load-sensing elements, but islimited because it does not take intoaccount any of the effects of the conveyorbelt movement and load distribution.

A more thorough calibration of a con-veyor belt scale is called a chain test. Here,a roller chain, with a known (and properlysized) weight per unit of length, is placedon top of a running conveyor belt to simu-late a material load on the conveyor in thearea of the weighbridge. This is more com-plete because it takes into considerationthe belt loading and conveyor influencessuch as alignment, tracking and belt ten-sion.

Due to the complex effects of the con-veyor forces on weighing accuracy, despitethe above calibration methods, the onlyway for a belt conveyor scale to be certifiedin the US is by conducting a materials test.This test requires passing a prescribedamount of material, which is weighedbefore or after on a separate scale that hasitself been certified within 24 hours, overthe conveyor scale. This can be expensiveand logistically complicated and is a signif-icant disadvantage of belt conveyor scales.

ConclusionThere are many options available when itcomes to the weight determination of bulkmaterials. Static weighing systems general-ly provide higher accuracies than dynamicsystems, but the stated accuracy of anyscale is only achievable with proper instal-lation and ongoing care. This includes spe-cial emphasis on housekeeping and cali-bration maintenance. Payment for anybulk material should only be done on a scale that is both certifiable and certified.______________________________�


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paul reagan, sampling associates international, us, and

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