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When counting thousands of parts every day, accuracy is key. Mistakes can be costly and translate in overfilling costs, unstable production processes, waste and a dam-aged reputation. Piece counting with scales is a simple and fast way to determine numbers of parts in receiving, warehousing, production or shipping. However, count-ing accurately requires a thorough counting solution.

Contents

1 Parts weights and deviation

2 The right counting scale system

3 Counting resolution vs. internal resolution

4 Benefits of scale calibration

5 The environment

6 What does “accuracy” mean for you?

7 Good Weighing Practice™ for Counting

This includes: •Anefficientcountingprocessforyouroperators•Adequatecountingscales•Operatortrainingand•Aplantomaintaintheoperationalreliabilityand accuracy of the scales.

What influences counting accuracy?Parts counting can be influenced by a multitude offactors.However,amongthemainimpactsarethe:•Partsweightsanditsdeviation•Theaccuracyofthecountingscales (counting resolution) •Theenvironment

Let’s take a closer look on each of those factors.

Best Practices in CountingHowtoAchieveAccurateCountingResults

2 METTLER TOLEDO - White Paper - Counting © 2012 Mettler-Toledo AG

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Yourcountingsystemshouldbespecifiedaccordingthecriticalpartsyouhavetocount.Therearecriti-calcomponentstoeverycountingprocess,whichincludecountingpartsthatareverylightand/orpartsthathavethegreatestdeviationinpieceweight.Eachparthasacertaindeviationthatcanbemeasuredand calculated with the standard deviation. For high-precisionparts,thestandarddeviationoftheaver-agepieceweight(APW)isabout0.5%.ThatmeansthatapartwithAPWof1.5gramshasadeviationof0.0225grams.Practicalexperienceshowsthatstan-darddeviationsvaryfrom0.5–1.5%.Therealsoarepartswithahigherstandarddeviation,suchassomeplastic-moldedparts.

Whatistheimpactoftheaveragepieceweightanditsdeviation on the counting accuracy? In summary, the higher the standard deviation, the higher the resulting counting error. In addition, the smaller the average pieceweight,thehigherthecountingresolutionofyourreferenceorsamplescaleneedstobetoachieveresults in a defined accuracy range. Therefore, when specifyinganewcountingsystem,askyoursupplierto take those two factors into account.

1 Parts Weights and Deviation

Tostartcountingwithascale,theaveragepieceweight first needs to be determined. Industrial count-ingscalescanstoretheaveragepieceweightinitsmemory.Todothis,firstweightensamplepartsandpressthesample10(orreference10)key.Nowthescalewillstoretheaveragepieceweighttoitsinternalmemory, a crucial key to achieving accurate counting results. Making a mistake here translates to big errors throughoutthecountingprocess.

Here are some key considerations when determining averagepieceweight:

1.Thesamplepartsyouuseneedtoberepresentativeofthepartsyouwanttocount.Foreachnewbatch,anewaveragepieceweightshouldbedetermineddueto changes in raw materials or machine settings.

2.Thenumberofsamplepiecesimpactsthecountingaccuracy.If,forexample,yourpartshaveahighstan-dard deviation, you can cut the counting error in half by increasingthenumberofsamplepartsfrom10to30.

3. The counting scale used to determine the average pieceweightneedstohavetheappropriatecountingresolutiontoaccuratelymeasurethesampleweight.Thiscanimplythatatwo-scalecountingsystemisneededwithahigh-resolutionsample(reference)scale connected to the counting (bulk) scale where theactualcountingtakesplace.

2 The Right Counting Scale System

3 METTLER TOLEDO - White Paper - Counting © 2012 Mettler-Toledo AG

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Notonlyisitimportanttoselecttherightequipment,butinstallingitcorrectlyandgettingoperatorstrainedisparamounttoputtinginplaceamaintenanceandcalibrationplan.Eventhoughindustrialscalesaredesigned to withstand harsh manufacturing environ-ments,theymustbeperiodicallytestedandmain-tained.Properscalecalibrationisespeciallyimportant

whenoperatingatwo-scalesystem.Ifthesampleandcountingscalearenotperiodicallycalibrated,chancesare the counting results are wrong. Qualified calibra-tion and maintenance makes sure your measuring deviceoperatesreliablyanddeliversaccurateresultsthroughout its lifetime.

4 Benefits of Scale Calibration

Howaccurateascalecandeterminethesampleweight is determined by its counting resolution. To achieveahighcountingresolutionthequalityoftheweighingelectronicintermsoflownoise,adequateintegrationandproperfilteringisofmajorimportance.

The counting resolution should not be mistaken with the so called “internal resolution” which has hardly any influence on the counting accuracy of a scale.

3 Counting Resolution vs. Internal Resolution

Ambientconditionsalsoinfluencecountingaccuracy.Toachieveanoptimumresult,theenvironmentmustbeasfreefromvibrationsaspossible.Forexample,drafts caused by fans or air conditioning systems mustbeprevented,asshoulddirectsunlightandfluctuationsintemperature.Thescaleitselfshouldbe

alignedhorizontally,especiallyhigh-resolutionscales.In each case, correct installation, user training and the scalescapabilityoffilteringnoisehaveapositiveef-fect on the counting result.

5 The Environment

Whatareyourprocesstolerances?Thisisanimpor-tantquestionthatneedstobeansweredduringtheevaluationofacountingsolution.Accuracyisnotthesame for everyone. To select the right counting scale,

it’skeytoknowifadeviationof+/-1%(e.g.+/-1partsof100parts)isacceptableorifitneedstobehigher.

6 What Does “Accuracy” Mean for You?

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BasedontheprinciplesoftheglobalWeighingStan-dard - Good Weighing Practice™ (GWP®)-METTLERTOLEDOhelpscustomerstodeterminetherightcount-ingsolutionforanyprocessrequirement.

With GWP®RecommendationforCounting*,youcanbe sure that:

Evaluation•Yourcountingprocessisevaluatedbydetermining theaveragepieceweightsanddeviationsofthe criticalpartsandyourprocesstolerances

Selection•Therightscalesandbalancesareselectedbased onyourrequirementsandtheoptimalsizeofsample (reference)piecesisspecified

Installation•Successfulinstallationandusertrainingis performed

Calibration•Scalesandbalancesaremaintainedandcalibrated regularlytopreserveaccuracyandtogeneratethe necessary documents for audits

Routine Operation•Traininghasbeenconductedensuringgood operation

7 Good Weighing Practice™ for Counting

Get more information on GWP®RecommendationforCounting* www.mt.com/gwp-rec-counting

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GWP® is the only science-based global standard for efficient lifecycle management of weighing systems,applicabletoallkindsofweighing systemsofanymanufacturer.Ithelpsyouto

•choosetherightbalanceorscale•calibrateandoperateyourweighing equipmentwithsecurity•complywithcurrentquality,compliance, laboratory and manufacturing standards

www.mt.com/GWP

*availableinselectedcountries