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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
Mettler-Toledo AG CH-8606GreifenseeSwitzerlandPhone +41 44 944 22 11Fax +41449443060
Subjecttotechnicalchanges©01/2013Mettler-ToledoAGMarCom Industrial
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
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