A belt weigher may be incorporated into the conveying line and the instantaneous mass flow rate of the material moving along the belt may be established. This
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A Guide to Dynamic Weighing for Industry Page 2 of 83 PANEL RESPONSIBLE FOR THIS GUIDE The Weighing & Force Measurement Panel reporting to the Learned Society Board of the Institute of Measurement and Control has prepared this Guide. The persons listed below served as members of the Weighing & Force Measurement Panel in preparing this Guide: Dr T Allgeier Flintec UK Ltd Mr J Anthony UK Weighing Federation Mr M Baker Sherborne Sensors Ltd Mr A Bowen AB Measurement and Control Solutions Mr P Dixon National Measurement Office Professor U Erdem Consultant, UE Consulting Mr P Harrison United Kingdom Accreditation Service Mr M Hopkins Procon Engineering Ltd Mr A Knott National Physical Laboratory Mr S Maclean Sherborne Sensors Ltd Professor J Pugh Glasgow Caledonian University Mr D Smith Avery Weigh-Tronix Mr A Urwin The Quality Scheme for Ready Mixed Concrete Mr C Whittingham M & C Engineers Mr B Yarwood Consultant, Dynamic Weighing Mr P Zecchin Nova Weigh Ltd The Weighing & Force Measurement Panel would like to express its thanks to the specialists who contributed to the preparation of the following sections: Section 3.2.1.3 Mr N Clark of Ishida Europe Ltd Section 5.2.1 Mr R W Stokes of Central Weighing Ltd Section 5.2.2 Mr D McLennan of Railweight Ltd Section 5.2.3 Mr D Pewter and Mr G Dorney of Herbert Industrial Ltd The Institute is grateful to members, specialists, and their organisations for providing relevant illustrations, and also to other providers of figures and photographs, including Eastern Instruments Inc and Gericke GmbH. This Guide is subject to review at any time by the responsible technical group of the Institute. The Institute welcomes all comments on this Guide and requests that these should be addressed to the Institute. DISCLAIMER This Guide represents the professional judgement of the members of the Weighing & Force Measurement Panel and the external contributors listed above. The Institute shall not be responsible to anyone for the use of or reliance upon this Guide by anyone. The Institute shall not incur any obligation or liability for damages, including consequential damages, arising out of or in connection with the use of, interpretation of, or reliance upon this Guide. Publication reference number WFMP1010
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A Guide to Dynamic Weighing for Industry TABLE OF CONTENTS Page 1. FOREWORD6 2. SCOPE OF DOCUMENT..7 3. DISCRETE MASS DELIVERY SYSTEMS.10 3.1 PROCESS BATCH WEIGHERS10 3.1.1 Application..11 3.1.2 Construction14 3.1.3 Typical Performance..14 3.1.4 Factors Affecting Accuracy.16 3.1.5 Calibration/Verification..17 3.2 GRAVIMETRIC FILLING MACHINES..17 3.2.1 Net Weighers17 3.2.1.1 Conventional Net Weighers..18 3.2.1.1.1 Application18 3.2.1.1.2 Construction19 3.2.1.1.3 Typical Performance..20 3.2.1.1.4 Factors Affecting Accuracy.20 3.2.1.1.5 Calibration/Verification..21 3.2.1.2 Weigh-Out Weighers21 3.2.1.2.1 Application21 3.2.1.2.2 Construction21 3.2.1.2.3 Performance21 3.2.1.2.4 Factors Affecting Accuracy.21 3.2.1.2.5 Calibration/Verification..21 3.2.1.3 Selective Combinational Weighers.22 3.2.1.3.1 Application22 3.2.1.3.2 Construction22 3.2.1.3.3 Typical Performance..25 3.2.1.3.4 Factors affecting Performance..26 3.2.1.3.5 Calibration/Verification..27 3.2.2 Gross Weighers..28 3.2.2.1 Application28 3.2.2.2 Construction28 3.2.2.3 Typical Performance..29 3.2.2.4 Factors Affecting Accuracy.29 3.2.2.5 Calibration/Verification..30 4. DISCONTINUOUS TOTALISING WEIGHERS31 4.1 SHIPPING AND RECEIVING WEIGHERS..31 4.1.1 Application..31 4.1.2 Construction31 4.1.3 Typical Performance..32 4.1.4 Factors Affecting Accuracy.32 4.1.5 Calibration/Verification..33 4.2 IN-PROCESS WEIGHERS.34 4.2.1 Application..34 4.2.2 Construction34 4.2.3 Typical Performance..34 4.2.4 Factors Affecting Accuracy.34 4.2.5 Calibration/Verification..35 Page 3 of 83
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A Guide to Dynamic Weighing for Industry 5. IN-MOTION WEIGHING SYSTEMS..36 5.1 CONTINUOUS WEIGHING SYSTEMS.36 5.1.1 Belt Weighers..36 5.1.1.1 Application36 5.1.1.2 Construction37 5.1.1.3 Typical Performance..38 5.1.1.4 Factors Affecting Accuracy.39 5.1.1.5 Calibration/Verification..39 5.1.2 Momentum Change Mass Flow Meters39 5.1.2.1 Application41 5.1.2.2 Construction41 5.1.2.3 Typical performance42 5.1.2.4 Factors Affecting Accuracy.42 5.1.2.5 Calibration/Verification..43 5.1.3 Weighed Feeders (Loss of Weight Feeders).43 5.1.3.1 Application44 5.1.3.2 Construction46 5.1.3.3 Typical Performance..48 5.1.3.4 Factors affecting Performance..48 5.1.3.5 Calibration/Verification..49 5.2 DISCRETE MASS WEIGHING SYSTEMS..50 5.2.1 Road Vehicle Weighing..50 5.2.1.1 Conventional Load Cell Weighbridges.51 5.2.1.1.1 Application51 5.2.1.1.2 Construction52 5.2.1.1.3 Typical Performance..52 5.2.1.1.4 Factors Affecting Accuracy.54 5.2.1.1.5 Calibration/Verification..55 5.2.1.2 Foundation-Less Weighbridges56 5.2.1.2.1 Application56 5.2.1.2.2 Construction56 5.2.1.2.3 Typical Performance..57 5.2.1.2.4 Factors Affecting Accuracy.57 5.2.1.2.5 Calibration/Verification..57 5.2.2 Rail weighbridges..58 5.2.2.1 Conventional Load Cell Weighbridges.59 5.2.2.1.1 Application59 5.2.2.1.2 Construction60 5.2.2.1.3 Typical Performance..62 5.2.2.1.4 Factors Affecting Accuracy.62 5.2.2.1.5 Calibration/Verification..63 5.2.2.2 Foundation-Less Weighbridges65 5.2.2.2.1 In-Track Weighbridges65 5.2.2.2.1.1 Application66 5.2.2.2.1.2 Construction66 5.2.2.2.1.3 Typical Performance..68 5.2.2.2.1.4 Factors Affecting Accuracy.68 5.2.2.2.1.5 Calibration/Verification..68 5.2.2.2.2 Active Sleeper Weighbridge.69 5.2.2.2.2.1 Application69 5.2.2.2.2.2 Construction69 5.2.2.2.2.3 Typical Performance..70 5.2.2.2.2.4 Factors Affecting Accuracy.70 5.2.2.2.2.5 Calibration/Verification..70 Page 4 of 83
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A Guide to Dynamic Weighing for Industry 5.2.2.2.3 Surface Mount Weighbridges..70 5.2.2.2.3.1 Application70 5.2.2.2.3.2 Construction71 5.2.2.2.3.3 Typical Performance..71 5.2.2.2.3.4 Factors Affecting Accuracy.71 5.2.2.2.3.5 Calibration/Verification..71 5.2.2.3 Portable Weighbridges72 5.2.2.3.1 Application72 5.2.2.3.2 Construction72 5.2.2.3.3 Typical Performance..73 5.2.2.3.4 Factors Affecting Accuracy.73 5.2.2.3.5 Calibration/Verification..73 5.2.3 Catch Weighing..73 5.2.3.1 Application74 5.2.3.2 Construction75 5.2.3.3 Typical Performance..77 5.2.3.4 Factors Affecting Accuracy (Categories X & Y).80 5.2.3.5 Calibration/Verification..80 6. BIBLIOGRAPHY.82 6.1 Useful Reading Material.82 6.2 Recommendations by the International Organisation of Legal Metrology (OIML) and Legislative Documents82 7. USEFUL ADDRESSES83 Page 5 of 83
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A Guide to Dynamic Weighing for Industry 1. FOREWORD This document has been compiled in recognition of the need to provide a comprehensive and authoritative description of every type of weighing machine or process involving a dynamic element. For the purposes of this document, these cases are defined as those where the product or object being weighed is in net motion relative to the weighing machine either while it is being weighed or directly before or afterwards, such that its motion impacts on the method and/or the accuracy of the weight measurement. The document is intended to inform the potential users and suppliers of such equipment about the salient issues that might be considered when evaluating a particular solution to suit a given weight measurement requirement. Many of the weighing machines described in the text are commonly used for the purposes of trade transactions and, as such, are almost always subject to Weights and Measures Legislation. It is not intended that this Guide should, in any way, conflict or substitute for relevant authorised regulations. Where appropriate, reference is made to the appropriate statutory and regulatory documents. The users and suppliers of such equipment need to familiarise themselves with the content of the correct statutory and regulatory documents and these take precedence in all cases where the use of equipment is governed by legislation. Page 6 of 83
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A Guide to Dynamic Weighing for Industry In the fiTypical performancefl sections, the weight range and accuracy figures stated are simply given as a guide to standard equipment regularly used in such applications Πthis is not to say that equipment is unavailable outside these weight ranges or that its performance may not be significantly better or worse than the values quoted. Calibration is simply the act of determining the accuracy of the indications of the instrument throughout its measuring range. It does not involve the adjustment of the instrument to improve the accuracy of the indications, although such adjustment may take place at the same time. The term Verification is used mainly in legal metrology Рthat is the use of weighing or measuring instruments for purposes which fall under the control of either national or European legislation relating to weighing or measuring for purposes of trade, law enforcement, and consumer protection. fiVerificationfl, in legal metrology terms, is a specific act, carried out to determine that an instrument performs within the permitted error allowances throughout its measuring range and that it also conforms to all relevant Weights and Measures legislation. Verification can be carried out by: A Weights and Measures Inspector; an Authorised Person; an Accredited Person, Manufacturer or Installer. Once the instrument has been tested, examined, and found to meet all the legislative requirements it is usually marked in some way, either with a stamp or irremovable sticker, identifying the organisation or person that has verified the instrument. Verification is necessary before an instrument can be taken into use for a legally controlled purpose. fiAuthorised Personfl is a term used specifically in the Non-automatic Weighing Instruments Regulations 2000 (SI 2000 No 3236). It means an inspector (of Weights and Measures) or some other person employed by a local weights and measures authority, who is authorised by the Chief Inspector of Weights and Measures of that authority to exercise functions under the regulations within the area of the local authority. Before these regulations were introduced, the enforcement and verification activities under previous regulations could only be exercised by an Inspector of Weights and Measures; these regulations allow for other persons, suitably trained, to carry out those functions. A selection of useful reading material is given in Section 6 Bibliography. Some of these may be given as a specific reference; in this case the reference number is given in the text in bold and in bold square brackets. Some of the dynamic weighing systems may be subject to legal metrological control. Where it is relevant, each section lists the appropriate informative references in the form of the European Measuring Instruments Directive (MID), International Organisation of Legal Metrology (OIML) Recommendations, and legal documentation. This Guide, however, should not be considered a complete source of regulations and further information should be sought from authoritative bodies such as National Metrological Institutes or Local Enforcement Offices. Page 8 of 83
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A Guide to Dynamic Weighing for Industry GUIDE TO THE DOCUMENT FOUNDATION-LESS WEIGHBRIDGES5.2.1.2CONVENTIONAL LOAD CELL WEIGHBRIDGES5.2.1.1SURFACE MOUNT WEIGHBRIDGES 5.2.2.2.3 ACTIVE SLEEPER WEIGHBRIDGES 5.2.2.2.2 IN-TRACK WEIGHBRIDGES 5.2.2.2.1 PORTABLE WEIGHBRIDGES5.2.2.3 FOUNDATIONLESS WEIGHBRIDGES 5.2.2.2 CONVENTIONAL LOAD CELL WEIGHBRIDGES 5.2.2.1 CATCH WEIGHING 5.2.3 RAIL WEIGHBRIDGES 5.2.2 ROAD VEHICLE WEIGHING 5.2.1 WEIGHED FEEDERS 5.1.3 MOMENTUM CHANGE MASS FLOW METERS5.1.2 BELT WEIGHERS 5.1.1 SELECTIVE COMBINATIONAL WEIGHERS 3.2.1.3 WEIGH OUT WEIGHERS 3.2.1.2 CONVENTIONAL NET WEIGHERS 3.2.1.1 GROSS WEIGHERS 3.2.2 NET WEIGHERS 3.2.1 PROCESS BATCH WEIGHERS 3.1 GRAVIMETRIC FILLING MACHINES 3.2 DISCRETE MASS WEIGHING SYSTEMS 5.2 CONTINUOUS WEIGHING SYSTEMS 5.1 IN-PROCESS WEIGHERS 4.2 SHIPPING & RECEIVING WEIGHERS 4.1 IN-MOTION WEIGHING SYSTEMS 5 DISCONTINUOUS TOTALISING WEIGHERS 4 DISCRETE MASS DELIVERY SYSTEMS3 DYNAMIC WEIGHING SYSTEMS STATIC WEIGHING SYSTEMS WEIGHING SYSTEMS Page 9 of 83
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A Guide to Dynamic Weighing for Industry 3. DISCRETE MASS DELIVERY SYSTEMS Discrete mass delivery systems cover a diverse range of weighing machines. What makes them unique is that they weigh defined masses of material into individual batches. Each batch may be either put into individual containers or combined with other weighed masses to make up a prepared formulation or batch mixture. Many dynamic weighing systems fall into this category from machines that prepare pre-packaged goods to pre-mixed materials for industrial batch processes. The weighed ingredients may be powders, granules, lumps, or liquids that are proportional by weight rather than volume. 3.1 Process Batch Weighers Many industrial processes are easier to operate if the materials are separated into discrete batches where the weights and mixing quality can be tightly controlled. The industries where batch weighing is most prevalent are as follows: Animal Feed Food Chemical Mineral Fertilizer Rubber & Plastics Pharmaceutical Glass They range from single weigh vessel systems often referred to as Cumulative Batching Systems where each ingredient is weighed sequentially and layered into the weigh hopper, to complex multiple weigh vessel systems also known as Simultaneous Batching Systems where ingredients are weighed simultaneously by separate vessels. This latter group encompasses a wide range of hybrids that weigh both simultaneously and sequentially. The vessels to be weighed are supported by load cells that are connected to a Batch Controller, which usually serves both to energise and condition the load cell signals as well as to perform the logic operations based on those signals and to provide outputs to other peripheral equipment and the operator. Systems often involve other process steps (such as heating or analytical control) interwoven into the overall control sequences, but for the purposes of this document the sequence actions considered are fundamentally related to weight measurement only and utilise dedicated batch weighing control instrumentation. The weighed vessels are basically static weighing devices that share the influences and considerations considered elsewhere (see section 6, WGC1099). The dynamic element in the measurement comes from the fact that the product being weighed is in motion before, possibly during, and after the measurement has taken place, and the effect of this motion is the focus of what follows in this section. The dynamic nature of the process leads to two main measurement and control issues that require consideration. When a material is being added to or removed from a vessel there is a delay in response between the observation that the required target weight has been reached and the termination of the addition. This delay or ‚in-flight™ time is a function of the overall measurement and control loop delays as well as the physical arrangement of the system. There will be additional material added (or removed) during this delay, and this will introduce an error that all batch weighing systems attempt to regulate and minimise. There is an additional measurement error caused by the change in momentum of the flowing material as it enters or leaves the weighed vessel and again batching systems need to account for this effect (see section 6, WGC1099). Page 10 of 83
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A Guide to Dynamic Weighing for Industry 3.1.1 Application Dedicated process batch weighing systems are beneficially used in applications where the manufacturing process is fundamentally centred on weight measurement coupled with the control of the plant sequences up and down stream of the weighing measurement, but with limited other process measurement or control requirements. In these applications the speed and expert knowledge facilities built into a dedicated system can bring technical advantages at a lower cost than that offered by more general process control solutions. Process batch weighing systems can have almost infinite configuration possibilities but, for the purposes of illustration in this document, fall into two broad categories of cumulative batching and simultaneous batching which may be combined and usefully referred to as combination batching. Cumulative Batching This technique requires minimum space and minimum materials handling and control equipment. As a result, costs are less than for the equivalent simultaneous batching system, see Figure 3.1.1. Figure 3.1.1 Schematic representation of cumulative batching The recipe can be set up within the batch controller, so that the target desired weights are set at: Set point 1 = Target weight, Ingredient 1 Set point 2 = Target weight, Ingredient 1 + Target weight, Ingredient 2 Set point 3 = Target weight, Ingredient 1 + Target weight, Ingredient 2 +Target weight, Ingredient 3 and so on. In order to avoid cumulative errors, current practice is to set the independent target weights and introduce a tare operation by the controller to zero the weight indication. When all ingredients have been weighed the batch is discharged to the next stage in the process such as a mixer. The use of one weighing system with a fixed weighing range may mean that acceptable accuracy may be impossible to achieve when very small ingredient amounts are added. Simultaneous Batching This type of system achieves the highest rate of production, as all materials are fed and weighed at the same time. Furthermore, higher weighing accuracies can be achieved as the capacity of each weigh hopper is designed to match the required range of each ingredient. Page 11 of 83
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