1. Introduction to Checkweighing

First introduced in 1976, the Principles of Checkweighing has been the industry standard guide for checkweigher users, Weights and Measures officials, and the academic community. This Third Edition expands on core checkweighing fundamentals and includes the latest developments in packaging and checkweighing technology.

What is a Checkweigher?

A checkweigher is an in-line system that weighs 100% of items as they pass through a production line, classifies items by preset weight zones, and ejects or sorts items based on their classification.

A physical checkweighing system typically consists of an infeed section, scale section, discharge section, rejector or line divider, and computerized control system. Components vary widely based on application, product type, and operating environment.

Simply stated, a checkweigher weighs, classifies, and segregates items by weight.

Core Objectives of This Guide

• Educate the packaging industry on the basics of checkweighing
• Describe the full range of uses for checkweighing systems
• Outline system requirements for different applications
• Guide users to select the optimal checkweigher for their specific application
• Serve as a comprehensive reference guide for checkweighing

2. Typical Uses of a Checkweigher

Checkweighers deliver value across every stage of packaging and production, with core applications including:

• Check for under and/or overweight filled packages
• Ensure compliance with net contents laws for prepackaged goods
• Detect missing components in a package (labels, instructions, lids, coupons, or product)
• Verify count by weight (missing cartons, bottles, bags, or cans in a case)
• Validate package mixes against weight limits to maintain solid-to-liquid ratio standards
• Reduce product giveaway by using checkweigher data to adjust filler settings
• Classify products into weight grades
• Ensure product compliance with customer, association, or agency specifications
• Weigh items before and after a process to measure process performance
• Fulfill USDA or FDA reporting requirements
• Measure and report production line efficiency

Statistical Value of Checkweighing

The primary value of checkweighing is 100% sampling, compared to intermittent off-line sampling. For a line running 100 packages per minute, hourly sampling of 15 packages only inspects 0.25% of total production. Modern checkweighers eliminate this gap, collecting full production data in real time.

Core Statistical Applications

• Analyze production by weight zone or classification (5+ zones for detailed fill data)
• Monitor production efficiency via total count, total weight, and items per minute
• Track standard deviation to alert operators/fillers of out-of-tolerance conditions
• Store production printouts as compliance records for management and regulators
• Analyze single and multi-head filler performance
• Print production totals by day, shift, hour, batch, or product run
• Monitor short and long-term filler performance via statistical analysis
• Provide Statistical Process Control (SPC) charts for manual and closed-loop automatic process adjustments
• Link packaging line data to upstream control and information systems
• Interface with PLCs for remote checkweigher control and production integration
• Reduce Quality Control labor costs

Integrated Inspection Capabilities

Checkweighers have evolved into comprehensive quality assurance stations, integrating additional inspection devices to check for:

• Open flaps on cartons or cases
• Missing caps
• Bar code label presence and readability
• Metal contaminants

3. Why Checkweighing Is Legally Necessary

Packaging and processing companies refer to the checkweigher as the "policeman" of the packaging line. It is the weight control center of the production line, protecting against non-compliant under/overweight packages reaching customers.

Core Regulatory Standards

The U.S. Department of Commerce NIST Handbook 133 defines binding net contents laws for prepackaged goods, including:

• Testing and sampling procedures for package weight verification
• Minimum number of packages to be inspected per lot
• Lot size definitions and tare weight testing requirements
• Maximum Allowable Variation (MAV) for underweight packages
• Number of underweight packages permitted in a lot

State Weights and Measures offices enforce these NIST regulations, with enforcement actions ranging from oral recommendations and warnings to formal legal action.

Non-Compliance Triggers

An inspection will find legal non-compliance if any of the following are present:

• One or more packages with unreasonably large minus weight errors
• An average minus error for the entire lot of packages
• Significant errors in selling price computations for weigh-price labeling

Legal Actions for Non-Compliance

• "Stop sale" / "off sale" orders prohibiting lot sale until official release
• Re-weighing or remarking orders for full lot correction before sale
• Criminal prosecution, with inspectors purchasing/confiscating samples as evidence of violation

Intelligent checkweighing and weight control virtually eliminate legal risk and consumer complaints about underweight packages, while reducing overweight costs via improved accuracy and filler feedback.

4. Static Scale vs. Automatic In-Motion Checkweigher

While both devices measure weight, static scales and dynamic checkweighers are fundamentally different in design, performance, and application.

Critical Note: Low-cost systems built from static scale bases, fast averaging indicators, and conveyors are not true checkweighers. Static bases cannot withstand continuous dynamic loading and vibration, leading to inaccurate readings and premature load cell failure.

5. Package Weight Control & Statistical Fundamentals

Legal standards require that the average weight of a lot of packages is equal to or greater than the label weight, and no single package has an unreasonable weight deviation from the label. Mastering statistical weight control is critical to meeting these requirements while minimizing production costs.

Normal Distribution (Standard Distribution)

Filling processes are subject to hundreds of random variables (air currents, voltage spikes, humidity, product density changes, mechanical wear). These random events cause fill weight to follow a Normal (bell curve) Distribution, with the following core characteristics:

• 68% of all data falls within ±1 standard deviation (σ) of the mean
• 95% of all data falls within ±2 standard deviations (σ) of the mean
• 99.7% of all data falls within ±3 standard deviations (σ) of the mean

Key Statistical Terms

Mean (μ / Mu)

The sum of all values divided by the number of values. The mean represents the average package weight, which is typically set slightly above the labeled weight to meet legal requirements. The mean alone cannot confirm legal compliance — it must be analyzed alongside standard deviation.

Standard Deviation (σ / Sigma)

Measures the spread of data around the mean for a normally distributed population. A smaller standard deviation means tighter, more consistent fill weights; a larger standard deviation means greater fill variation.

In production, the standard deviation of product weight is primarily determined by filler performance. The core goal of checkweighing and SQC is to define mean and standard deviation, then optimize the filling process to set the mean as close to the label weight as possible while remaining legally compliant.

6. Checkweigher Accuracy: Definition, Testing, and Influencing Factors

Core Components of Accuracy

Checkweigher accuracy is defined by two critical metrics: Linearity and Repeatability.

• Linearity (Mean Error): How close the checkweigher's measured weight is to the actual static weight of the package. The difference between actual and indicated weight is the error; lower error equals better linearity. Linear error is easily compensated for via calibration.
• Repeatability: Measured via standard deviation, it describes the weight variance from multiple weighments of the same mass. A lower standard deviation equals better repeatability (precision). When manufacturers reference checkweigher accuracy, they almost always refer to repeatability.

Formal Accuracy Definition: The sum of the standard deviation and the mean error of weighments from repeated runs of a single item across the scale. Industry standard is to define accuracy at 2σ (95% of weighments fall within the stated accuracy range).

Accuracy Testing Method

1. Weigh a representative product on a calibrated static reference scale to record the actual weight
2. Run the same product across the checkweigher multiple times, recording every indicated weight
3. Calculate the mean (average) and standard deviation of the checkweigher readings
4. Calculate mean error as the absolute value of (actual weight - checkweigher mean weight)
5. Final accuracy = standard deviation + mean error (defined at 1σ, 2σ, or 3σ)

Factors That Affect Checkweigher Accuracy

Environmental Factors

• Extreme temperature fluctuations (damages strain gauges, causes condensation)
• Debris/dust buildup on the scale platform (offsets tare/zero settings)
• Vibration from nearby machinery (introduces "noise" to the weight signal)
• Air currents/drafts (critical impact on high-precision pharmaceutical checkweighers)
• Electrical noise (ESD, RFI from radios, cell phones, and machinery)
• Moisture/caustic environments (corrodes load cells and components; aluminum load cells are unsuitable for wet/corrosive settings)
• Physical abuse (stepping on the weigh platform, over-torquing bolts, improper cleaning)

Product Factors

• Unstable contents (shifting liquid/granular product extends settle time)
• Product spillage on the scale platform (causes incorrect zero readings)
• Unstable package geometry (high height-to-width ratio, small footprint causes wobbling)
• Variable package shape (inconsistent footprint for polybags/irregular items)
• Inconsistent product orientation (triggers early/late weighing via the photoeye)
• Reflective packaging (confuses photoeye, causes missed/incorrect weighments)
• Insufficient product spacing (multiple packages on the scale at once)

Minimum Required Accuracy Calculation

Filling Applications

Higher accuracy directly reduces product giveaway. The optimal checkweigher delivers the highest possible accuracy for your environment and application, paired with a low-standard-deviation filler.

Counting / Missing Component Detection

Where:
Acw = Checkweigher accuracy
Wcomp = Weight of the smallest component to be verified
STDtotal = Standard deviation of the total package weight (including all components and packaging)

Prerequisite: 3 × STDtotal must be less than the weight of the smallest component. If this is not met, weight-based component verification is not possible, even with a perfect checkweigher.

Weight Zone Limits

Zone limits are the weight values that define the cut-off between weight classification zones (typically 3 or 5 zones). For legal compliance, zone limits must be set to virtually eliminate the chance of accepting non-compliant under/overweight packages.

• 3-Zone System: Underweight (Reject), Acceptable, Overweight (Reject)
• 5-Zone System: Underweight (Reject), Marginal Under (Warning), Acceptable, Marginal Over (Warning), Overweight (Reject)

For 95% classification accuracy, tighten zone limits by 2σ of checkweigher accuracy from the legal maximum/minimum acceptable weights. For 99.7% accuracy, use a 3σ adjustment.

7. How to Choose the Right Checkweigher

Four core application considerations define your checkweigher requirements: system environment, required accuracy, line rate, and package specifications.

1. Environmental Requirements

2. Weigh Cell Technology Selection

3. Line Rate & Package Handling

• Speed vs Accuracy Tradeoff: Line rate is inversely proportional to accuracy. Higher conveyor speeds reduce weigh time, lowering achievable accuracy. For high throughput, split the line across multiple checkweighers to reduce per-machine line rates.
• Golden Rule of Spacing:
  Belt Speed = Packages Per Minute (PPM) × Pitch
  Pitch = distance between the leading edge of consecutive packages.
• Package Handling Design:
  • Rigid, uniform packages (cans, cartons) perform best on chain conveyors
  • Flexible/malleable packages (bags) perform best on belt conveyors
  • Tall/unstable products require guiderails, timing screws, or side transport belts for stability
  • Small/short products require small-diameter pulleys for smooth transfers

4. Rejector System Selection

5. Control System & Feature Requirements

• Reporting: Onboard printing, serial/Ethernet data output, compatibility with PLC/SCADA systems, custom report generation
• Process Control: PLC integration, closed-loop filler feedback, single/multi-head filler performance monitoring, drift correction
• User Interface: Intuitive operation, minimal keystrokes for product changeover, remote control capability, large readable weight display
• Product Changeover: Onboard product memory (25-100+ products), tool-less guiderail adjustment, variable speed drives, barcode-driven automatic setup
• Safety: Emergency stops, guarded pinch points, Lockout/Tagout compliance, UL/CE/ANSI B155.1 agency approvals
• Serviceability: Quick-disconnect connections, tool-less change parts, local service support, preventive maintenance contracts

8. Core Checkweigher System Components

A complete checkweigher system consists of five integrated functional sections, working in tandem to deliver accurate, reliable in-line weighing:

1. Infeed Section

   Matches the speed of the upstream production line, times and spaces products to ensure consistent, stable delivery to the weigh section. Includes timing conveyors, spacing belts, timing screws, and product guiderails.

2. Weigh Section

   The heart of the checkweigher, consisting of the weigh pan, weigh cell (load cell/MFR), conveyor, and photoeye trigger. The photoeye signals the start of the weigh cycle, and the weigh cell captures the dynamic weight reading as the product passes across the pan.

3. Discharge Section

   Transports weighed products to the reject station, maintaining consistent speed and timing to ensure precise rejection of off-weight items.

4. Rejector / Line Divider Receives signals from the control system to remove non-compliant products from the line, or sort products into weight-based grade lanes. Includes timing controls to ensure the correct product is rejected.
5. Computerized Control System

   The "brain" of the checkweigher, responsible for weight signal processing, product classification, rejector timing, data collection and statistical analysis, user interface, and integration with upstream/downstream production equipment.

9. Checkweigher Cost Savings & Payback Calculation

The primary financial benefit of a checkweigher is reduced product giveaway from overfilling, paired with lower labor costs and eliminated compliance risk. Use the calculation below to determine your projected savings and payback period.

Checkweigher Savings Calculation

Real-World Example

A line fills 200 cans per minute, 3 x 8-hour shifts per day, 5 days per week. Average overfill is 10g per can, product cost is $0.0001 per gram. A $19,000 checkweigher with 2g accuracy at 2σ is installed.

• Annual packages: 200 × 60 × 8 × 5 × 52 = 24,960,000 cans
• Current annual overfill cost: 24,960,000 × 0.0001 × 10 = $24,960
• Overfill cost with checkweigher: 24,960,000 × 0.0001 × 2 = $4,992
• Annual savings: $24,960 - $4,992 = $19,968
• Payback period: $19,000 / $19,968 ≈ 0.95 years (11.4 months)

Note: This calculation only includes direct overfill savings. Additional savings from reduced QC labor, eliminated compliance fines, and lower customer complaints will shorten the payback period further.

10. Checkweighing Glossary