Checkweigher

What is a Checkweigher

A checkweigher is an automatic weighing instrument that determines the mass of discrete, non-identical loads, typically finished packages, one after another as they pass through it, then compares each result against preset weight limits and acts on packages that fall outside those limits. In legal metrology terminology this device is an automatic catchweighing instrument: it weighs items of differing mass automatically, in contrast to a continuous totalizing belt weigher that integrates a flowing bulk stream, or a non-automatic scale that requires an operator. The catchweigher distinction matters because it places the device under OIML R51 and, in the European Union, under Annex VIII of the Measuring Instruments Directive when the weighing result has legal or trade significance.

The core function is acceptance classification. A package is weighed in motion, the measured mass is compared against a lower limit and an upper limit, and the result drives one of three outcomes: accept and pass through, reject as underweight, or reject as overweight. The two purposes this serves are complementary. The first is legal and reputational protection: underfilled packages breach weights-and-measures law and erode customer trust. The second is cost control: every gram of product above the target is given away free, so a checkweigher that feeds fill data back to the filler can drive the average down toward the legal minimum and recover material cost on high-volume lines.

Structurally a dynamic checkweigher is built from three conveyor zones in series. The infeed, or metering, conveyor accepts packages from the upstream process and adjusts their speed and spacing so that only one package sits on the scale at a time, with a clean gap between consecutive items. The weigh conveyor is a short belt mounted directly on the weighing transducer; this is where the mass is captured during the brief window in which the package is fully supported by the weigh bed. The outfeed and reject zone carries accepted packages onward and diverts rejects into a bin or parallel lane through a reject device. Surrounding this mechanical core is the weighing controller, which performs analogue-to-digital conversion, digital filtering, zero tracking, classification, statistics, and communication.

The lineage of automatic checkweighing traces back to the broader history of automatic weighing in the early twentieth century, but the modern in-motion checkweigher matured alongside high-speed packaging in the postwar food and pharmaceutical industries, when manual spot-check weighing could no longer keep pace with filling lines running hundreds of units per minute. Two technology shifts then reshaped the category: the move from mechanical and strain gauge transducers to electromagnetic force restoration weigh cells, which cut settling time to the order of a millisecond, and the arrival of digital signal processing, which let controllers separate a true mass from conveyor vibration in software rather than by mechanical damping alone.

In terms of operating envelope, checkweighers span an enormous range. Pharmaceutical blister and vial lines weigh items of a few grams to a hundred grams with resolutions in the milligram region; food packing lines handle tens of grams to several kilograms; and end-of-line case and parcel weighers reach tens of kilograms on logistics conveyors. No single machine spans this whole range, because the weigh cell, belt length, frame, and reject device must all be matched to the package mass band and the line speed. Correct selection is therefore an exercise in mapping a specific package, throughput, and accuracy class onto a specific weighing technology and mechanical configuration.

Architecture and Checkweigher Types

Checkweighers are grouped along three axes: by weighing mode (static versus dynamic), by package mass band, and by integration level (standalone versus combination with contamination inspection). The most fundamental split is static versus dynamic. A static checkweigher requires the package to stop and settle on the weigh platform before reading, behaving like an indexed bench scale; it is simple and accurate but slow, suited to low-volume, high-value, or manual stations. A dynamic checkweigher weighs the package while it crosses the moving weigh belt, with no stopping, which is what enables line rates of hundreds of packages per minute. Most industrial checkweighers are dynamic; static units survive in laboratories, sample stations, and very heavy single-pallet checks.

Within dynamic machines, the package mass band is the primary configuration driver, because it sets the weigh cell type, the belt length, the frame strength, and the practical throughput ceiling. The table below summarizes the broad classes by mass band, with representative throughput and weighing technology. Throughput figures are catalogue maxima for favourable small, light products and fall sharply for longer or heavier packs.

The third axis is integration. A standalone checkweigher does weighing only. A combination system places weighing in series with contamination inspection in a single hygienic frame and a shared controller, most often checkweighing plus metal detection, or checkweighing plus x-ray inspection. Combination units save line length and consolidate reject handling and data logging, and they are common on regulated food and pharmaceutical lines where both weight verification and foreign-body detection are mandatory. The penalty is that the combined unit must reconcile reject logic from two inspection stages, so the reject confirmation and binning design becomes more involved.

A further mechanical variant is the multi-lane checkweigher, in which two or more parallel weigh belts run side by side under one controller. Multi-lane configurations multiply effective throughput where a single lane cannot reach the required rate, and they are used on very high-speed pharmaceutical and confectionery lines. The trade-off is that each lane needs its own weigh cell and reject device, and lane-to-lane consistency must be calibrated and verified independently.

Three mechanical constants underlie every dynamic checkweigher regardless of class. The infeed must singulate and gap the packages so that exactly one load rests on the weigh bed during the measurement window. The weigh belt length must exceed the package length by enough margin that the package is fully on the bed for a usable settling window at the line speed. And the transfer between belts must be smooth, because every transition injects an impulse that the controller has to filter out. Failures in throughput or accuracy are far more often traced to spacing, belt length, and transfer geometry than to the weigh cell itself.

Weighing Principles and Reject Systems

The heart of a checkweigher is the weighing transducer, and two principles dominate: the strain gauge load cell and the electromagnetic force restoration (EMFR) weigh cell. A strain gauge load cell measures the elastic deflection of a metal spring element. Bonded foil gauges in a Wheatstone bridge convert that strain into a millivolt signal proportional to the applied force. Strain gauge cells are rugged, inexpensive, and tolerant of overload and shock, which makes them the workhorse for heavy package, case, and parcel weighing. Their limitations are slower settling, coarser resolution, and a larger temperature coefficient, which restrict them on the fastest, most precise lines.

An EMFR weigh cell works on a different principle. The weigh pan is held at a fixed null position by a coil suspended in a permanent magnetic field; when load is applied, a servo loop increases the coil current just enough to restore the pan to its reference position, and that restoring current is proportional to the applied force. Because nothing has to deflect and recover, the cell settles extremely fast, on the order of a millisecond, with sampling at high rates, and it resolves far finer mass increments with better temperature stability than a strain gauge cell. This is why high-speed and high-precision checkweighers, including Mettler-Toledo systems built around its FlashCell EMFR technology, Wipotec weigh cells, and Minebea Intec and Bizerba precision units, are almost universally EMFR based. The table below contrasts the two principles on the metrics that matter for selection.

Whatever the cell, the controller never sees a clean step. The weigh belt is vibrating, the package lands with an impulse, and the support transfers from infeed to weigh belt to outfeed within a fraction of a second. The controller therefore samples the cell continuously and applies digital filtering, often combined with vibration-cancelling or anti-floor-vibration sensing that measures frame motion and subtracts it, to recover the true static-equivalent mass from inside the measurement window. The usable window is bounded by the settling-time relationship: the time a package is fully and only on the weigh bed shortens as the conveyor speeds up, as the package lengthens, and as the weigh belt is made longer relative to the package. This three-way coupling between speed, package length, and belt length, not the cell alone, sets the real throughput-versus-accuracy ceiling.

Once a package is classified out of tolerance, a reject device removes it. The choice of rejector is governed by package mass, shape, stability, and line speed. An air-blast nozzle fires a short pulse of compressed air to blow light packs off the belt; it is contact-free and very fast, ideal for sachets, pouches, and small cartons up to roughly 1 kg. A pusher or arm rejector drives a pneumatic plate or sweep across the belt to push heavier or unstable packs sideways into a bin, handling product up to several kilograms. A drop or flip-gate (trapdoor) opens a hinged section of the belt so the reject falls through, giving gentle handling for fragile or top-heavy items. A diverter or sweep arm steers rejects onto a parallel lane for re-work rather than scrap. The table below maps rejector type to package profile.

On regulated lines the reject subsystem carries as much specification weight as the weigh cell. A lockable reject bin with a full-bin sensor, reject-confirmation photocells, and air-pressure monitoring prevents a missed or failed reject from going undetected. An enhanced fail-safe design forces the line to stop, rather than to pass product, if the rejector or its air supply fails, so that a non-conforming package can never reach the accept stream unnoticed. For pharmaceutical and audited food lines this fail-safe behaviour and a tamper-evident audit trail are mandatory features, not optional extras.

Standards, Accuracy Classes, and Average Weight

Because checkweighing usually has legal and trade consequences, its specifications are anchored in legal metrology rather than in marketing accuracy claims. The governing international recommendation is OIML R51, Automatic catchweighing instruments, issued in two parts (metrological requirements and the test report format). In the European Union the equivalent requirements appear in Annex VIII (MID-006) of the Measuring Instruments Directive 2014/32/EU, with general non-automatic weighing fundamentals defined in EN 45501. A checkweigher placed on a regulated line is therefore type-approved and verified against these documents, and a procurement specification should state the required class explicitly.

OIML R51 sorts automatic catchweighing instruments into categories by use. Category X covers checkweighers proper, the in-motion instruments that sort packages by weight to verify prepackaged goods. Category Y covers weight-labelling and price-labelling duty, where the instrument prints or assigns the weight rather than merely sorting it. Within category X the accuracy is expressed as an accuracy class written X(x). The factor x is selected by the manufacturer from the series 1, 2, or 5 multiplied by a power of ten; a smaller x denotes a tighter maximum permissible error and a tighter permitted standard deviation for a given verification scale interval e. So, ordered from most to least demanding, X(0.5) is stricter than X(1), which is stricter than X(2). The class that a line actually needs follows from the package mass band and the applicable trade tolerance, not from the headline display resolution. The table below summarizes the framework.

OIML R51 verification is statistical, not single-shot. A representative procedure for category X is to weigh a reference test load repeatedly, for example sixty or more passes of a known weight, and to evaluate both the mean error and the standard deviation of the errors against the limits set by the chosen class and verification interval. This reflects the physical reality that a dynamic weight is a distribution, not a point value, so an honest checkweigher specification reports an accuracy or standard deviation that is always tied to a stated throughput, package length, and target weight. A bench-scale style single accuracy figure with no operating conditions attached should be treated with suspicion.

Separate from instrument metrology is the prepackaged goods regime that determines how much product must actually be in the pack. In the European Economic Area this is set by Directive 76/211/EEC, implemented in national law, and signalled to the consumer by the lower-case e mark. It defines the three packers' rules that a batch must satisfy at the moment of packing: first, the average actual content of the batch must not be less than the nominal quantity printed on the pack; second, no more than 2.5 percent of packs, that is no more than 1 in 40, may fall short of nominal by more than the tolerable negative error (TNE, also written T1); and third, no individual pack may be short of nominal by more than twice the TNE. The TNE itself is a tabulated function of the nominal quantity.

A checkweigher is the instrument that enforces all three rules at line speed. It rejects any pack lighter than nominal minus twice the TNE so that the absolute-floor rule can never be broken; it counts packs that fall into the band between TNE and twice TNE so that the 2.5 percent inadequate-pack limit is tracked; and it maintains the running batch mean to keep the average safely above nominal. The same statistics that prove compliance also expose giveaway, the surplus product handed out for free when the average sits too far above nominal, which is the economic lever that justifies tightening the fill and is addressed in Chapter 6.

Key Specification Parameters

A checkweigher data sheet may list dozens of figures, but only a handful drive a sound selection. The most important are weighing range and resolution, throughput, accuracy or standard deviation at conditions, settling and belt geometry, ingress protection and hygiene, and reject and interface options. Each is explained below, with the recurring caution that a single number quoted out of context is meaningless on this category of instrument.

Weighing range and resolution. The range is bounded by the minimum and maximum capacity and by the verification scale interval e or the display division d. Representative product families illustrate the spread: the Ishida DACS-GN series covers configurations from roughly 600 g in 0.5 g graduations up to 6,000 g in 2 g graduations, while smaller DACS-W variants weigh from a few grams up to a few hundred grams. The key selection rule is to size the range so the target weight sits comfortably inside it with headroom, and to confirm that the resolution is fine enough to discriminate the TNE band that the average weight rules require.

Throughput. Throughput is quoted in packages per minute (ppm) and is the most over-read figure on any data sheet. EMFR systems reach high rates on small, light products: the Ishida DACS-GN is rated up to 600 ppm, Wipotec HC-A up to 600 values per minute, and Mettler-Toledo FlashCell up to 800 ppm for small light products such as a cereal bar. Heavy cases and bulk packs run at perhaps 30 to 120 ppm. The catalogue maximum applies only to a favourable short, light product; the achievable rate on your line falls as the package lengthens, as the target weight rises, and as the required standard deviation tightens.

Accuracy and standard deviation at conditions. Dynamic weighing accuracy must always be read together with the throughput, product length, and target weight at which it was measured. EMFR systems are commonly characterized by the standard deviation of repeated dynamic weighings, and improvements are quoted relative to a defined product and speed, for example a stated reduction in standard deviation for a 35 g cereal bar at a given ppm while staying inside the MID limits. When comparing two machines, normalize their accuracy claims to the same product, speed, and target before drawing any conclusion.

Settling time and belt geometry. The weigh belt must be long enough that the package is fully supported on the bed for a usable measurement window at the line speed, yet short enough not to waste window length. The governing relationship couples conveyor speed, weigh-belt length, and package length: the available settling window shrinks as speed rises and as the package grows relative to the spare belt length. Vibration management matters too, so anti-vibration sensing and a stable, isolated frame are part of the accuracy specification, not cosmetic options.

Ingress protection and hygienic design. Food and pharmaceutical lines impose washdown duty. Hygienic checkweighers from Wipotec and other makers are offered with frames and weigh cells rated to IP65 for splash duty and up to IP69K for high-pressure, high-temperature washdown, with stainless steel construction, sloped surfaces, and tool-free belt removal for cleaning. The protection rating must match the cleaning regime: an IP65 frame will not survive routine IP69K washdown, and an over-specified IP69K machine on a dry line is wasted capital.

Reject, interfaces, and data. The reject device, its monitoring (full-bin, air-pressure, reject confirmation), and its fail-safe behaviour are core spec items as described in Chapter 3. On the data side, modern controllers offer Ethernet-based interfaces such as Ethernet/IP, PROFINET, and OPC UA, alongside legacy serial and discrete I/O, so the checkweigher can log statistics to a line MES, exchange feedback with the filler, and provide an audit trail. The output list below summarizes the interfaces typically available:

• Discrete I/O: Reject solenoid, photocell inputs, line-stop and alarm contacts for direct wiring to the conveyor and rejector.
• Serial (RS-232 / RS-485): Legacy links to printers, labellers, and older line controllers.
• Industrial Ethernet (Ethernet/IP, PROFINET): Real-time exchange with a PLC for line integration and feedback fill control.
• OPC UA / MQTT: Statistics, batch reports, and audit data to an MES or historian in smart-factory architectures.
• USB / network export: Batch records, Cpk reports, and event logs for quality and regulatory documentation.

Selection Decision Factors

To turn the preceding chapters into a specific machine, follow the decision sequence below. As with most line equipment, selection mistakes rarely come from one wrong number; they come from deciding throughput or weigh cell before the package, accuracy class, and hygiene duty have been pinned down. These eight steps can serve as a fixed RFQ template.

1. Package envelope and mass band: First fix the minimum and maximum package weight, length, width, height, and stability on the belt. This sets the weigh cell type, belt length, and frame class before anything else, per the Chapter 2 mass-band table.
2. Required accuracy class and trade tolerance: Decide the OIML R51 / MID category X class, X(0.5) versus X(1) versus X(2), from the trade tolerance and the average weight TNE for your nominal quantity. The class, not the headline resolution, defines the legal duty.
3. Throughput at real conditions: State the line rate in ppm for your actual product, then demand a vendor guarantee of accuracy or standard deviation at that ppm, that product length, and that target weight, not the catalogue maximum.
4. Weighing technology: Choose EMFR for high speed and fine resolution (food, pharma, precision) and strain gauge for heavy cases and parcels where ruggedness and cost dominate, per the Chapter 3 comparison.
5. Reject mechanism and fail-safe: Match air-blast, pusher, drop gate, or diverter to package mass and fragility, and specify lockable bin, full-bin and air-pressure monitoring, reject confirmation, and enhanced fail-safe line-stop for regulated lines.
6. Hygiene and ingress protection: Set IP65 for splash or IP69K for high-pressure washdown, with stainless construction and tool-free cleaning, matched exactly to the cleaning regime so the rating is neither under nor over specified.
7. Integration and data: Decide standalone versus combination with metal detection or x-ray, and specify the control interface (Ethernet/IP, PROFINET, OPC UA) needed for line integration, feedback fill control, and audit-trail reporting.
8. Feedback fill control and giveaway target: If giveaway reduction is a goal, require the checkweigher to compute moving statistics (mean, standard deviation, Cpk) and feed the trend to the upstream filler so the fill setpoint can be lowered toward the legal minimum.

One dimension that is routinely underweighted at the purchasing stage is serviceability and verification support: availability of accredited calibration and re-verification service, local spare weigh cells and belts, software for batch-report retrieval, and a documented procedure for periodic legal re-verification. A checkweigher is a verified instrument with a finite verification interval, so a machine that is cheaper to buy but lacks local calibration support can cost more across its life. Major suppliers for high-speed food and pharmaceutical duty include Mettler-Toledo (Garvens and Hi-Speed lines, FlashCell EMFR cells), Wipotec-OCS, Ishida (DACS series), Minebea Intec, Bizerba, and Thermo Fisher, while Anritsu and Loma Systems are strong on combination checkweighing with metal detection or x-ray inspection. Confirm that the chosen supplier holds current type approval for your region and maintains a verification and spare-parts presence near your plant.