Specifying a checkweigher on throughput alone is the most common route to a warranty fight: a 400 ppm line running 50 g stick packs at ±50 mg needs a different load cell, conveyor, and reject device than the same line running 5 kg cartons at ±2 g, and the same vendor will quote two different frames [S2].
The decision is largely mechanical and electrical engineering, not commercial — belt width, roller height, conveyor speed window, and ingress protection determine the cabinet footprint and frame cost more than the brand label on the load cell. The selection guide workflow at major OEM application desks is a structured intake form for exactly those numbers [S2].
For a process engineer, the checkweigher is the final regulatory weight-of-record before a legal-for-trade or cGMP line releases product, so the acceptance numbers — not the brochure — are what the purchase order has to defend. Buyers who skip the spec gates end up retrofitting a heavier frame or a higher-resolution cell after FAT, which is where most checkweigher budgets break [S2].
Throughput and line-speed window: the first filter
Throughput sets the conveyor speed window, the number of weigh-heads in series, and the reject device type, and is the first number an OEM application desk will demand on the intake form [S2]. Pharma bottling lines at 200–400 bottles/min typically specify dynamic checkweighing with a single weigh head and a multi-air-jet reject, while bulk carton lines at 20–60 ppm can accept a heavier-duty belt with roller-top reject [S2].
Operating speed must stay inside the load-cell sample window: a 50 ms filter time on the weigh head caps the usable belt speed for a given product length, and pushing past it inflates standard deviation rather than throughput. The METTLER-TOLEDO XS3 Checkweigher datasheet illustrates the dynamic — reversible transport direction, flexible height adjustment, and a wheeled mobility package designed to relocate the unit between bottleneck points on a pharma line [S3].
Lines that swing between SKUs — 50 g sachets and 500 g bags on the same conveyor — should be specified for the worst-case product length, not the most common, because the longest product dictates the minimum spacing between weigh head and reject station. Stops or changes shorter than the spacing window force a no-read reject [S3].
Weight range and accuracy class: the load-cell decision
Weight range and required accuracy together pick the load cell — typically a strain-gauge or electromagnetic force-restoration (EMFR) cell with resolution chosen so the displayed increment is ≤1/10 of the legal tolerance. Pharma and food lines commonly run ±50 mg at 50 g (Class I-equivalent resolution), while case-weigher lines target ±2 g at 5 kg (OIML R76 Class III-equivalent) [S2].
Total weight range has to span empty conveyor, lightest product, and heaviest product in a single frame; under-rating the range forces a two-scale install and adds a transfer conveyor that itself becomes a new reject point. Over-rating by a factor of 10× to gain resolution is a frequent cost mistake — a 6 kg cell on a 500 g product will read 2–3× worse than a 600 g cell because the signal-to-noise ratio drops off near the lower end of the load range [S2].
Repeatability is what pays the bill, not absolute accuracy: published spec is usually given as ±1σ under controlled conditions, and on a real line the 3σ figure is roughly 2–3× the 1σ number. Expect the actual standard deviation on a stainless pharma checkweigher to be 2–3× the controlled bench sheet [S3].
Belt width, roller height, and frame footprint
Belt width is sized to the largest product dimension plus 50–100 mm clearance on each side, and it locks in the conveyor roller diameter, motor power, and frame footprint. 100 mm belt systems dominate stick-pack and small-ampoule pharma, 200–300 mm belt systems are typical for cartons and pouches, and 400+ mm belts show up in case-weigher and pallet-in-feed applications [S2].
Roller height — the height of the weigh conveyor above floor — must match upstream and downstream conveyors to within ±5 mm or the line will need ramps that smear product spacing. Variable-height frames cover most of this, but the lift range is typically 700–1100 mm and outside that range the OEM builds custom legs, which adds lead time and cost [S3].
Frame footprint is driven by belt length, which in turn is set by the product-spacing window at line speed. A 400 bottles/min line with 100 mm product pitch needs a weigh zone of 200–300 mm plus a settle zone of equal length, so the cabinet is roughly 800–1000 mm long before the reject station. Lining these numbers up at the intake form is the single biggest defence against an oversized or undersized cabinet at site acceptance [S2].
Environment, IP rating, and washdown spec
Ingress protection decides cabinet material, window material, and cable gland spec, and is the spec gate that most often forces a stainless upgrade over a painted mild-steel frame. Food and pharma washdown zones typically demand IP65 minimum, dairy and meat zones with caustic cleaning chemicals push to IP66 or IP69K, and dry-pack zones can stay at IP54 [S2].
Stainless 304 frames with polycarbonate windows cover most food-grade washdown; stainless 316 with sealed load-cell pockets and welded conduit is the standard for chemical and aggressive-CIP environments. The METTLER-TOLEDO XS3 spec sheet lists a reversible transport direction and adjustable height specifically so the same frame can be moved between wet and dry zones without re-anchoring — a washdown spec, not a marketing option [S3].
For flammable-dust or solvent environments, the checkweigher cabinet must be specified for the ATEX zone or NEC class before any other option, because retrofitting explosion protection on a delivered frame is a full re-design. The intake form on every major OEM includes a zone rating field for this reason [S2].
Integration: protocol, reject device, and data output
Integration is the spec gate that most often gets left to the controls integrator, and that is where most FAT delays originate. Common industrial fieldbus options are PROFINET, EtherNet/IP, and PROFIBUS, with OPC UA emerging as the new spec for greenfield pharma 4.0 lines; serial RS-485 and discrete I/O are still required as a hard-wired fallback for safety stops [S2].
Reject device is part of the checkweigher scope, not the line builder's: multi-air-jet, pusher, drop-flap, and roller-top reject each have different reaction times and product-mass limits. Air-jet works down to ~20 g single-product mass; pusher takes over from 50 g up to a few kg; drop-flap handles bags and cartons up to 10 kg; roller-top is the case-weigher standard [S2].
Data output — weight logs, audit trail, batch reports, and statistical process control (SPC) feeds — has to be specified against the plant MES or historian before the unit ships, not after. A checkweigher that only exports CSV over a shared drive is not legal-for-trade in most regulated jurisdictions and will not pass pharma data-integrity audits [S3].
Selection matrix: a six-criteria scoring pass
Side-by-side scoring against the same six gates (throughput, weight range/accuracy, belt/footprint, environment/IP, integration, reject device) is the only way to compare two checkweighers without one brand's marketing number masking another's engineering limit [S2].
Entry-level pharma checkweighers (e.g. METTLER-TOLEDO XS3-class) typically score well on accuracy and footprint but are limited to single weigh-head, single-reject configurations; mid-range units add multi-head in-motion weighing for higher throughput at higher cost; heavy-duty case weighers score on weight range and belt size but trade away fine resolution at the low end of the cell [S3].
Spec-driven selection for adjacent handling equipment
The same spec-gate discipline — throughput, product mass, accuracy, footprint, environment, integration — applies to upstream and downstream units, and lining them up at the same time prevents a bottleneck at the checkweigher that no checkweigher spec can solve. For bulk upstream feeds, pneumatic conveying selection criteria follow a parallel six-criteria logic on dilute-vs-dense and vacuum-vs-positive pressure that mirrors the checkweigher decision tree. For palletised downstream outputs, pallet rack selection criteria use the same capacity-and-compliance-first ordering — load class, frame type, seismic zone — that should drive the case-weigher frame choice. And where the line terminates in a warehouse, stacker crane price and system cost runs on a comparable spec-first total-system cost pass that has to be in the same capex review as the checkweigher itself. [S1]
First tracking signal: most OEM application desks publish a 6–10 question intake form that mirrors the six gates above; demanding that form at RFQ stage is the cheapest spec-discipline step a buyer can take [S2]. Second signal: the intake form also forces the ATEX/IECEx zone, the legal-for-trade class, and the data-integrity standard (e.g. FDA 21 CFR Part 11 / EU Annex 11) into the quote, so the cross-vendor comparison is on the same axis before any commercial terms are discussed.
For component-level specifications, see checkweigher, pressure transmitter, and flow meter.