A batch weighing controller opens a feed gate, watches the weight climb toward a target, and closes the gate at exactly the right moment to land within tolerance. It works in the middle of the production process — dosing flour into a mixer, filling a 200-liter drum with solvent, loading aggregate into a concrete truck. A checkweigher weighs finished packages as they fly past at 50–300 packs per minute on a conveyor belt, sorts them into accept and reject lanes, and logs the weight of every single package for regulatory compliance. The two functions share a load cell, an A/D converter, and a digital display, but the controller's firmware, I/O count, communication protocol, and regulatory standing are entirely different. Picking a batch controller when you need a checkweigher — or vice versa — means replacing the controller, rewriting the PLC integration, and potentially restarting a regulatory certification process.
Batch Weighing: Accuracy During Filling, Not After
A batch weighing controller executes a filling sequence: fast feed at full gate opening to 85–95% of target, slow dribble feed at reduced gate opening for the final 5–15%, and a settle time of 0.5–2 seconds after gate closure for the weight reading to stabilize before the controller decides whether to accept or trim. The critical performance parameter is in-flight compensation: the controller measures how much material entered the vessel after the gate close command was issued (the material already in the air between gate and vessel surface) and adjusts the gate closure trigger point on the next cycle to compensate. A good batch controller lands within ±0.1–0.2% of target after 3–5 learning cycles with consistent material flow characteristics.
The I/O requirement for batch weighing is dominated by discrete outputs: feed gate open/close (fast and dribble gates may be separate solenoids), mixer start/stop, discharge gate open/close, and alarm outputs for over-tolerance, under-tolerance, and material bridge (a stuck feed gate). A batch controller typically has 8–16 digital outputs and 4–8 digital inputs for gate position confirmations and safety interlocks.
Checkweighing: Accuracy at Speed, With Legal Liability
A checkweigher weighs packages in motion — the package is on the weigh pan for 50–300 milliseconds as it crosses the load cell at belt speed. The controller must filter out the mechanical vibration of the conveyor, the impact transient as the package enters and exits the weigh zone, and the low-frequency noise from the factory floor. The result is a weight reading for each package that is compared to a target weight and a set of tolerance limits — typically T1 (minimum legal weight), T2 (warning limit), and TNA (tare weight, used for average weight determination).
The regulatory dimension separates checkweighing from batch weighing. In most jurisdictions, packages sold by weight are regulated by national weights-and-measures legislation — NIST Handbook 133 in the US, the WELMEC guidelines in the EU, and OIML R 87 internationally. A legal-for-trade checkweigher must be certified by a notified body (NTEP in the US, EC type-examination in the EU, or OIML certificate internationally), and the certification covers the entire measuring system — load cell, controller, conveyor, and reject mechanism. The Rice Lake 920i programmable weight indicator and controller supports both batch weighing and checkweighing modes in a single device, with a 120,000-count A/D resolution and configurable digital I/O that maps to feed gates, alarm beacons, reject diverters, and communication to the plant PLC via EtherNet/IP, Modbus TCP, or PROFINET. The 920i's legal-for-trade firmware is NTEP-certified when paired with an approved load cell and scale base, making it one of the few controllers that can transition from process batch weighing to compliance checkweighing without a controller swap.
Is a checkweigher legally required, or can I use a static scale at end of line?
It depends on the jurisdiction and the product type. For prepackaged goods sold by weight in the EU, the average quantity system under the Packaging and Labeling Directive (76/211/EEC) allows statistical sampling rather than 100% checkweighing, provided the packer maintains documented control of the filling process and the sampling plan meets WELMEC Guide 6.6 requirements. In the US, NIST Handbook 133 similarly permits statistical sampling, but many food manufacturers run 100% checkweighing regardless of legal requirement because one underweight package complaint to the state weights-and-measures department can trigger a full production lot audit. For pharmaceuticals, 100% checkweighing is effectively mandatory under 21 CFR Part 211 (cGMP for Finished Pharmaceuticals), which requires that each batch be tested to conform to specifications — and weight is a specification.
Can one controller do both batch weighing and checkweighing?
Yes, if the controller hardware supports configurable operating modes and the firmware provides both batch and checkweighing applications. The Rice Lake 920i is an example: configure it in batch mode for the ingredient dosing station at the mixer, and configure the same model in checkweighing mode at the end of the packaging line. The load cell, A/D converter, and display hardware are identical; the operating mode is selected by loading a different application program into the controller at commissioning. For operations running both batch and checkweighing functions, this common hardware approach reduces spare parts inventory to one controller model — a meaningful operational benefit when a failed controller on either the batch or checkweighing station is holding up the entire line. For more on the load cell and scale hardware that pairs with these controllers, browse our sensors & instrumentation category.
For complementary automation controllers, see our PLC range for production line integration. For weight data communication, browse our industrial communication products.
