How an Automatic Checkweigher Works
An automatic checkweigher is a high-speed dynamic weighing system. Unlike a static scale, it weighs products in motion — the package passes across a weigh belt at line speed, and the load cell reading is taken during the brief transit time across the weigh platform.
The weighing sequence:
- Infeed conveyor — separates products into single-file with a defined gap between packages; a minimum gap of one package length is required for individual weight readings
- Weigh belt — the package crosses the weigh platform; the load cell captures a reading; typical weighing time is 0.1–0.5 seconds depending on belt length and line speed
- Signal processing — the controller applies a digital filter to the load cell signal, removing vibration noise; the filtered reading is compared against the programmed upper and lower limits
- Accept or reject signal — if the reading is within limits, the package passes to the outfeed; if outside limits, a rejection signal triggers a downstream device
- Rejection — the rejecter (pusher, air blast, flipper or diverter) physically removes the out-of-spec package from the line
The critical parameter is the weighing time window — how long the package spends on the weigh belt. A 200 mm weigh belt at a line speed of 30 m/min gives a 0.4-second window. A shorter belt or higher speed compresses that window, reducing the number of load cell samples available and degrading accuracy. There is a direct trade-off between line speed and checkweigher accuracy that the specification sheet does not always make explicit.
Checkweigher vs. Bagging Machine: What Each Does
These two pieces of equipment are frequently confused in procurement conversations because both involve weighing and both are part of a packaging line. They do fundamentally different jobs:
| Packing Scale / Bagging Machine | Automatic Checkweigher | |
|---|---|---|
| Function | Controls fill weight — stops filling at target weight | Verifies fill weight — confirms each filled bag meets spec |
| Position in line | At the fill point | Downstream of fill and seal, before palletising |
| Weighing mode | Static — weighs the bag while stationary | Dynamic — weighs the bag in motion |
| Accuracy | ±0.1–0.5% of fill weight | ±0.5–2 g absolute (independent of fill weight) |
| What it catches | N/A — it controls fill, not verifies it | Underfill, overfill, missing product, double-filled bags |
A bagging machine with good fill accuracy still needs a checkweigher — because the bagging machine catches fill drift, but does not catch mechanical failures: a bag that slipped off the fill spout during fill and received only 40% of the target weight, or a bag that jammed against the bag clamp and filled to 130% before the operator noticed. The checkweigher catches these outliers; the bagging machine’s fill control catches drift. When we supply both as part of an automatic packaging line, we integrate the feedback loop between them so the checkweigher’s drift signal adjusts the filler set-point automatically.
Rejection Systems: Choosing the Right Type
The rejection device is chosen based on package type, line speed, and whether the rejected package needs to be recoverable or can be discarded:
Pusher rejecter
a pneumatic arm sweeps the package off the side of the conveyor; fast (response time 30–50 ms), suitable for rigid packages (cans, cartons); can damage soft bags if the push force is too high
Flipper / drop flap
a trap door in the conveyor drops the rejected package into a bin below the line; suitable for products where the bag can survive a 300–500 mm drop; simple mechanism with good reliability
Air blast rejecter
a valve opens a blast of compressed air that deflects lightweight packages off the conveyor; suitable for packages under 500 g; not effective for heavy bags (25 kg) where air force is insufficient
Diverter belt
a section of the outfeed belt pivots to direct the package into a reject lane; gentle, suitable for fragile products; slower response time (100–200 ms) limits maximum line speed
For 25 kg flour or grain bags on a high-speed line, a diverter belt is the most appropriate rejection system — it handles the weight without damaging the bag, and it deposits the rejected bag onto a recovery conveyor where the operator can inspect and rework it rather than discarding it.
Line Speed and Accuracy: Realistic Numbers
Checkweigher specifications quote maximum speed (packages per minute) and accuracy (±X g). Here is what those numbers mean in practice:
Maximum speed is typically tested with rigid, stable packages on a vibration-free test bench. On a production line with conveyor vibration, irregular package spacing and temperature variation, reduce the quoted maximum speed by 15–25% to get a realistic operating speed. A checkweigher quoted at 120 ppm (packages per minute) will run reliably at 90–100 ppm in production.
Accuracy (±X g) is the 2-sigma (95th percentile) reading error under controlled conditions. At line speed and with ambient vibration, expect 1.5–2× the quoted figure. A checkweigher specified at ±1 g will typically achieve ±1.5–2 g in production on a 25 kg bag — which is still well within legal tolerance limits for declared-weight goods in most markets.
When Is a Checkweigher Legally Required?
In the EU, the Weights and Measures Directive (76/211/EEC, updated by 2009/23/EC) requires producers of pre-packed goods sold by declared weight to implement a system that ensures average fill weight meets the declared value and limits the number of short-weight packages. A checkweigher is the standard implementation of this requirement — but the directive does not mandate checkweigher use specifically; it mandates the outcome (statistical compliance).
In practice: if you are selling declared-weight products into EU retail, a checkweigher is the only practical way to demonstrate compliance during an audit. Running without one and relying on bagging machine fill control alone creates a compliance gap that a trading standards inspector will identify.
For food applications, HACCP plan requirements may also mandate a checkweigher as a Critical Control Point for underfill detection, depending on your product and customer specification.
Frequently Asked Questions
Can a checkweigher detect metal contamination?
No — a standard checkweigher detects weight only. Metal detection requires a separate metal detector, typically positioned upstream of the checkweigher on the same line. Some suppliers offer combined checkweigher and metal detector units in a single frame; these are more compact but the two functions remain independent. If you need both (as most food lines do), check whether a combined unit or two separate machines gives better throughput and maintenance access on your specific line layout.
What is the minimum package weight a checkweigher can handle?
Entry-level checkweighers are specified down to 5 g minimum package weight for pharmaceutical and small-format food applications. For industrial bagging applications (10–50 kg bags), the minimum is not the constraint — the maximum capacity is. Confirm that the checkweigher weigh cell is rated to at least 120% of your maximum package weight to avoid overload damage from product surges.
How does a checkweigher integrate with a bagging machine?
Most modern bagging machines and checkweighers support a feedback signal: if the checkweigher detects a consistent drift (e.g., 10 consecutive bags underweight by 30 g), it sends a signal to the bagging machine controller to adjust the fill target upward. This closed-loop feedback reduces fill waste on overfill and prevents batches of underfill bags reaching the rejection point. Not all bagging machine controllers support this — confirm compatibility before purchasing checkweigher and bagging machine from different suppliers. When we supply both, we configure and test this feedback loop before shipment.
Add a Checkweigher to Your Line
We supply automatic checkweighers as standalone units or integrated into a complete automatic packaging line. Tell us your bag weight range, line speed (bags per hour) and package type, and we will specify the right checkweigher and rejection system. We sell direct — no distributors — and our engineers handle installation and commissioning. If your application also involves upstream weighing, see our guide to automatic batching systems.