Direct answer
What does a checkweigher add to a filling line?
A checkweigher measures packs in motion after filling and can classify or reject packs against configured limits. It may also provide trends or controlled feedback to the filler. The design must define whether the purpose is quality monitoring, 100% inspection, legal quantity control, filler adjustment or a combination, because each requires different evidence and safeguards.
- Define pack dimensions, weight range, centre of gravity and tare variation.
- Match conveyor speed, belt length, spacing and transfer to stable weighing.
- Specify reject method, confirmation, bin security and full-bin response.
- Agree data ownership, recipes, permissions, alarms and batch records.
- Bound any automatic correction and prove fault and recovery states.
Separate four functions that are often called “feedback”
| Function | What it does | Key control question |
|---|---|---|
| Measurement | Produces a pack-weight result under defined dynamic conditions. | Is the pack stable, correctly spaced and within the configured measuring range? |
| Classification or rejection | Diverts a pack that falls outside the configured zone or has no valid result. | Can the system prove that the correct pack reached a secure reject destination? |
| Trend monitoring | Shows movement in mean, spread, reject rate or other process indicators. | Are the data grouped by the correct product, recipe, filler head and production state? |
| Automatic correction | Applies a bounded adjustment to the filler when a validated trend rule is met. | How are delay, isolated outliers, head differences, limits and loss of communication handled? |
Design the conveyor route for a stable measurement
The pack should enter, travel across and leave the weighing belt without collision, bridging or external contact. Guide rails, belt transitions, air movement, vibration, wet product, flexible packs and a high centre of gravity can all disturb the result. The line must create enough pitch for the checkweigher to identify individual packs and enough downstream distance for the selected reject action.
Where multiple filler heads feed one lane, record head identity if head-by-head feedback is required. A combined average can hide one drifting head while the overall line remains near target.
Prove the complete reject sequence
Reject integrity includes detection, tracking, actuation, confirmation, secure collection and response to a full or unavailable reject container. Define what happens to packs with no valid weight, closely spaced packs, a failed reject device or loss of air. The line should enter a controlled state rather than allowing suspect product to continue unrecorded.
Test accepted, underweight, overweight and invalid-measurement packs at the real speed and spacing. Include startup, stop, restart, blocked outfeed and containers already between measurement and rejection.
Use trends and bounded correction rather than chasing every pack
Individual packs include normal variation and measurement noise. Automatic adjustment should use a defined rule, minimum evidence and upper and lower limits. It should consider the physical delay between a filler adjustment and the affected pack reaching the checkweigher; otherwise the loop can overcorrect or oscillate.
Agree whether the correction applies to all heads, one identified head or a recipe value. Record every automatic and manual change, and stop correction when communication, recipe identity or measurement validity is uncertain. Review the wider controls and data integration requirements before fixing the interface.
Define tare and quantity-control responsibility
Rigid containers may still vary in empty weight; flexible packs can change presentation on the belt. Decide whether the checkweigher measures gross weight only, uses a representative tare or receives another pack reference. For trade or packaged-goods compliance, obtain competent metrology advice and keep the engineering role of the line distinct from the packer’s legal responsibility.
Use the calibration and verification guide to document reference checks and the accuracy guide to define the process acceptance method.
Checkweigher integration checklist
- Pack length, width, height, weight range and centre of gravity.
- Tare range, product movement and container stability.
- Required sustainable line rate and minimum pack pitch.
- Accepted, underweight, overweight and invalid-result zones.
- Reject method, confirmation, bin security and full-bin logic.
- Recipe, batch, head identity and data-retention requirements.
- Feedback rule, delay, correction limits and communication faults.
- Reference checks, challenge packs and FAT/SAT scenarios.