Knowledge centre · Accuracy
Understanding ±0.5% filling accuracy — and what affects it
Accuracy is not a single number printed on a datasheet. It is the result of a metering principle, a mechanical tolerance and a set of production variables that either hold or drift.
What the number means
A stated accuracy of ±0.5% with an error of ≤1 g describes deviation from the target dose under defined conditions. Two things matter when you read such a figure: the conditions (product, temperature, container) and the sampling (how many consecutive fills were weighed, and over what period). A figure measured on a stable product in a test bay is a starting point; the question for a buyer is what it does in the eighth hour of a shift.
Why piston fillers drift
- Wear: piston seals and valve seats wear, changing displaced volume progressively.
- Product change: a change in viscosity alters fill time and slip, with no change in the mechanical setting.
- Solids: particles trapped at the valve seat cause intermittent short or long fills.
- Compressibility: products with entrained air behave as a compressible medium — the piston stroke no longer equals the volume delivered.
A servo-driven rotor pump removes the first and fourth of these mechanically: displacement is controlled by rotation, and there is no piston seal to wear. Viscosity and particle effects remain — but they are managed by configuration rather than by re-adjusting the machine.
The variables that decide repeatability in production
| Variable | Symptom when uncontrolled | Control |
|---|---|---|
| Product temperature | Weight drifts during the shift | Upstream temperature control; heated hopper |
| Entrained air | Random light fills | Degassing; agitated hopper; gentle transfer |
| Nozzle/container fit | Spillage, apparent underfill | Filling station built to the actual container |
| Line pressure and supply | Inconsistent refill of the hopper | Level control and consistent feed |
| Check-weighing | Drift goes unnoticed | Statistical check-weighing with feedback to the servo |
Practical takeaway
Buy accuracy as a system, not as a component: metering principle + filling valve matched to the product + container-specific filling station + a check-weighing routine. Where those four are present, ±0.5% is a working figure rather than a laboratory one.