Gravimetric Filling and Dosing Systems
Gravimetric filling doses product by weight instead of volume. A scale measures the container while the controller switches from a fast coarse feed to a slow fine feed and closes the feed just before the target, allowing for material still falling. WPL builds filling software and weighing electronics for food and chemical products, plus the R10 indicator and WeightControl software.
What a gravimetric filling machine does
A gravimetric filling machine doses product by weight: a scale measures the container or a weigh hopper during filling, and a controller stops the feed when the target weight is reached. Filling by volume depends on the density of the product, which changes with temperature, aeration and particle size; filling by weight measures the quantity that is stated on the pack directly.
Every gravimetric filler has four parts:
- a weighing platform or weigh hopper with load cells;
- a feed device, such as a valve, screw conveyor, vibratory feeder or pump;
- weighing electronics and a controller that decide when to switch the feed;
- outputs and data connections to valves, a PLC, a printer or a reporting system.
WPL Industries, established in 1994, has built filling software and weighing electronics for food and chemical products, from single filling stations to lines connected to industrial PCs and PLCs.
Coarse feed, fine feed and material in flight
Most fillers use two speeds: a fast coarse feed fills most of the container, and a slow fine feed brings it to the target. The fine feed makes the final part of the fill small enough that the controller can stop it at the right moment.
The controller has to close the feed before the scale shows the target, because of material in flight: product that has already left the valve or screw but has not yet landed in the container. With a 25 kg bag, for example, a few hundred grams can still be falling at the moment the valve closes, and the exact amount changes with flow rate, drop height and product. A filler that learns from the result of previous fills adjusts its cut-off point, so the final weight stays close to target when conditions drift.
Speed depends on how quickly the reading settles once the feed stops. WPL states that its weighing electronics sample up to 1200 times per second, and WPL filling systems use custom algorithms, including flow estimation, to predict the right moment to close the feed and limit product giveaway.
| Phase | Feed | What the controller watches |
|---|---|---|
| Tare | Closed | Empty container weight, stable reading |
| Coarse feed | Fast | Switch point to fine feed |
| Fine feed | Slow | Cut-off point, allowing for material in flight |
| Settle and check | Closed | Final net weight against tolerance |
| Correction | Closed | Adjustment of the next cut-off |
Tolerance, giveaway and the ℮ mark rules
Giveaway is the product that ends up in a pack above its stated quantity, and every filler has to balance it against the risk of underfilling. Because fills always vary a little, the average must be set somewhat above the nominal quantity; the smaller the variation, the closer to nominal that average can be. Reducing giveaway explains how to measure it.
For prepackages carrying the ℮ mark, Council Directive 76/211/EEC sets three rules: the average content of a batch may not be below the nominal quantity, only a small share of packs may fall short by more than the tolerable negative error, and no pack may fall short by more than twice that error. The directive gives these tolerable negative errors:
| Nominal quantity | Tolerable negative error |
|---|---|
| 5 to 50 g | 9 % of nominal |
| 50 to 100 g | 4.5 g |
| 100 to 200 g | 4.5 % of nominal |
| 200 to 300 g | 9 g |
| 300 to 500 g | 3 % of nominal |
| 500 to 1000 g | 15 g |
| 1 to 10 kg | 1.5 % of nominal |
The packer is responsible for meeting these rules. WeightControl SQC records sample checks of filled packs against tolerance zones, with mean, standard deviation and reports.
WPL products for filling and dosing
The right equipment depends on how automated the filling is, from an operator pouring to a target to a line controlled by a PLC.
| Level of automation | Suggested WPL equipment | What it does |
|---|---|---|
| Manual dosing by an operator | WeightControl Base with a scale | Dosing function with target weight, tolerance, bar indicator and a visual and audible signal at target |
| Multi-component batches | Base with WeightControl Formula | Step-by-step recipe weighing, batch scaling, PDF and CSV batch records |
| Semi-automatic filling station | R10 weighing indicator with a platform such as the SSP or BPS | Filling application, 50 presets with under limit, target and over limit, relay outputs and digital I/O |
| Automatic line | WPL filling electronics and software, specified per project | Custom algorithms, connection to industrial PCs and PLCs |
| Quality control of filled packs | Base with WeightControl SQC | Sample checks under the ℮ mark rules |
The R10 reads up to three scales and has four relay outputs (dry contact, 30 V DC or 250 V AC, 3 A) and 13 digital I/O channels, which can accept a start and stop switch, signal limits or run a filling sequence. Larger valves and motors are usually switched through a contactor or the PLC. Options include a 12-bit analogue output and Profinet, Profibus, Modbus TCP, EtherNet/IP or DeviceNet interfaces, and after each cycle the R10 can send date, time, ID and weight over the serial port or Ethernet. Scale interfaces explained helps you choose a connection.
All software modules are listed under weighing software. For a complete filling machine or an integration with an existing line, see custom weighing solutions.
Practical tips for a filling scale
Most filling errors come from the installation rather than the electronics, so check these points first.
- Keep the load path free. Hoses, filling nozzles and cables must not touch the container or platform, or they carry part of the weight.
- Isolate vibration. Mount the platform on a rigid base away from pumps and vibratory feeders, or separate them mechanically.
- Tare every container. Empty cans, drums and bags vary in weight; tare each one instead of using a fixed value where the variation matters.
- Check with test weights. Place a known weight on the platform at the start of each shift and after cleaning. Every WPL scale can be supplied with an EURAMET cg-18 calibration certificate as an option; see EURAMET cg-18 calibration.
- Record per batch. Store fill weights with batch number and time, so drift becomes visible; see weighing data logging.
Related knowledge
Veelgestelde vragen
Why fill by weight instead of by volume?
The quantity on a pack is usually stated as a weight, and the density of many products changes with temperature, aeration or particle size. A volumetric filler then delivers a different weight for the same volume. Weighing measures the stated quantity directly, which makes it easier to control giveaway and document fill weights.
How fast can a gravimetric filler work?
It depends on the product flow, the container, the drop height and how quickly the reading settles after the feed stops. WPL states that its weighing electronics sample up to 1200 times per second. Send WPL the product, fill weights and required output per hour, so the cycle can be assessed for your application.
Can giveaway be reduced to zero?
No. Every filling process has some variation, and under the ℮ mark rules the batch average may not fall below the nominal quantity, so the target has to sit slightly above nominal. Reducing the variation, through a fine feed, correction of the cut-off point and a stable installation, lets that margin become smaller.
Can a WPL filling scale work with our PLC?
Yes, several routes are possible. The R10 has relay outputs and digital I/O, and offers Profinet, Profibus, Modbus TCP, EtherNet/IP or DeviceNet and an analogue output as options. For complete lines, WPL builds filling systems with an open structure to industrial PCs and PLCs, specified per project.
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