What is a dosing system?
A dosing system feeds a measured quantity of powder, granules or liquid to a process in a controlled way. This happens volumetrically, based on volume per revolution or unit of time, or gravimetrically, based on weight measured by load cells. The choice depends on the required accuracy, product behaviour, batch or continuous process and dust-free requirements.
Key points
- Volumetric dosing is simple and economical; suited to a constant bulk density
- Gravimetric dosing measures weight and corrects deviations; needed for high accuracy
- Common dosing devices: feeding screw, dosing gate (rotary valve), dosing valve and vibratory feeder
- Poorly flowing powders need extra measures such as agitators or aeration
- For toxic or explosive powders, containment and ATEX matter
Volumetric or gravimetric?
| Aspect | Volumetric | Gravimetric |
|---|---|---|
| Principle | Fixed volume per revolution or time | Weight measured by load cells |
| Accuracy | Depends on constant bulk density | Higher; corrects product variation |
| Investment | Lower | Higher |
| Suited to | Free-flowing products, less critical recipes | Critical recipes, varying products |
| Variants | Feeding screw, rotary valve, vibratory feeder | Loss-in-weight, gain-in-weight, batch weighing |
Dosing techniques for powders and granules
Feeding screw
A screw in a trough or tube discharges the product; speed determines the quantity. Suits many powders, including slightly cohesive ones. See our feeding screws.
Dosing gate
A rotary cell wheel doses free-flowing product and seals between two pressure zones. See the dosing gate.
Dosing valve
A valve that opens, throttles or closes discharge from a silo, big bag or container. Suited to coarse dosing and batch filling with fine adjustment. See dosing valves.
Vibratory feeder
A vibrating trough gives an even flow for free-flowing and fragile products.
Batch or continuous dosing?
Batch dosing weighs a fixed quantity per charge, for example for a mixer. Continuous dosing feeds a constant flow to a continuous process, such as an extruder. Batch is easier to control per recipe; continuous needs a stable mass flow and often gravimetric control. Read more in powder mixing.
Dosing difficult powders
Cohesive, sticky or bridging powders discharge poorly from a hopper. Solutions include a steeper or asymmetric hopper, an agitator above the screw, aeration or a twin screw. Testing the product beforehand is advisable; product behaviour is hard to predict.
Dust-free and containment dosing
For active ingredients or toxic powders, dosing must be dust-free. This requires closed connections, for example split butterfly valves, and a system matching the required containment level. For combustible dusts, ATEX rules apply.
Applications by sector
- Pharma: APIs and excipients, often closed and cleanable; see pharmaceutical industry
- Chemicals: additives and pigments; see chemical industry
- Food: macro, minor and micro ingredients; see food industry
Frequently asked questions
What is the difference between dosing and weighing?
Weighing measures a quantity; dosing feeds that quantity in a controlled way. A gravimetric dosing system combines both.
What is loss-in-weight?
A feeder mounted on load cells that continuously measures how much weight it loses. The control adjusts the speed so mass flow stays constant.
Which dosing system suits my product?
That depends on bulk density, flow behaviour, required accuracy and quantity. A product test gives the most certainty.
Can a dosing system operate dust-free?
Yes, with closed connections and a suitable containment level.
Conclusion
Volumetric where simplicity and cost count, gravimetric where accuracy counts. The right choice starts with the product. See our dosing systems for powders and granules and the flexible dosing system (FDS), or contact us for advice.
Author: Joost van Velzen, Managing Director Hecht Technology B.V. Updated: .