Every powder dosing system faces the same core problem: the material you are feeding today is not exactly the same material you calibrated for last week. Bulk density shifts with humidity, particle size distribution drifts between batches, and flowability changes with temperature. A volumetric feeder — which assumes a constant mass per screw revolution — will silently over-dose or under-dose as these variables drift. A loss-in-weight feeder detects the error in real time and corrects for it. The trade-off is cost, complexity, and the operational discipline required to keep a gravimetric system running accurately.
How Volumetric Feeding Works — And Where It Drifts
A volumetric feeder meters powder by controlling the speed of a screw, vane, or belt. If the system is calibrated to deliver 100 grams per screw revolution and the bulk density of the powder stays at 0.50 g/cm³, the delivered mass matches the setpoint. But when a new batch of the same powder arrives with a bulk density of 0.48 g/cm³ — a 4% shift, entirely normal in practice — the same screw speed now delivers 96 grams per revolution. No alarm fires. The feeder has no way to know.
Volumetric feeding is the right choice when: the material has tight bulk density specifications and consistent flow properties, the dosing tolerance is ±3% or wider, and the capital budget or operational complexity of gravimetric control cannot be justified. It is also the default for continuous processes where weigh hopper refill interrupts would disrupt downstream operations.
How Loss-in-Weight Feeding Closes the Loop
A loss-in-weight (LIW) feeder mounts the entire feed hopper, screw, and drive on a precision load cell platform. A controller continuously monitors the declining total mass and calculates the instantaneous mass flow rate — typically 5–20 times per second. When the measured flow rate deviates from the setpoint, the controller adjusts screw speed to compensate. This closed-loop gravimetric control maintains dosing accuracy to ±0.25–0.5% of setpoint even as bulk density, fill level, and material flowability change during the batch. Controllers like the ESIT ECI batch weighing controller achieve 24-bit ADC resolution for fill and dosing applications, providing the measurement precision required for sub-gram dose accuracy in pharmaceutical and specialty chemical compounding.
The operational cost of LIW feeding is the refill cycle. When the hopper mass drops to a preset minimum, the controller switches from gravimetric to volumetric mode, opens a refill valve from a surge bin, and refills the hopper typically in 5–15 seconds. During refill, control is open-loop — the system relies on the last known screw calibration. Once the refill gate closes and the mass signal stabilizes, the controller transitions back to closed-loop gravimetric control. The refill duration and frequency are key design parameters: too frequent short refills degrade accuracy by increasing the fraction of time spent in open-loop mode; too infrequent refills require a larger, more expensive load cell platform.
Side-by-Side: When Each Mode Wins
| Application Factor | Loss-in-Weight | Volumetric |
|---|---|---|
| Accuracy under density variation | ±0.25–0.5% (self-correcting) | ±2–5% (drifts with bulk density) |
| Capital cost | 2–4× volumetric equivalent | Lower; simpler mechanical design |
| Refill interruption | Open-loop during refill cycle | No interruption (continuous) |
| Recalibration frequency | Annually (load cell verification) | Per batch or after material change |
| Best for | High-value actives, narrow tolerance | Bulk fillers, wide-tolerance recipes |
For processes handling multiple ingredients at varying dose sizes, a hybrid configuration is common: loss-in-weight feeders on the high-value, low-dose-rate active ingredients where accuracy drives product quality and cost, and volumetric feeders on the bulk carriers and fillers where a ±3% variation is economically acceptable. Understanding which ingredients fall on which side of this line is the practical skill that separates a well-designed dosing system from one that generates endless quality deviations. For broader process control architecture, see our process controller selection and flow meter options for integrated batching systems.
