In an asphalt plant, the quality of the mix is decided largely at the scales. Aggregate, filler and bitumen are weighed on separate scales and combined in the mixer. If one scale is off by a few kilos, the binder content or the grading of the mix drifts away from the recipe — and finding that out after the road is built is very expensive. This page explains how an asphalt plant weighing system works, what the automation takes over and what to watch out for.
How does weighing work in an asphalt plant?
In a batch-type asphalt plant the aggregate is first dried and heated in the dryer drum, lifted by the elevator to the screen and separated into the hot bins by fraction. Weighing starts from these hot bins:
- Aggregate scale: Hot bin gates open one after another and the fractions are weighed on the same scale on top of each other, i.e. cumulatively. Each fraction’s target is added to the running total.
- Filler scale: Filler is fed by screw conveyor into a separate scale. Because the quantity is small, even a few kilos of deviation is large in relative terms.
- Bitumen scale: Heated bitumen is pumped into its scale. It has the biggest effect on the cost and durability of the mix, so this is the most sensitive weighing.
When all three scales are ready, aggregate and filler are discharged into the mixer first; after a short dry mix the bitumen is added, and when the wet mixing time is over the mix is discharged into the truck or skip. Every step needs timing and sequence — and the real job of the automation is to run that sequence the same way in every batch.
Load cells, tare and calibration
Scales measure with load cells. The load cell signal goes to a weighing module or analogue input on the PLC and the software converts it to kilograms. Three things need regular checking:
- Zero and tare: Material or bitumen left on the scale after discharge is added to the next weighing. Without an automatic tare after discharge, the error accumulates batch after batch.
- Calibration: Calibration with known weights proves the scale reads the true value. Repeat it after replacing a load cell, after mechanical work and at the start of the season.
- Mechanical freedom: A pipe touching the scale body or a tight cable creates a force the load cell cannot see. The software may calculate correctly, but the measurement is wrong.
In-flight material and jog feeding
When a bin gate closes, the scale seems to have reached its target, but material is still falling between the gate and the scale. This is called in-flight material or free fall. Without compensation, every fraction ends up slightly over target. Good weighing logic closes the gate slightly before the target and uses short open/close pulses (jog feeding) near the target. We explain this in detail in what is in-flight material and how to set it.
What to look for in weighing automation
- Sequence rules: no new weighing while the scale is discharging into the mixer; no new batch into the mixer before it has discharged. Having these rules in software removes operator error.
- Running out of material: if the gate is open but the weight does not increase, the bin is empty or the material is bridging. The software must detect it and stop production.
- Records: target and actual values of every batch should be logged, and out-of-tolerance batches should stand out in the report.
- Recipe discipline: recipe changes should be restricted to authorised users, and it should always be possible to trace which batch was produced with which recipe.
How DKT APlants handles weighing
DKT APlants asphalt plant control software weighs aggregate, filler and bitumen automatically according to the recipe. The bitumen scale is filled to the recipe quantity and discharged into the mixer; empty-bin signals are shown on screen. Production records and an event log are kept and the session is recovered after a power failure. The software talks to the PLC over Modbus TCP; the panel and field wiring stay untouched.
Frequently asked questions
Why is aggregate weighed cumulatively in an asphalt plant?
Weighing the fractions on top of each other on one scale means a single scale and a single discharge gate are enough. Each fraction’s target is added to the previous total; the software calculates these totals from the recipe.
How important is a small error in bitumen weighing?
Bitumen is the most expensive component and the one with the biggest effect on durability. Even a small relative deviation affects cost and pavement life, so the bitumen scale’s tare and calibration should be checked regularly.
How often should scales be calibrated?
Always after a load cell replacement or mechanical work on the scale body; otherwise at the start of the season and at regular intervals using known weights.
Can I renew the software without changing my plant’s weighing system?
Yes. Load cells and weighing modules stay in place; what changes is the PC software that talks to the PLC. We check compatibility with the plant’s IO list together.
Related pages
- Asphalt plant control software (DKT APlants)
- Asphalt plant cold feed and dosing
- Burner and dryer drum control
- What is in-flight material and how to set it?
Send us your plant’s IO list
The plant’s input/output list and PLC model are enough. We check compatibility together, set up DKT APlants for your plant and show it running in simulation. WhatsApp: +90 547 909 99 90