Why Your Scale Reads Heavy
During a feed, a hopper scale can display hundreds of pounds that aren't in the hopper. It isn't a broken load cell — it's physics. Here's what your batch panel is really measuring.
Batching physics · with Willaman Solutions LLC
First, an Experiment You Can Feel
Have someone drop a baseball onto your open hand from a few inches away. You mostly feel the weight of the ball — the force on your palm is pretty much just its mass.
Now have them drop it from a third-story window. The instant it lands, the “weight” you feel is far higher than the ball's. Only once your hand and arm absorb the motion — and the ball is still — does the force settle back to the ball's true weight.
A weigh hopper is your hand. Thousands of pounds of aggregate falling ten feet is the ball. And the scale reports every bit of that landing force as weight.
The Baseball Test
Force on palm
From a few inches
steady — just the ball's weight
Force on palm
From the third story
spikes far heavier, then settles
Same ball, same weight — the only difference is how fast it's moving when it lands. Your weigh hopper feels falling aggregate exactly the same way.
Dry Materials: the Scale Leads the Target
When the gate opens, aggregate leaves the bin at nearly zero velocity and accelerates all the way down. By the time it hits the weigh hopper it's moving fast — and stopping it takes force. The scale can't tell that force apart from weight, so the displayed value leads the true contents for as long as material is flowing.
The moment the gate closes, whatever is still in the air — the freefall, or preact — keeps landing, while the impact force vanishes. That's why the reading overshoots, then settles back to the true batched weight.
v = √(2gh)
velocity at impact
25.4 ft/s
sand after a 10 ft drop
+790 lb
extra reading at 1,000 lb/sec
Batch Cycle Simulation
Displayed
7,520 lb
Actual on scale
6,730 lb
Try it: impact force at your plant
Drag the sliders to see how drop height and flow rate change what your scale reads while material is falling.
Velocity at impact
25.4 ft/sec
v = √(2 × 32.2 × 10)
Extra weight displayed
+788 lb
F = (1,000 ÷ 32.2) × 25.4
While material flows, the scale reads this much above the weight actually in the hopper. The moment the gate closes and the last of the stream lands, it disappears.
Encapsulated Cement Scale
Aeration
2–4 PSI · ON
Vent capacity
~50% blocked
Phantom weight
+62 lb and climbing
Cementitious Materials: Air Is Part of the Batch
Cement scales are sealed vessels, and that changes everything. Cement flowing in at 400 lb/sec displaces roughly 7,360 in³ of air every second. All of it has to escape through the dust collector — and a moderately dirty one might only pass a third of that.
The excess air has nowhere to go, so pressure builds inside the vessel and pushes on its walls. Add the 2–4 PSI aeration system that keeps cement flowing, and the scale reads phantom weight — force from air, not material. Filling from a tanker truck has the same effect, but worse.
As the trapped air slowly vents after the gate closes, the displayed weight drifts back down. Panels compensate by targeting extra material so the settled value lands on spec — known as negative preact.
100–200 lb
force from 2–4 psi on an 8" valve
~225 lb
phantom weight, typical buildup
~350 lb
with a severely restricted vent
Try it: phantom weight from trapped air
Air pressure acting on the gate valve area shows up on the scale as weight that was never batched.
Phantom weight displayed
+63 lb
1.25 psi × 50 in²
Field estimates during a 400 lb/sec fill
Moderately dirty dust collector: ~225 lb of phantom weight. Severely restricted vent (6–8 psi buildup): ~350 lb.
Blocked venting makes it worse: the air slowly escapes after the gate closes, so the scale value drifts down as the material settles — what operators know as negative preact.
Speak the Language of the Panel
Four terms that explain almost every “why doesn't the scale match?” conversation at a batch plant.
- Impact force
- The momentum of falling material read as weight. Present only while material is flowing; it disappears the instant flow stops.
- Freefall / preact
- The material still in the air when the gate closes — it hasn’t landed yet, but it’s coming. On batch panels, the freefall value is actually the quantity in the air less the impact force from the flow rate.
- Phantom weight
- Apparent weight created by trapped air pressure in an encapsulated cement scale. It vents away after the fill, so the reading drifts down as material settles.
- Negative preact
- Deliberately targeting more than the setpoint so that after settling and venting, the true amount on the scale lands on spec.
Physics and original diagrams courtesy of Willaman Solutions LLC. © 2026 Willaman Solutions LLC. All rights reserved. Values shown are illustrative field estimates; actual forces depend on plant geometry, materials, and equipment condition.
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