Cement batches always overweight or unstable
Symptom
Cement is always over target, overshoots badly when the silo is full, the scale "inflates" with dust from the joints, or the reading goes negative during discharge.
Likely causes
- Screw run-on after the stop, or in-flight value too small.
- Aerated cement flooding through the screw when the silo is full.
- Silo butterfly valve or scale discharge valve leaking.
- Scale vent filter blocked, so air pressure builds up during feeding.
- Mixer vent blocked, pushing air back into the scale at discharge.
Most likely and cheapest to check first.
Checks
- Watch the scale after the screw stops. A rise means run-on or a leaking valve.
- Compare overshoot with the silo full and nearly empty.
- Check aeration timing and the scale vent filter.
- Watch the outlet sock during discharge - sucked in or blown out?
Fix
- Adjust the in-flight value; check the screw motor brake.
- Stop silo aeration during weighing; close the silo valve.
- Replace the worn valve seat.
- Clean or replace the scale vent filter and the mixer dust filter.
Parts you may need
Cement is the most expensive error
Under EN 206, cement must be within ±3 % per load (stricter rules apply under ASTM C94 and many project specifications). Overweight cement gives cement away for free on every m³; underweight cement gives weak concrete and claims.
FIELD TIPA scale is least accurate at the bottom of its range. A 3,000 kg cement scale weighing 150 kg for a small batch works at 5 % of its capacity. That is a design problem, not a calibration problem.
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