Cement and Water Scale Problems
This guide covers 16 faults on cement and water scales (weigh hoppers) related to weighing, discharge and the air system, with the symptoms, possible causes and checks and fixes for each.
Safety warning: The checks and interventions in this guide must only be carried out by trained maintenance personnel. Before entering the mixer or working on the hydraulic, pneumatic or electrical system, stop the plant, isolate the power and apply the LOTO (lockout/tagout) procedure; release the pressure in the hydraulic and pneumatic lines. If in doubt, contact MEKA technical service.
Scale discharges slowly and briefly shows a negative value
Symptom: The scale flows very slowly when discharging into the mixer, and the kg value on screen briefly drops below zero (-kg) before recovering.
Checks and fixes:
- Check that the scale discharge flap/butterfly valve opens fully, and check the bypass
- Inspect the discharge line for blockages, material bridging or build-up restricting the flow
- Check the load cells and their connections for mechanical binding, impact damage or incorrect mounting
- Check the scale calibration and zero settings
- Check the load cell signal cables and connection points for interference, looseness or breaks
- Check whether vibrations during discharge affect the scale reading
- Check the discharge time, valve opening speed and automation parameters
- After adjustments and maintenance, test the scale's discharge accuracy
Mixer inlet sock collapses inward when the flap opens
Symptom: When the scale discharge flap opens, the mixer inlet canvas sock is sucked inward (vacuum).
Checks and fixes:
- Check the mixer venting line and dust filter
- Check for filter blockage, restricted air passage or insufficient venting
- Check for air flow problems that create a vacuum inside the mixer during discharge
- Check the inlet sock connections and air passage points
- Check the scale discharge flap opening speed and the material discharge rate
- Check the mixer cover, seals and enclosed areas for airtightness
- If necessary, increase the mixer venting capacity or clean/replace the filter
- After maintenance, test the air flow and sock movement during discharge
Cement does not flow for 2–3 seconds, then dumps all at once
Symptom: Although the scale flap is commanded to open, cement does not flow at all for the first 2–3 seconds and then discharges all at once.
Checks and fixes:
- Check the flap opening movement and its full-opening time
- Inspect the pneumatic actuator air pressure, the solenoid valve and the air line
- Check the flap shaft, bearings and mechanical linkages for binding
- Check the scale outlet for cement bridging or material jams
- Check the flap opening speed settings (speed control valve) to prevent sudden opening
- Inspect the silo/scale for moisture-related lumps or material sticking
- Check the air flow and venting in the discharge line
- After adjustment and maintenance, carry out a controlled cement discharge test
Flap under the scale does not open fully and cement flows slowly
Symptom: The pneumatic butterfly valve/flap under the scale does not open fully and cement trickles out slowly.
Checks and fixes:
- Check the opening and closing movement of the pneumatic valve/flap
- Check the air pressure, the pneumatic actuator and the connecting parts
- Clean out cement residue causing build-up or mechanical binding in the valve
- Check that the butterfly valve/flap reaches the fully open position
- After adjustment and maintenance, test the cement flow rate
Air pressure in the mixer pushes the scale upward during discharge
Symptom: While the scale is discharging, high air pressure (back pressure) inside the mixer pushes the scale upward.
Checks and fixes:
- Check the mixer venting system and dust filters for blockage
- Check the pressure relief capacity inside the mixer during discharge
- Inspect the mixer inlet line, filter and air outlets for restricted air flow
- Check the flexibility and fitting of the canvas sock/compensator between the scale and the mixer
- Check the scale suspension, load cells and mechanical fixings against recoil forces
- Check whether the discharge rate is too high and review the flap opening parameters
- Check for sealed enclosed volumes that cause sudden pressure rises in the mixer
- After filter cleaning, venting improvements and mechanical checks, test the system
Weighing value drifts when the mixer motors start
Symptom: When the mixer motors start, the scale weight readings suddenly drift in the negative or positive direction.
Checks and fixes:
- Check how vibration and mechanical shocks from the mixer motors starting affect the scale system
- Check the load cell connections, mounting points and mechanical fixings
- Inspect the distance between the load cell signal cables and the motor power cables, the cable shielding and any electromagnetic interference
- Check the terminals, sockets and signal cables for looseness, oxidation or breaks
- Check the load cell junction box connections and signal balance
- Check the scale indicator/amplifier calibration and filter settings
- Measure the current spikes in the mixer motor contactors and drive circuits
- Check the scale's mechanical connections for contact or friction that motor vibration could stress
- After improving the electrical insulation, cabling and calibration, test the weighing stability
Scale does not return exactly to zero after discharge
Symptom: After the scale discharges, the kg value on the automation screen does not return exactly to "0" and stays between -3 kg and +5 kg.
The load cell mounting bolts or the scale's external suspension chains are mechanically binding due to vibration. Check that the empty scale hangs freely and clean the load cell mounting kits.
Tight inlet bellows stop the scale from moving
Symptom: The scale's top inlet bellows (screw outlet connections) are so tight that they stop the scale from moving down.
The screw outlet bellows have hardened because of cement dust build-up and act as a mechanical restraint. Remove the bellows, clean out the hardened cement and check their flexibility.
Worn flap seal lets material into the mixer while the scale fills
Symptom: The cement scale flap seal is worn and material leaks into the mixer while the scale is filling.
The butterfly valve or flap seal has worn and lost its sealing. Because material leaks into the mixer early during filling, the automation registers an under-weight and mortar quality suffers. Replace the flap seal.
Scale indicator fluctuates while the mixer runs
Symptom: The cement scale indicator keeps showing fluctuating values while the mixer is running.
Mixer vibration is being transmitted to the scale. Check the load cell mounting pads (anti-vibration rubbers) and replace any worn ones.
Hardened dust bellows have spoiled the weighing accuracy
Symptom: The cement scale's dust bellows have hardened and weighing accuracy has been lost.
The bellows fabric has hardened because of cement dust and moisture. Remove the existing bellows and replace them with highly flexible genuine silicone or antistatic dust bellows. Renew the seals.
Discharge flap stays open and the automation does not start the next cycle
Symptom: The cement scale discharge flap stays open and the automation does not move on to the next cycle.
The pneumatic cylinder rod that drives the flap is binding because of mortar dust. Clean the rod, lubricate it lightly and check the flap limit switch setting.
Blocked vent filter makes the scale inflate
Symptom: The vent filter on the cement scale's top cover (above the bellows) is blocked and the scale is inflating.
Air coming from the screw conveyor is trapped inside the scale. Remove and clean the mini filter bag on top of the scale, or replace it with a highly air-permeable antistatic filter fabric.
Pneumatic vibrator runs too often and for too long
Symptom: The cement scale's pneumatic vibrator runs too often and for too long.
Optimise the vibrator run times in the automation. Running pneumatic vibrators for long periods leads to high air consumption and can lower the system air pressure.
Water scale keeps taking water after reaching the target weight
Symptom: The water scale keeps taking in small amounts of water after reaching the target weight.
The pneumatic flap/valve at the water scale inlet is not sealing fully. Replace the flap seal set, or check and tighten the cylinder stroke setting.
High silo jet filter pressure makes the scale air pressure drop
Symptom: The silo jet filter air regulator is set to too high a pressure; during filter pulses the scale air pressure drops.
Check the silo filter cleaning pulse pressure (ideally 4–5 bar). Setting it too high draws too much air from the main air system during pulsing and lowers the scale pressure.
Still not solved? If the fault persists after these checks, you can reach the MEKA technical service team 24/7. For the genuine parts you need, see Concrete Batching Plant Spare Parts.
Other concrete batching plant troubleshooting guides:
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