How to Calibrate Weighing Feeders and Belt Scales

Calibration ensures accurate metering for graphite grinding workshops (critical for product consistency and explosion safety). Below is a standardized, step-by-step guide for weighing feeders (loss-in-weight / screw-type) and electronic belt scales, with graphite-specific safety and precision requirements.

🎯 Core Calibration Principles

• Priority Order: Static calibration → Dynamic calibration → Daily verification
• Allowed Errors (graphite grinding, industrial grade):
  • Weighing feeders: ±0.5% of set flow
  • Belt scales: ±0.25%~0.5% (trade settlement: ±0.25%; general production: ±0.5%)
• Key Precondition: All systems must be explosion-proof (Ex d IIB T4) and electrically grounded (resistance ≤10 Ω) to prevent graphite dust ignition.

1. Calibration of Weighing Feeders (Loss-in-Weight / Screw-Type)

Common for graphite feeding: loss-in-weight (LIW) feeders for precision, screw-type feeders for high throughput.

1.1 Pre-Calibration Preparation

1.2 Static Calibration (Core: Zero & Span)

Step 1: Zero Calibration (Eliminate Tare Weight)

1. Empty the feeder hopper; confirm no residual material.
2. Start the feeder in manual mode at 0 speed; run for 5–10 minutes to stabilize.
3. Enter the Weighing Indicator menu and execute Auto Zero; repeat 3 times and record the average value (should be 0 ±0.1% of full scale).

Step 2: Span Calibration (Verify Full-Scale Accuracy)

1. Load the feeder hopper with segmented standard weights (20%, 50%, 80%, 100% of full scale).
2. For each weight, set the feeder to the corresponding flow rate; record the displayed value vs. actual weight.
3. Calculate error: Error = (Displayed Value – Actual Weight) / Actual Weight × 100%
4. Adjust the gain coefficient in the Weighing Indicator if error exceeds ±0.5%; recheck until all segments meet requirements.

Step 3: LIW Feeder Special Calibration

1. Calibrate the level sensor to ensure accurate material level detection.
2. Test the feed motor speed vs. flow rate curve: record speed at 30%, 60%, 100% flow; adjust PID parameters for stable flow control.

1.3 Dynamic Calibration (Final Validation, Real-World Condition)

Option A: Loss-in-Weight (LIW) Feeder – Material Calibration

1. Set the feeder to the target flow rate (e.g., 50 kg/h for graphite); run stably for 10–15 minutes.
2. Use a precision scale to weigh the total material consumed during the test period.
3. Calculate actual flow: Actual Flow = Total Consumed Weight / Test Time
4. Compare with set flow; adjust PID parameters if error >±0.5%; repeat 3 times for average verification.

Option B: Screw-Type Feeder – Physical Calibration

1. Place a standard collection container (e.g., 500 kg capacity) at the feeder discharge.
2. Start the feeder at the target flow; run until the container is ~80% full.
3. Weigh the collected material; calculate actual flow and error.
4. Adjust the Weighing Indicator calibration coefficient; re-test until error ≤±0.5%.

1.4 Routine Verification Schedule

2. Calibration of Electronic Belt Scales

Widely used for graphite conveying; core methods: chain code calibration (fast) and physical calibration (high precision).

2.1 Pre-Calibration Preparation

2.2 Static Calibration

Step 1: Zero Calibration

1. Run the belt empty for 15 minutes to eliminate belt tension/tare weight.
2. Execute Auto Zero in the Weighing Indicator；repeat 3 times (displayed value = 0 ±0.1% FS).

Step 2: Interval Calibration (Span Check)

1. Place standard weights (50–100 kg) on the weighing section; record displayed value vs. actual weight.
2. Adjust gain coefficient if error >±0.5%; confirm linearity across 20–100% FS.

2.3 Dynamic Calibration (Two Methods)

Method 1: Chain Code Calibration (Most Common, Fast)

1. Measure belt circumference: Run the belt, record time for 3 full revolutions; calculate circumference = (Total Distance / 3).
2. Lay chain code: Cover the entire weighing idler section (center of belt); secure both ends to prevent slipping.
3. Calculate theoretical flow: Theoretical Flow (t/h) = Chain Code Weight (kg/m) × Belt Speed (m/s) × 3.6.
4. Run test: Start the belt; run for ≥3 minutes (or 1 full belt revolution).
5. Calculate error: Error = (Displayed Flow – Theoretical Flow) / Theoretical Flow × 100%
6. Adjust chain code coefficient if error >±0.5%; repeat 3 times for consistency.

Method 2: Physical Calibration (Highest Precision, Final Validation)

1. Place a precision floor scale at the belt discharge end.
2. Set the belt to target speed/flow; run until 500–1000 kg of material is collected on the scale.
3. Record the scale’s actual weight vs. belt scale’s displayed total.
4. Calculate error: Error = (Displayed Total – Actual Total) / Actual Total × 100%
5. Adjust calibration coefficient if error >±0.25% (trade settlement) or ±0.5% (general production); re-test 3 times.

2.4 Speed Calibration (Critical for Flow Accuracy)

1. Measure belt speed with a tachometer at target operating speed (e.g., 1.5 m/s).
2. Input the measured speed into the Weighing Indicator；verify displayed speed matches actual (error ≤±0.1 m/s).

2.5 Routine Verification Schedule

3. Common Issues & Troubleshooting

4. Graphite Grinding Workshop Special Requirements

1. Cleanliness: Daily wet mopping (no dry sweeping) to prevent dust accumulation (ignition hazard).
2. Explosion-Proof: All calibration tools and equipment must meet Ex d IIB T4 rating; no open flames or static sparks during calibration.
3. Dust Control: Ensure workshop slight negative pressure (-5~-10 Pa) during calibration to prevent dust diffusion.
4. Documentation: Record all calibration data (date, operator, error, coefficient) for traceability; retain certificates for 1 year.

✅ Quick Calibration Checklist

1. Pre-check: clean, grounded, no dust accumulation
2. Static zero → span calibration
3. Dynamic chain code/physical calibration
4. Adjust coefficients, re-test 3 times
5. Record results, save certificate
6. Daily zero check before production