Unstable or drifting scale reading — load cells, cables and junction box
Symptom
The reading jumps when a motor starts or at random, the zero creeps over days or jumps after an impact, or one cell reads very differently from the others.
Likely causes
- Electrical noise - load cell cable run next to motor or inverter cables, or a poor shield earth.
- Water inside a load cell or the junction box.
- Overload or shock, such as a rock falling into an empty hopper.
- Welding current passed through a cell.
- Loose terminal or cut or crushed cable.
- Vibration through worn anti-vibration pads.
Most likely and cheapest to check first.
Checks
- Wiggle the cables while watching the reading; open the junction box and look for water.
- Disconnect each cell at the junction box and test input, output and insulation resistance against its datasheet.
- Compare the cells with each other. A faulty cell stands out.
- Corner test - put the same weight on each corner and compare.
Fix
- Run load cell cables in their own tray, crossing power cables at 90°. Earth the shield at one point only.
- Dry or replace the junction box; seal glands; add silica gel.
- Replace the damaged cell and verify the scale.
- Replace worn anti-vibration pads.
Stop the equipment if
The scale cannot return to within zero tolerance. A bypassed zero interlock is a stop condition.
Parts you may need
Why small faults make big errors
A typical load cell gives about 2 mV per volt of supply at full load. With 10 V supply that is only 20 mV at full load. A little moisture or a noisy cable is enough to move the reading by kilograms.
| What kills load cells | Prevention |
|---|---|
| Overload and shock | Never drop material from height into an empty hopper; fit overload stops |
| Welding current | Remove cells before welding, or clamp the earth so current cannot pass through them |
| Water | Tight glands, sealed junction box, drip loops, no pressure washing on cells |
| Lightning and surges | Earth the structure; surge protection on long cable runs |
CAUTIONNever shorten a load cell cable. Its length is part of the cell's temperature compensation. Coil the extra length in the junction box.
FIELD TIPKeep the calibration sheet with the spare load cell. When it is fitted, you need the old settings and the last verification to compare.
CAUTIONA non-matching replacement load cell (different linearity, temperature compensation or output) can look correct at first and then drift with temperature. Fit the same model and rated output, or recalibrate the whole scale across its range.
⚠ Every check inside a machine starts with full lockout. Safety disclaimer