A platform on four load cells reads differently depending on where the test weight sits, because no four cells and no frame are perfectly matched. Corner calibration equalises the contribution of each cell so that the same weight reads the same anywhere on the deck.
Before touching the trimmers
Check that the deck is level, that all four cells carry load (a cell with a gap under it reads zero and cannot be trimmed), and that the junction box is dry. Then calibrate the system span with a weight at the centre. Corner adjustment alters the effective sensitivity slightly, so doing it before span calibration means doing span twice.
The sequence
Place a test weight of at least 20% of capacity over corner one and note the reading. Move the same weight to the other corners. Adjust the trimmer of the corner that reads high, not the ones that read low, because turning a trimmer down is repeatable while turning three up compounds the error. Iterate until all four corners agree within the target, typically 0.1% of the applied load for a commercial platform.
When trimmers are not enough
If one corner needs more than about 5% correction, the cells are badly mismatched or one is damaged. On digital systems, per-cell gain and offset in software replace the potentiometers and give a cleaner result, but the underlying mechanical checks still apply.
Kevin Huang