Load-cell Check Out Procedure

Symptom:
Scale readings on the scale head display do not return reliably to zero, or drift, or test weights give different values based on their placement on the scale.

This test is done by using Scale Readings

(click to go to test using a volt meter)

Scale failures:
Loadcell hysteresis: a "stretched" cell cannot return to same "resting" value
Loadcell non-linearity: loadcell returns a non-linear response throughout its range.
Scale binding: non load-cell failure due to couplers, tight electrical/hydraulic

The SEARCH FOR BAD LOADCELL(S) should include tests for HYSTERESIS  and LINEARITY. Multiple loadcell scales should be tested at each cell location.

HYSTERESIS

A.  SINGLE LOADCELL Scales:

1. Note scale tare reading.

2. Lift up on scale, then release. Note reading.

3. Load down scale, remove load. Note reading.

Readings which do not return to tare within a few counts, may indicate a 'sprung' cell (one that has been mechanically over stressed) or there may be a mechanical bind i.e. bad scale bearings or binding electrical and/or pneumatic hoses.

B.   MULTIPLE LOADCELL Scales:

If after pulling and pushing the scale using the SINGLE CELL test (above), the readings do not return to the tare value, ISOLATE the faulty cell by:

Pulling and/or pushing at each cell location - the bad cell will be worst at returning to zero.

If a cell is in doubt, perform the Hysteresis test while testing the loadcell signal values with a voltmeter. See instructions for testing by looking at loadcell signals with a voltmeter.

If possible, apply stresses as close as possible to the cell being evaluated.

LINEARITY

A.  SINGLE LOADCELL Scale readings should be taken at several points in its load range. Non-linearity can be caused by sprung cells, but it can also be caused by bad mechanical pivots, binding due to incorrectly routed electrical or pneumatic lines, or other mechanical connections to the scale which do not permit free movement.

B.   MULTIPLE LOADCELL scales should be tested as in single cell systems. Additional testing by tensing and compressing cells at each point, may show up problems not found by normal loading from a central point. A properly balanced scale, should read correctly, regardless of which loadcell is stressed. Un-even results between different cell locations are a strong indicator of a problem with a specific cell.