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 a milli-volt meter

(click to go to test using the display readings)

You should test for HYSTERESIS  and LINEARITY.

HYSTERESIS

1)   SINGLE CELL SCALES:
a.   take a tare - reading to tenths of a mv.
b.   put cell under tension -- remove load, note tare in mv.
c.   put cell in compression -- remove load, note tare in mv.
d.   Tare readings must return to original within 0.1 mv.
      If not, cell is damaged, or the scale bearings are bad.
 
2)   MULTIPLE CELL SCALES:
ISOLATE signal from cells, then follow as in single cell systems except take readings for each cell.
To isolate signals from paralleled cells:

A.  Determine the return signal wire colors.
      Colors for loadcells:                               REVERE/H.B.M.           Most Others
      +    EXCITATION to cell                           Green                            Red
      -     EXCITATION to cell                            Black                            Black
     +    SIGNAL from cell in tension          Red                                White
      -     SIGNAL from cell in tension          White                            Green

B.  Separate the signal wires at the junction box.
C.  Verify that the excitation is still applied to each cell.
D.  Apply excitation (system on), verify 8-15 volts d.c. (typical)
E.  Write down “empty scale” tare milli-volt readings from each cell.
F.  Add weight to scale progressively, write down readings for each cell.
      Where possible, apply stresses at cell under test.

LINEARITY

These examples assume 30mv/volt loadcells with 10 volt excitation.

1)   SINGLE CELL SCALES

should progress evenly in signal value returned for load applied
i.e. this example shows readings from a single cell scale with a 1500# loadcell:

LOAD                    SIGNAL               COMPUTED WEIGHT ON CELL

Tare Only:           12 mv.                  12/30mv. x 1500# = 600# tare

300#                      18mv.                   18/30mv. x 1500# = 900#

600#                      24mv.                   24/30mv. x 1500# = 1200#

900#                      30mv.                   30/30mv. x 1500# = 1500#

2)   MULTIPLE CELL SCALES,

example of a 3-cell scale using 3 x 500#  cells and a scale with a 600# tare.

LOAD  CELL-1 / weight              CELL-2 / weight            CELL-3 / weight

Tare:    12 mv.        200#             11.4 mv.      190#         12.6 mv.     210#

300#     18 mv.        300#             17.1 mv.      285#         18.9 mv.     315#

600#      24 mv.       400#             22.8 mv.     380#          25.2 mv.     420#

900#      30 mv.       500#             28.5 mv.     475#          31.5 mv.     525#

NOTE: SIGNALS were measured, WEIGHTS were computed and should total up to actual, i.e. Tare  =  200 + 190 + 210  =  600#.

               EVALUATION OF CELL DATA FOR SCALE ACCURACY

This can be best summarized by documenting the working range of the cell for its LOADING.

1)   SINGLE LOADCELL Scales:

Used (for example) with 10 volts excitation and a 3 mv. /volt response (0mv. no-load, 30mv. full load),  should be tested for tare and full scale signal readings.

- If the full-scale minus tare value is less than 20% (6mv.), gain and accuracy will be impaired.

- A good solution is to mechanically balance out the tare in the scale so that a smaller cell can be used.

2)   MULTIPLE LOADCELL Scales:

The cells should be isolated from one another in the return signals and be tested as in the single cell applications for tare and loaded readings.