1) SINGLE CELL
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.
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.
loadcells: REVERE/H.B.M. Most Others
+ EXCITATION to
cell Green Red
- EXCITATION to
+ SIGNAL from cell in
tension Red White
- SIGNAL from
cell in tension
B. Separate the signal wires at the junction
C. Verify that the excitation is still
applied to each cell.
D. Apply excitation (system on), verify 8-15 volts d.c.
E. Write down empty scale tare milli-volt
readings from each cell.
F. Add weight to scale progressively, write down readings for
apply stresses at cell under test.
These examples assume 30mv/volt loadcells with 10 volt
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#
LOAD SIGNAL COMPUTED WEIGHT ON CELL
Tare Only: 12
mv. 12/30mv. x 1500# = 600#
18mv. 18/30mv. x 1500# = 900#
24mv. 24/30mv. x 1500# = 1200#
30mv. 30/30mv. x 1500# = 1500#
MULTIPLE CELL SCALES,
example of a 3-cell scale
using 3 x 500# cells and a scale with a 600#
LOAD CELL-1 / weight CELL-2 / weight CELL-3
Tare: 12 mv.
200# 11.4 mv. 190# 12.6 mv. 210#
300# 18 mv.
300# 17.1 mv. 285# 18.9 mv. 315#
mv. 400# 22.8
mv. 380# 25.2 mv. 420#
mv. 500# 28.5 mv. 475# 31.5 mv.
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
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.
should be isolated from one another in the return signals and be tested as
in the single cell applications for tare and loaded readings.
- The cells should read within 5 mv. of each other
or the cell placement should be examined for even load distribution.