Rice Lake Weigh Modules/Mount Assemblies - Manual (Page 72)

4-9
INSTALLATION & SERVICE TIPS Load Cell Troubleshooting
Bridge Resistance
Before testing bridge resistance, disconnect the load cell from the digital weight indicator.
Find the positive and negative Excitation leads and measure across them with a multimeter
to find the input resistance. Don’t be alarmed if the reading exceeds the rated output for the
load cell. It is not uncommon for readings as high as 375
Ω
for a 350
Ω
load cell. The
difference is caused by compensating resistors built into the input lines to balance out
differences caused by temperature or manufacturing imperfections. However, if the
multimeter shows an input resistance greater than 110% of the stated output value (385
Ω
for a 350
Ω
cell or 770
Ω
for a 700
Ω
cell), the cell may have been damaged and should be
inspected further. **
If the Excitation resistance check is within specs, test the output resistance across the
positive and negative Signal leads. This is a more delicate reading, and you should get 350
Ω
±
1% (350
Ω
cell). Readings outside the 1% tolerance usually indicate a damaged cell.
Now comes the tricky part. Even if the overall output resistance test was within normal
specifications, you could still have a damaged load cell. Often when a load cell is damaged
by overload or shock load, opposite pairs of
resistors will be deformed by the stress—
equally, but in opposite directions. The
only way to determine this is to test each
individual leg of the bridge. The Wheat-
stone Bridge diagram, right, illustrates a
load cell resistance bridge and shows
the test procedure and results of a sample
cell damaged in such a manner. We’ll call
the legs that are in tension under load
T
1
and T
2
, and the legs under compression
C
1
and C
2
.
With the multimeter, we tested each leg and
got the following readings:
■ T
1
(–Sig, +Exc) = 282
Ω
■ C
1
(–Sig, –Exc) = 278
Ω
■ T
2
(+Sig, –Exc) = 282
Ω
■ C
2
(+Sig, +Exc) = 278
Ω
Note, when testing leg resistance, a reading
of 0
Ω
or
∞
means a broken wire or loose
connection within the cell.
In a good load cell in a “no load” condition,
all legs need not have exactly equal resis-
tance, but the following relationships must
hold true:
1. C
1
=T
2
2. T
1
=C
2
3. (C
1
+ T
1
) = (T
2
+ C
2
)
In this damaged load cell, both tension legs
read 4
Ω
higher than their corresponding
compression legs. The equal damage mim-
ics a balanced bridge in the output resis-
tance test (3 above), but the individual leg
tests (1, 2 above) show that the cell must
be replaced.
**NOTE: On multiple-cell applications for
matched millivolt output, excitation resis-
tance values may be higher than 110%.
If the load cell has passed all tests so far but
is still not performing to specifications,
check for electrical leakage or shorts. Leak-
age is nearly always caused by water con-
tamination within the load cell or cable, or
by a damaged or cut cable. Electrical short-
ing caused by water is usually first detected
in an indicator readout that is always un-
stable, as if the scale were constantly “in
motion.” The wrong cell in the wrong place
is the leading cause of water contamination.
Almost always, these leaking cells are “envi-
ronmentally-protected” models designed for
normal non-washdown, not the “hermeti-
cally-sealed” models that would have stood
up to washdown and other tough applica-
tions. Another cause is loose or broken solder
connections. Loose or broken solder con-
nections give an unstable readout only when
the cell is bumped or moves enough so the
loose wire contacts the load cell body. When
the loaded scale is at rest, the reading is
stable.
To really nail down electrical leakage prob-
lems though, test resistance to ground with
a low-voltage megohm-meter. Use caution;
a high-voltage meter that puts more than 50
VDC into the cell may damage the strain
gauges. If the shield is tied to the case, twist
all four leads together and test between
them and the load cell metal body. If the
shield is not tied to the case, twist all four
leads and the shield wire together and test
between them and the body. If the result is
not over 5000 M
Ω
, current is leaking to the
body somewhere.
If the cell fails this test, remove the shield
wire and test with only the four live leads to
the metal body. If this tests correctly (over
5000 M
Ω
), you can be reasonably sure cur-
rent is not leaking through a break in the
cable insulation or inside the gauge cavity.
Minor water infiltration problems can some-
times be solved outside the
factory. If you are sure that water
contamination has occurred and if you are
sure that the cable entrance seal is the entry
point, try this remedy: remove the cell to a
warm, dry location for a few days, allowing
the strain gauge potting to dry. Before
putting the cell back into service, seal with
silicone around the cable entry point in the
load cell body. This prevents the reentry of
water vapor into the cell.
Resistance to Ground
278
Ω
282
Ω
282
Ω
278
Ω
- Sig + Sig
- Exc
+ Exc
C
1
T
2
T
1
C
2