Rice Lake Z6 Single-Ended Beam, SS Welded-seal, IP67, OIML C3 - Manual (Page 21)

Load Cell Troubleshooting 17
With the multimeter, we tested each leg and got the following
readings:
• T1(–Sig, +Exc) = 282Ω
• C1(–Sig, –Exc) = 278Ω
• T2(+Sig, –Exc) = 282Ω
• C2(+Sig, +Exc) = 278Ω
NOTE: When testing leg resistance, a reading of 0Ω 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 resistance, but the following relationships
must hold true:
1. C1=T2
2. T1=C2
3. (C1 + T1) = (T2 + C2)
In this damaged load cell, both tension legs read 4Ω higher
than their corresponding compression legs. The equal
damage mimics a balanced bridge in the output resistance
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 resistance values may be higher than 110%.
11.4 Resistance to Ground
If the load cell has passed all tests so far but is still not
performing to specifications, check for electrical leakage or
shorts. Leakage is nearly always caused by water
contamination within the load cell or cable, or by a damaged
or cut cable. Electrical shorting caused by water is usually first
detected in an indicator readout that is always unstable, 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
“environmentally-protected” models designed for normal
non-washdown, not the “hermetically sealed” models that
would have stood up to washdown and other tough
applications.
Another cause is loose or broken solder connections. Loose
or broken solder connections 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 problems though, test
resistance to ground with a low-voltage megohm-meter. Use
caution; a high-voltage meter that puts more than 50VDC 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 5000MΩ,
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 5000MΩ), you can be reasonably sure current is not
leaking through a break in the cable insulation or inside the
gauge cavity.
Minor water infiltration problems can sometimes 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.