Rice Lake Weigh Modules/Mount Assemblies - Manual (Page 71)

INSTALLATION & SERVICE TIPS
4-8
Load Cell Troubleshooting
Load Cell Troubleshooting
Here are some easy-to-follow steps to help you troubleshoot potential load cell problems. Before you begin you will need a good quality
digital multimeter, at least a 4 1/2 digit ohm meter. The tests are: physical inspection, zero balance, bridge resistance and resistance to
ground.
Zero Balance
This test is effective in determining if the load cell has been
subjected to a physical distortion, possibly caused by overload,
shock load or metal fatigue. Before beginning the test, the load cell
must be in a “no load” condition. That is, the cell should be removed
from the scale or the dead load must be counterbalanced.
Now that the cell is not under any load, disconnect the signal leads
and measure the voltage across the negative signal and positive
signal. The color code for determining negative- and positive-signal
leads is provided on the calibration certification with each load cell.
The output should be within the manufacturer’s specifications for
zero balance, usually
±
1% of full scale output. During the test, the
excitation leads should remain connected with the excitation volt-
age supplied by the digital weight indicator. Be certain to use
exactly the same indicator that is used in the cell’s daily operation
to get a reading accurate to the application.
The usual value for a 1% shift in zero balance is 0.3mV, assuming
10 volts excitation on a 3 mV/V output load cell. To determine your
application’s zero shift, multiply the excitation volts supplied by
your indicator by the mV/V rating of your load cell. When perform-
ing your field test, remember that load cells can shift up to 10% of
full scale and still function correctly. If your test cell displays a shift
under 10%, you may have another problem with your suspect cell,
and further testing is required. If the test cell displays a shift greater
than 10%, it has probably been physically distorted and should be
replaced.
How does it look? If it is covered with rust, corroded or badly
oxidized, chances are the corrosion has worked its way into the
strain gauge area as well. If the general and physical condition
appears good, then you need to look at specifics: sealing areas, the
element itself, and the cable.
All areas of the load cell are sealed to protect the contents from
contamination by water and chemicals. To see if any seals have been
degraded, get right up close to the cell and look at the strain gauge
seals (points A). Is rust concentrated on a part of the cover weld? If
there is no cover, do you see any tiny holes in the potting? These are
indications that there has been contamination to the gauge area.
Check the load cell cable entrance (point B) for signs of contamina-
tion.
Physical Inspection
Other items to look for: metal distortion or cracks, metal rippling,
cracks in the weld, or abrasions in the metal. It may be necessary
to remove the load cell and check it for physical distortion against
a straight edge.
No inspection would be complete without thoroughly inspecting
the cable. Cable should be free of cuts, crimps and abrasions. If your
cable is cut and in a wet environment, water or chemicals can “wick”
up the cable into the strain gauge area, causing load cell failure.
If your physical inspection fails to uncover any identifiable damage,
a more detailed evaluation is required.
B
A