following are some easy-to-follow steps to help you with Rice Lake load cell
troubleshooting when your load cell isn’t working properly. Before you begin,
you will need a high-quality, digital multimeter and at least a 4.5-digit ohm
meter. There are four tests: Physical inspection, zero balance, bridge
resistance and resistance to ground.
How does your
load cell look? If it is covered with rust, corroded or badly oxidized on its
exterior, corrosion has likely worked its way into the strain gauge area, as
well. If the general and physical condition appear to be in good health, then
you likely need to look at specifics: sealing areas, the element itself and the
In most load cells, 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, look closely at the strain
gauge seals. 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
for signs of contamination, as well.
Other items to look for include: 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 reviewing the cable. The 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.
This test is effective in determining if the load cell has been
subjected to a physical distortion, which can be caused by overload, shock load
or metal fatigue. Before beginning the test, the load cell must be in "no
load" condition. In other words, either the cell should be removed from
the scale or the dead load should be counterbalanced. 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.
Once the cell is no longer 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 voltage supplied by the digital weight indicator. Be certain to use
the exact same indicator that is used for the cell’s daily operation to get a
reading specific to the application.
value for a 1% shift in zero balance is three-tenths millivolt, assuming
10 volts excitation on a three millivolt per volt output load cell. To
determine your application’s zero shift, multiply the excitation volts supplied
by your indicator by the millivolt per volt rating of your load cell. When
performing your field test, remember that load cells can shift up to 10% of
full scale output and still function correctly. If your test cell displays a
shift under 10%, you may have another problem with your suspect cell, meaning
further testing is required. If the test cell displays a shift greater than 10%,
it is likely it has been physically distorted and should be replaced.
Before testing bridge resistance, disconnect the load cell from the digital
weight indicator. Measure across the positive and negative signal leads using a
multimeter to find the input resistance. If the reading exceeds the rated
output for the load cell, don’t be alarmed; it is not uncommon to receive
readings as high as 375 ohms for a 350-ohm 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 ohms for a 350-ohm cell or 770 ohms for a 700-ohm cell), the
cell may have been damaged and should be inspected further.
If the excitation resistance check is within specifications, test the
output resistance across the positive and negative signal leads. This is a more
delicate reading, and you should get 350 ohms ±1% (350-ohm cell). Readings
outside the 1% tolerance usually indicate a damaged cell.
However, even if the overall output
resistance test is within normal specifications, you may 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 Wheatstone Bridge diagram 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 T1
and T2, and the legs under compression C1 and C2.
With the multimeter, we tested each leg and got the following readings:
- T1(–Sig, +Exc) = 282 Ω
- C1(–Sig, –Exc) = 278 Ω
- T2(+Sig, –Exc) = 282 Ω
- C2(+Sig, +Exc) = 278 Ω
NOTE: When testing leg resistance, a reading of 0 ohm or eight means a broken wire or loose connection within the cell. In a functioning load cell in a “no load” condition, all legs need not have exactly equal resistance, but the following relationships must hold true:
- (C1 + T1) = (T2 + C2)
In this damaged load cell, both tension legs read four ohms 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 percent.
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 by 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 could have stood up to
washdown and other tough applications.
cause is loose or broken solder connections. Loose or broken solder connections
give an unstable readout only when the cell is bumped or moved enough for the
loose wire to contact the load cell body. When the loaded scale is at rest, the
reading is stable.
electrical leakage problems, test resistance to ground with a low-voltage
megohmmeter. Use caution—a high-voltage meter that puts more than 50 volts of
direct current 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 together with the shield wire and test between them and the body. If the
result is not over 5,000 megohms, current is leaking to the body
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 5,000 megohms), 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 that the cable entrance seal is the entry point, try the following remedy: Move 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.