The following four tests can be used to assist in troubleshooting load
cells. A precision power supply and multimeter are necessary.
Test 1: Mechanical Inspection
Each load cell should be inspected for physical damage. A
thorough inspection of the entire length of any cables should be
done. Nicked or abraded cables can short out a load cell. Inspect
for distortion or cracks on all metal surfaces. Flexure surfaces, if
any, should be parallel to each other and be perpendicular to
both end surfaces. If the load cell has been physically deformed
(bent, stretched, or compressed from original configuration) it is
non-repairable. Consult Aerocon for load cell application
information.
Test 2: Zero Balance (electrical output with no load)
Changes in the zero balance are usually caused by residual
stress in the sensing area. Residual stresses result from
overloading the cell, and /or from millions of cycles.
With a millivoltmeter, measure the load cell's output under a 'no
load' condition, (no weight on the load cell). The output of a
trimmed cell should typically be within +/- .1% of rated output.
Always consult published specifications to obtain the correct
values.
EXAMPLE: If the typical load cell output =2 mV/V and the typical
power supply output =10V, then the full scale output would be
=20 mV. Typical zero balance =+/-.1% of rated output. The zero
reading should then be within +/- .02 mV.
If the output is greater than the zero balance tolerance, the cell is
damaged but may still be useable. When the zero reading
exceeds the zero balance tolerance but is less than 50% of the
full scale output, the load cell may be electronically 're-zeroed' if
the electronic readout with the cell allows it; however, there is no
guarantee that the load cell will remain linear. If a load cell has
more than 50% full scale zero balance shift, it may require 'regauging'.
Test 3: Bridge Resistance
Out of tolerance bridge resistance values are usually caused by
the failure of a compensating element. Another cause is a
broken or burned bridge wire. These failures commonly result
from electrical transients or lightening strikes.
TO TEST FOR BRIDGE RESISTANCE: Measure the resistance
across each pair of input and output leads. The input and output
resistance is typically 350 +/- 3.5 ohms, however, there are
many variations, depending on each load cell's published
specifications. Consult the published specifications of the subject
load cell for proper values and tolerances. If these readings are
'out of spec', the load cell requires repair.
Test 4: Resistance to Ground
Current flow from the load cell's electronics to the load cell body
is usually caused by water contamination within the load cell or
its cable.
TO TEST FOR LEAKAGE: Connect all of the input, output,
sense and ground leads together and measure the resistance
between the load cell body and the leads with a megohmmeter.
Make sure good contact is made with the metal load cell sensor.
The reading should be at least 5000 Megohms. If the load cell
fails this test, retest without the ground wire. If the load cell
passes this test, a problem may exist with the load cell cable. If
the test fails, the load cell requires repair.