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LOAD CELL CHECKS
With the help of an accurate digital multimeter and a ohm meter the following
information will assist in carrying out primary load cell system checks,
either prior to proceeding with calibration of the load cells and instrumentation,
or to determine the general health of a system.
We hope you find our fault finding guide useful. But if you need further help, please click on the button above for more information.
Mechanical Checks
Check the physical installation of the cells onto the application and any
other connections or structures attached to it, prior to system calibration.
Any component causing unexpected resistance to free downward movement of
the cells and the structure under loading will induce accuracy errors to
the display
reading and to any calibration process.
Inspect the physical condition of each cell and its mounting. Look for visible
signs of overloading, such as deformation, bending or cracks. Any components
found in this condition are not recoverable and must be replaced.
Electrical Checks
Determine if the load cells are performing correctly by measuring the output
mV reading from the load cells and compare it to what would be expected for
a given load, supply voltage and load cell mV/V.
The first test would be to measure the sum (combined value) of the load cell
outputs, at the junction box and at the input to the instrumentation. Procedure
as follows:
• Disconnect the summed signal wires from the instrument (leave the supply
[excitation] wires connected). Using the digital Multimeter measure across the
signal wires.
The reading
should represent the expected output if the scale weight and load value are known and
if the load cell mV/V is 2, and an excitation voltage of 10 Volts is applied,
a reading
of
approximately 20mV would be expected with a full load (Capacity of the
scale).
At ZERO around 0-3 mv depending on the dead load of your scale. Check each load
cell, if you fine one that is much higher than the others. It may be the probrlem.
The formula can be transposed to determine any single unknown value.
The same test can be carried out at the output of the junction box to identify
a possible fault in interconnection cabling. If the measured value of the above
test is not as expected, there is a good possibility that one or more of the
load cells is faulty, or that there is a cabling fault.
If the same mV output test is carried out at each individual load cell, (measurement
taken from one load cell at a time at the junction box), it will identify uneven
loading or a faulty cell.
If after carrying out the individual cell output tests, the mean (average) of the individual readings does not equate to the result of the first (combined) test, it would suggest problems in the junction box itself (the termination board) or cabling between the junction box and instrument.
If an individual load cell is suspected, the following tests can be carried
out to confirm its condition:
Resistance Test
Set the Multimeter to OHMS (W) and check input and output resistance as follows:
Disconnect all load cell wires at junction box
Connect meter across load cell excitation wires and read the input resistance
as shown above.
Connect meter across load cell signal wires and read the output resistance
Please refer to individual data sheets for resistance values of you load cell.
To check resistance to ground (caused by moisture or breakdown of internal
insulation) twist all the wires together (including the screen) and then
connect a ohm meter between the wires and the load cell body.
Unloaded Output Test
Reconnect load cell excitation wires at junction box. *Remove all load from
the suspected cell (including the loading assembly if fitted), then carry out
the milli Volt output test. The measured output between should be approximately
zero mV (allow up to + or - 0.5 milli Volts). Any appreciable mV measured suggests
a fault such as overload, short or ingress. *The above test would only be
feasible if facilities are available to safely take the load off from the
load cell.
There are other tests that can be carried out to determine the exact nature
of a fault, but the tests detailed above will be enough to determine where
in the system a fault lies.
All the above tests assume that the instrumentation is in satisfactory working
order.
CAUSES OF DRIFT AND INSTABILITY
There are many possible causes for instability and drift, such as:
Mechanical influence
Load Cell degradation
Electrical Influence (noise)
Cabling & Termination
Compromised Calibration
Instrumentation - Meter / Junction box
Mechanical Influence
If the instrument displayed value is only unstable when the platform or vessel
is unloaded it is possible that the problem is mechanical. So look for resistance
to free movement (or too much movement) of the mounting arrangements and load
cells including any external connections to the structure.
Determine if any vibration or influence is present caused by other systems
operating close by or connected.
Load Cell Degradation
Normally if a load cell has an internal fault that causes irregular values
or drift then it would be expected to affect the system whether loaded or unloaded,
however the fault in a cell (such as poor cabling connection).
Electrical Influence
If electrical noise is influencing the system it would normally be present
regardless of loading on the platform etc., however check for screen continuity
through the system and ensure it is terminated at the end of the line (normally
at instrument dedicated terminal).
To further minimize the possibility of electrical noise interference it is
worth remembering the following:
Keep the length of cabling between load cell and instrument as short and direct
as possible.
Avoid grouping the load cell and instrument communication cabling with other
cables, particularly those that generate or induce high voltages or currents,
and do not allow the cabling to cross over or run close to other cables.
Electrical
Power Source Influence
Power source can also cause problems. Make sure that the source of power meets
manufactures specs.
Cabling Termination
Check integrity of wiring especially at points of termination including junction
box and at instrument. Again, if wiring is causing problems due to loose or
poor contact it would normally be present regardless of loading, however if
movement or vibration is only present when the platform is empty this could
be a contributory factor.
Calibration
Has calibration or internal settings been recently carried out or altered?
If so consider how the calibration was achieved, which method was used. Make
sure
you are
using
at
leased the minimum amount weight recommended for your system. In most cases
10% of the total capacity will work.
| Welding Notes: |
| • Make sure the current dose not run though the load cell when welding. |
| • Always place ground on the same side of load cell when welding. |
| • Keep load cell cool at all times. Place a wet rag over the load cell when welding. After a weld has been done, run water over the load cell to keep it cool. |
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