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.
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
eg if the vessel weight and load value are known and for example together they are 65Te, and four load cells of 20Te
capacity each are fitted to the vessel, and if the load cell mV/V is 2, and an excitation voltage of 9.8 Volts is
applied, a reading of approximately 15.93mV would be expected. The following is the formula to show how this is
arrived at:
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 (Ω) 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, which should be approximately 380Ω (+ or - 30Ω) for the T61, T62, T63, T95 and T90/93 models. VC3500 range should read 785Ω +/- 20Ω. The 350, T66 and 650 models' input resistance is 400Ω +/- 20-30Ω, while the T30 input resistance should read 750Ω +/- 30Ω and model 740 should be 800Ω +/- 5Ω.
Connect meter across load cell signal wires and read the output resistance, which should be 350Ω +/- 1.5-3Ω for the T61, T62, T63, T66, T90/93, T95, 350 and 650 models. The VC3500, T30 and 740 models should read 700Ω +/- 1.5-5Ω
Please refer to individual data sheets for full resistance values or for details of other models not specified above.
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 megohmeter between the wires and the load cell body. If all is well a reading of at least 5000 Megohms (MΩ) should be returned. Test again without the screen wire to eliminate or incriminate it.
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
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) could
be exacerbated by movement taking place around it (refer to load cell checks page).
Electrical Influence
If electrical noise is influencing the system it would normally be present regardless of loading on the
platform, 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 minimise 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.
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 been recently carried out or altered? If so consider how the calibration was achieved,
which method was used.
Instrumentation
Checks for mounting integrity should be made.
