E012 - Electrical
The electrical requirements of a loadcell can have a significant effect on the measurement accuracy that can be achieved.
If a loadcell is fitted with a standard four core cable the cable length should not be changed after the loadcell is calibrated. The information in the table below shows the typical errors for a 10 metre increase in cable length for two common types of cable with a 700Ω bridge loadcell. A lower bridge resistance will increase the errors and a higher bridge resistance will reduce the errors.
|Cross sectional area per core||0.22||0.5||mm2|
|Resistance per core||0.092||0.04||Ω/metre|
|Loadcell output change for 10m increase in length of cable||-0.26||-0.11||% of load|
|Loadcell output change for 1°C temperature increase on a 10m length of cable||-0.001||-0.0005||% of load|
The cable errors can be reduced to zero by using voltage sensing. This requires loadcell instrumentation that is equipped with this function and two additional cores in the loadcell cable. The instrumentation uses the additional cores to measure the loadcell supply voltage at the loadcell end of the cable and it then makes the required correction to the loadcell signal.
Loadcell accuracy can be reduced by the effects of electrical interference. It is difficult to quantify this but a few simple precautions normally reduce the effect of any potential interference to negligible levels:
- Choose a loadcell with a full range load that is as near to the required measurement range as possible.
- Always use good quality screened cable for loadcells.
- Always connect the screen as directed by the instrumentation manual.
- Do not route loadcell cables next to power cables or cables carrying high power signals. The minimum cable spacing should be 300mm, but a larger distance is better.
- If a loadcell cable must cross a power cable do this at 90°.
- Keep loadcells away from other sources of interference.
- Earth the metalwork that the loadcell is mounted on.
- Avoid using mobile phones or other transmitters near the loadcell when taking critical measurements.
Inevitably some compromises will have to be made but meeting as many of the requirements as possible can avoid a lot of system commissioning problems.